[Aircraft Electrical Systems]
[From the U.S. Government Publishing Office, www.gpo.gov]


WAR DEPARTMENT
TECHNICAL MANUAL
AIRCRAFT ELECTRICAL SYSTEMS
NON-HfRCULATING ™
Reprint of Technical Manual No. 1-406
PUEII^SHERS
2328 WEST SEVENTH STREET LOS ANGELES, CALIFORNIA

LIBRARY
OF
NORTH TEXAS
STATE TEACHERS COLLEGE
DENTON, TEXAS
TM 1-406
WAR DEPARTMENT
h
TECHNICAL MANUAL
AIRCRAFT ELECTRICAL SYSTEMS
Reprint of Technical Manual No. 1-406
2328 WEST SEVENTH STREET
LOS ANGELES, CALIFORNIA
This reprint of Technical Manual No. TM 1 -406 Published by Aero Publishers, Los Angeles, Calif.
Lithoprinted in U.S.A.—1943
*TM 1-406
108481
TECHNICAL MANUAL!	WAR DEPARTMENT,
No. 1-406 J	Washington, April 16, 1942.
AIRCRAFT ELECTRICAL SYSTEMS
Paragraphs
Section I. Fundamentals	of electricity____________________ 1-13
II.	Direct current	circuits__________________________14—20
III.	Magnetism and magnetic devices___________________21-33
IV.	D-c electrical	measurements______________________34—40
V.	Condenser________________________________________41-^5
VI.	Generation of electromotive force by electromagnetic induction_________________________________,____46-56
VII.	Aircraft storage batteries_______________________57-64
VIII.	Generator and regulator	systems__________________65-76
IX.	Ignition systems_________________________________77-87
X.	Starting systems_______________________________  88-99
XI.	Lighting, landing gear, warning, and retracting systems_____________________________________________ 100-105
XII.	Wiring systems_________________________________106-108
Page
Index_______________________________________________________ 185
Section I
FUNDAMENTALS OF ELECTRICITY
Paragraph
General--------------------------------------------------------------- 1
Energy and work------------------------------------------------------- 2
Atoms and their structure_____________________________________________ 3
Molecules and chemical reactions-------------------------------------- 4
Conduction and conductivity------------------------------------------- 5
Separation of charge-------------------------------------------------- 6
Static electricity---------------------------------------------------- 7
Potential differences and potential levels---------------------------- 8
Electromotive force--------------------------------------------------- 9
Electrical current___________________________________________________ 10
Electrical resistance------------------------------------------------ 11
Resistors----------------------------------------------------------   12
Electric circuit---------------------------------------------------   13
1. General.—a. Electricity is	used on aircraft for many purposes.
It serves, broadly, to furnish light, heat, and power. Some common applications of electricity in aircraft are to,start an engine; ignite
♦This manual supersedes TM 1—406, October 18, 1940.
449339°—42---1
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the fuel charge in cylinders; operate navigation, landing, instrument lights, etc.; and operate instruments, radio equipment, and signal devices. The advantage in the use of electricity as a medium for transfer of energy lies in ease of control, adaptability to a multiplicity of functions, and the comparative lightness in weight of distribution systems.
Z>. A background of special knowledge is required for intelligent maintenance of electrical equipment. The essentials of this background are included in the first six sections of this manual. Important technical words are printed in italics at that point in the text where they are explained. A clear conception of basic principles will enable the mechanic to understand modern electrical equipment and will help him to grasp the principles of newly developed apparatus.
c. Analogies have occasionally been drawn in this manual between electrical phenomena and situations which are more familiar. Interpretations closer than those suggested in the text are not to be placed upon these analogies, for in many cases they are inaccurate when carried to minute analysis.
2. Energy and work.—a. Energy is intangible, yet its effects are evident. It may be defined as the capacity for doing work. Its effects on matter are such as to produce heat, light, sound, or motion. Energy can assume various forms, such as mechanical energy, chemical energy, electrical energy, magnetic energy, and heat energy. The law of conservation of energy states that energy can be neither created nor destroyed. This means that when energy in one form is converted into one or more other forms, the amount obtained is exactly equal to the amount supplied.
b. Kinetic energy is energy possessed by a body by virtue of its state of motion, which may be movement along a line, or spinning motion. Potential energy is energy possessed by virtue of position or condition. Work is defined as the product of an effective or moving force multiplied by the distance through which the force acts. In ordinary situations work is measured in foot pounds. If an object is moved through a distance of 1 foot by an average force of 1 pound, 1 foot pound of work is done. In the metric system, which is used for measurement of electrical quantities, the unit of work is the joule. It is equivalent to slightly less than three-fourths of a foot pound. Inasmuch as work creates energy and energy can do work, they are measured in the same units.
c. When a man is struck by a moving automobile he discovers that it possesses kinetic energy because it does work on him. If a brick which weigh 5 pounds is raised from the floor to a table top 3 feet above the floor, the work performed is 15 foot pounds, or approxi
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mately 20 joules. At the level of the table top the brick possesses, with respect to floor level, 15 foot pounds, or approximately 20 joules of mechanical potential energy. If the brick is dropped without being pushed, at the moment of release it possesses no kinetic energy because it is motionless. It gains speed and kinetic energy as it falls. In this case, inasmuch as only a negligible amount of energy is transformed into sound and heat (caused by friction between the air and the brick) and because the law of conservation of energy holds true, the brick, at the instant of touching the floor, still possesses practically all of the 15 foot pounds or 20 joules of energy which it had when it started downward but it is now all in the form of kinetic energy. As the brick hits the floor, all of the kinetic energy, in accordance with the principle of conservation, is converted into sound, heat, and work of denting the floor. If a rubber band is stretched, work is done to overcome the internal forces which tend to hold the particles of rubber together. In its stretched condition, the rubber possesses energy. If released, the stored (potential) energy is changed first into kinetic energy of motion and then into sound and heat.
3. Atoms and their structure.—a. The elementary units of which all matter is composed are called atoms. Atoms are approximately 10,000 times too small to be seen with the most powerful optical microscope. Nevertheless, a substantial amount of evidence has been accumulated which gives a fairly clear picture of atomic structure.
Z>. Each atom in the universe resembles the solar system in structure. At the center of the atom is a very tiny and extremely dense material known as the nucleus. One to 92 relatively light and less dense substances known as electrons are found moving on orbital paths about the nucleus, in somewhat the same manner as the planets of the solar system revolve about the sun. The distances between the nucleus and its planetary electrons are relatively great, just as relatively great distances exist between the sun and its planets.
c.	The planets of the solar system are restrained from flying off into space and are held on their orbital paths by the gravitational pull of the sun; the planetary electrons in the atom are held on their orbital paths by electrical forces. Two kinds or states of electrical charge are found in nature. For reasons to be shown later, the names “positive” (+) and “negative” (—) have been applied to them. Bodies which are oppositely charged are attracted toward each other. This effect is known as an electrical force. Every electron carries a negative charge. Every nucleus is positively charged. The nu-
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cleus of an atom is so massive (from 1,835 to 440.000 times heavier than an electron) that it remains stationary and swings the planetary electrons of the atom around it. Only the lightest nuclei show any evidence of a slight wobbling motion in reaction to the pull of the electrons on them. The terms “electrical charge” and “electrical force” have been presented without explanation as to cause, for science has none to offer at present. Nor is there an explanation for the gravitational force which holds things to the earth and the earth on its orbit. Both forces are acknowledged because their effects are familiar, and because scientists understand the laws under which the forces operate.
d.	All electrons are composed of equal quantities of matter and are therefore said to be equal in mass. Also, they carry equal amounts of negative charge. The charge of an electron has been called an electronic unit of charge, which is useful in measuring the quantity of electrical charge.
e.	Nuclei vary widely both in mass and amount of positive charge carried. Each known atom may be classified according to type on the basis of the number of positive charges carried by its nucleus. These types are called the elements of matter, of which there are 92. Some of the elements are called metals. A distinguishing feature of a metallic atom is that one or more of its outermost planetary electrons is easily detached and may become a “free” electron, capable of moving among the other atoms of the metal. Carbon, which is not a metallic element, also has some detachable electrons.
4. Molecules and chemical reactions.—a. When two or more atoms of the same or different elements are linked together by interaction of their planetary electrons into a more or less stable group, the combination is called a molecule. The stability of a molecule depends upon the ability of its atoms to resist separation. Energy would have to be used to pull a molecule apart and sort out the nuclei and electrons into separate atoms, just as energy is required to pull apart a rubber band.
b. When the atoms of two or more molecules are rearranged so as to form different molecular combinations, the process of rearrangement is known as a chemical reaction. The electrodes of a battery cell obtain their electrical charge by means of a chemical reaction within the cell. When a dry cell is assembled in manufacture its chemicals immediately begin reaction. The central carbon electrode is robbed of a large number of its detachable electrons, which are moved over to the zinc can and crowded in among the detachable electrons of the zinc. The chemical reaction and the movement of
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electrons cease as soon as the ‘‘pressure” of the free electrons in the zinc has reached a certain value.
5. Conduction and conductivity.—a. If the ends of a metal wire are touched to the terminals of a dry cell, some of the electrons which were crowded into the zinc instantaneously begin to push their way in among the less crowded free electrons of the wire. Simultaneously, the electron-hungry carbon electrode starts taking free electrons from the end of the wire attached to it. A drift of electrons begins almost simultaneously all along the wire; the electrons are passed from atom to atom. Simultaneously, the chemicals inside the cell, finding that the concentration of free electrons which they had built up within the zinc is being depleted, resume reaction, transferring electrons from the carbon electrode to the zinc can in an effort to keep up the concentration of electrons which had been previously established. A sustained and continuous flow of electrons around the circuit is the result. This is an electric current.
b.	A conductor is a substance which is able to transport electrons with comparative ease. Metals and their alloys are the best conductors.
c.	Nonconductors are substances in which there are practically no detachable electrons because of the tenacious hold which the atoms and molecules have upon their planetary electrons. Good examples of nonconducting solids are sulfur, mica, glass, and bakelite. Gasoline is a nonconductor. Distilled water is a poor conductor. However, a surface film of moisture, or water to which acid or salt has been added, is quite conductive. Gas molecules will not transport charge in appreciable amounts except after disruptive break-down of the molecules.
d.	Insulators and insulation are composed of nonconductive materials. A wire is insulated to prevent electrons from flowing onto or away from the wire if it rests against another piece of metal which would provide a path for flow of electrons. If, however, the electrical force between the wire and some surface outside the insulation is made great enough, one of the electrons in the insulation will ultimately be detached from a molecule and, by collision, will start an avalanche of charged particles; this is referred to as break-down of the insulating material.
6. Separation of charge.—a. When a body possesses equal amounts of positive and negative charge it is said to be electrically neutral. When the two kinds of charge are close together, they tend to cancel the effect of each other. This accounts for the choice of the words “positive” and “negative” as designations for the two kinds of charge.
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Each atom in its normal state possesses equal quantities of positive and negative, and is therefore normally neutral. Since all substances are composed of atoms, it follows that they too are normally neutral.
b.	The state of neutrality may be upset by adding or subtracting electrons. A body which possesses an excess of electrons is said to be negatively charged. One which is deficient in electrons is positively charged. The carbon and zinc electrodes of the dry cell discussed in paragraph 4 are examples of charged bodies. The quantity of excess or unneutralized charge residing on a body, in practical situations, is measured in coulombs. One coulomb is equal to the combined charge of 6.28 X1018 electrons.
c.	Work must be done in forcing an excess of electrons onto, or in extracting them from, a neutral body so as to charge it negatively or positively. Conversely, when electrons flow from a negatively charged body into a conductor, or are drawn into a positively charged body from a conductor, they do work or give up energy as they move. The charging of a body represents the creation of electrical potential energy; its discharge represents the release of this energy.
d.	Separation of two kinds of charge, that is, the removal of electrons from their parent atoms and the storing of the two kinds of charge in separate places, ready to do work in getting back together again, is performed usefully in aircraft by the dry cell (par. 4), storage battery (par. 57), and dynamo or generator (par. 51). Another kind of separation of charge associated with aircraft is undesirable and will now be discussed.
7. Static electricity.—a. If two substances are placed in close contact, some of the electrons of one substance may mingle with those of the other and remain when the two substances are separated. Each substance becomes “charged” (one positively and the other negatively) with respect to each other. Energy is required to pull the materials apart, for oppositely charged bodies attract each other. Combing one’s hair on a dry day may result in an accumulation of charge on the comb, and the hair becomes oppositely charged. The snapping sparks represent the recombination of the charges and the release of the energy when the comb is pulled away from the hair. If one slides off of the upholstered seat of a car on a dry day, steps to the ground, and then tries to touch the car, a spark may snap between the hand and the car. This may be prevented by holding onto the door handle while getting out; the charges then may flow back to the car and be neutralized as fast as they are separated.
b.	Static electricity is an accumulation of negative or positive electrical charge caused by an excess or shortage of electrons, respec
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tively. Clouds often become highly charged. Lightning is evidence of the movement of electrons to, away from, or between clouds by means of a disruptive break-down of air molecules. The luminous path of the lightning discharge is quite conductive.
c.	Aircraft in flight may become highly charged by contact with charged particles or molecules. When a large metal aircraft, equipped with rubber tires, lands on a dry day, the charged state may persist. One might draw a spark, or receive a shock, from contact with the aircraft. If the neutralization of the charge were to occur via the gasoline hose for filling the tanks, a disastrous spark might occur. Therefore, a ground wire is used to discharge the aircraft. The wire also prevents further building up of charge. In some aircraft the rubber tail wheel contains metallic particles to render it conductive. As a safety precaution, a gasoline dispensing hose is made with a wire within it which electrically connects the nozzle of the hose with the tank at the other end; the gasoline truck is grounded by means of a dragging chain.
d.	Metal parts of an aircraft which are not in good electrical contact with each other are bonded, by use of wire braid. The bonding improves the return path for the electrical circuits on the airplane, prevents the possibility of hazardous static spark discharge between parts, minimizes static and ignition interference in the radio apparatus, and improves signal strength of the radio transmitter.
8. Potential differences and potential levels.—a. In the example of the brick in paragraph 2, the position of the brick at any time in its fall could be described by stating its instantaneous distance above the floor level. The points in an electrical circuit are not designated in terms of distance. They are described in terms of their potential level with respect to some reference level in the circuit. The potential of any point in an aircraft electrical system is measured in terms of the amount of energy a coulomb of charge either loses or gains, as the case may be, in moving between the point in question and the metal framework of the aircraft, which is used as a reference energy level. For each joule of energy gained or lost by a coulomb of charge in so moving, it is said to move through 1 volt of potential energy level. One joule of energy is released or gained by 1 coulomb of charge when it moves between two points which are 1 volt apart in energy level. For example, when 20 coulombs of charge have moved through an instrument lamp whose terminals are 3 volts apart in potential level, 60 joules of energy have gone into the lamp.
b.	It follows from the above discussion that there is always a drop in potential between the terminals of any “load” device which
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is absorbing energy from an electrical circuit. The more heavily the unit draws energy from the flowing charge, the less energy the charge will have upon emergence, and therefore the greater the voltage drop between its terminals. If the circuit is “open” so that the charge is at rest and is not losing energy, the terminals of the unit will be found to be at the same potential.
c.	The earth’s crust is a fairly good conductor. The earth is so immense that a considerable quantity of charge can flow onto or away from it without changing its total charge appreciably. It is, therefore, used both as a reference potential level called ground potential^ and also as a means of neutralizing or preventing accumulations of charge on aircraft when on the ground, as described in paragraph 7.
d.	On aircraft, the metal framework is used as a reference potential level, frequently referred to as “ground” potential. For instance, if a wire becomes grov/nded, it is in contact with a metal part of the framework. Some point in the circuit might be referred to as being “10 volts above ground potential,” meaning above the energy level of the framework of the aircraft. In all such cases it is understood that when in flight there may be thousands of volts difference in potential between the aircraft structure and the earth, due to accumulation of static charge. However, this is of no consequence in the electrical circuits on the aircraft; all points in these circuits retain their usual relative potential levels with respect to the aircraft structure.
9. Electromotive force.—a. The moving electrons in an electric circuit are under the influence of a physical force which pushes them constantly. If the force were not continuously present on the electrons, they would slow down and stop because the resistance they encounter robs them of their energy. There is no convenient way of determining the magnitude of this physical force; however, there is no need to know it. Circuit calculations are made by employing an energy-level measurement as an indirect indication of how much force a given source can apply to the electrons in a circuit. If all of the load is removed from a generator or battery and a measuring device which indicates amount of difference in potential energylevel (voltmeter) is connected to the terminals, the voltage observed measures the maximum amount of energy which the source can impart to the electrically charged particles within it under ideal conditions, that is, with no charge flowing from the source. This value of potential difference is known as the electromotive force, or e.m.f., of the source. When a load exists, the potential difference
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between the terminals is somewhat less than the e.m.f., and the potential difference then is to be regarded either as a measure of the energy being actually gained by the electrons within the source under load conditions, or as the total loss of energy that they experience as they flow around the attached circuit.
When two or more sources of e.m.f. are connected in series, the e.m.f. they can apply to a circuit is the sum of each e.m.f.
—11—{I—ll—11—ll—11—
—	4-
------• —’---12 VOLTS-► •-----
Q NORMAL CELL ARRANGEMENT
—I—I —I—I-I—I—
------• —----8 VOLTS--— •-----
(2) ONE CELL REVERSED
—’I—’HI—H1—I1-
♦	—----0 VOLTS-*■	•----—
(3) THREE CELLS OPPOSING THREE CELLS
Figure 1.—Effect of cell arrangement on e.m.f.
Figure 1(1) represents by means of symbols the six cells of an air . craft storage battery connected together in series. Each cell has an e.m.f. of approximately 2 volts. Consequently the e.m.f. between the points representing the terminals of the battery is approximately 12 volts. In figure 1@ the relative location of one cell has been reversed. There are now 10 volts of e.m.f. in one direction and 2 volts in the other. The result is tpartial cancelation of electromotive force. The net e.m.f. between the terminals is 10 volts minus 2 volts, or 8 volts. If this battery were to be connected to a load
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circuit, the current would flow as if an 8-volt battery were in use. The cell connected backward would be charged instead of discharged, since the current would flow through it backward. The situation is comparable to that of five men having a tug of war with one man. The net pull would be that of four men, and the one on the weaker side would be pulled in the direction opposite to that of his effort. In figure 1(3), three cells are opposing three cells. The e.m.f. of each group of three cells exactly cancels that of the other; the net sum is zero. The terminals are at the same potential. If a load circuit is connected to them, no current will flow. The situation is similar to that of three men pulling against three equally strong men; no motion results.
c. E.m.f. and potential differences in aircraft electrical systems are both measured by use of the voltmeter, the construction of which is described in paragraph 37. In use, two wires (leads) are attached to the terminals of the voltmeter, and the other ends of the wires are brought in contact with the two points between which the potential difference is to be determined.
10. Electrical current.—a. A “convention” is a practice which is followed by general agreement. A number of years ago, before the real nature of electrical current had been discovered, it was generally imagined that electrical flow consisted of positively charged particles moving in the direction shown in figure 2, out of the positive terminal of a source of e.m.f. and into the negative terminal. Even though it is now known that this is not what really takes place, this conception of conventional current is still used for several reasons: It has been proven experimentally that the magnetic effects of an electron moving in one direction are exactly the same as those of a positive unit of charge moving in the reverse direction. Also, on the conventional basis of current flow, the charge flows from a point which is at a “higher” or relatively positive potential level toward one which is at a “lower” or relatively negative potential level. In this manual, unless otherwise specified, the word “current” refers to the conventional flow of imaginary positively charged particles.
I). The rate of flow of charge is measured in amperes. When charge is flowing past a given point in a wire at the rate of 1 coulomb of charge per second, this is one ampere of current. The amount of current may be measured in amperes by means of the heat produced by the flow of charge; by the amount of chemical action it can produce; or by the magnetic effects of the movement of charge. The magnetic effect is employed in the operation of the ammeter (or ampere meter), used in aircraft electrical systems (par.
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36)	. In the use of the ammeter, the circuit is opened at some point and the ammeter is inserted so that the charge will flow through it.
c. Electrical current occurs commonly as—
(1)	Direct current (d-c) which is a flow of charge in one direction.
(2)	Alternating current (a-c) which is a flow of charge which reverses direction periodically, under the influence of an e.m.f. periodically alternating in direction (pars. 48 and 49).
CARBON	—	—_______
g)
DRY CELL ZINC
Figure 2.—Direction of flow of conventional current.
11	. Electrical resistance.—a. Electrons experience collisions as they progress inside a wire. These collisions impede their progress. The electrical resistance of a conductor is a measure of the difficulty which electrons experience as they try to find their way through the conductor. Heat is always developed in a conductor as a consequence of its resistance, and its temperature will rise as a result of current flow if the heat which is developed is not carried away.
6.	The electrical resistance of a substance is measured in the unit of resistance called the ohm. When a conductor by its resistance causes
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1 coulomb of charge to lose 1 joule of energy while passing through it, its resistance is 1 ohm. If the potential drop between the terminals of a conductor is 1 volt when charge flows at the rate of 1 ampere, the value of its resistance is 1 ohm. It will be realized after some consideration that the foregoing definitions are really equivalent statements.
c.	The resistance of a conductor depends upon several factors as follows:
(1)	Dimensions.—A 2-foot length of No. 18 wire has twice as much resistance as a 1-foot length of No. 18 wire. A thick piece of wire 1 foot in length has less resistance than a thin wire 1 foot in length. Since it is desirable to minimize the resistance of wires which make up electrical circuits, the wires are made as short as possible and as thick as is practicable without adding too much weight to the aircraft.
(2)	Nature of conductor.—Table I lists the resistivities of various substances. The resistivity of a substance is the resistance in ohms between opposite faces of a cubic centimeter of the substance. The factor “IO-7” found in table I may be interpreted to mean, “move the decimal point 7 places to the left.” This would give silver a resistivity of .0000016 ohm. It will be noted that silver has the least resistivity and it is therefore the best conductor. The resistivity of copper is but slightly higher than that of silver, and is lighter and cheaper, hence its extensive use in electrical circuits.
Table I.—Electrical resistivities
Substance	Resistivity in ohms per cm cube
Silver		16X10-3 * * * 7
Copper		17X10-7
Aluminum		28X 10-7
Iron 		100X10-7
German silver		330 X10"7
Manganin		440X10-7
Carbon		3,485X10-7
(3) Temperature of conductor.—The resistivities shown in table I
were measured with the substances at room temperature. The resis-
tivity of each metal listed increases with rise in temperature. The
resistivity of a copper wire will increase approximately 4 percent for
each rise in temperature of 10° Centigrade.
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d.	The resistivity of carbon behaves abnormally with change in temperature when compared with metals. Its resistance decreases, rather than increases, with rise in temperature, decreasing approximately 5 percent for each rise in temperature of 100° Centigrade.
12. Resistors.—a. In many cases resistive materials are purposely used in electrical units, as in heating, current-limiting and voltagedropping devices. The resistive materials commonly used on aircraft include nichrome (an alloy of nickel and chromium), german silver (an alloy of copper, zinc, and nickel) and carbon.
b. A resistor is a unit placed in a circuit for the purpose of introducing electrical resistance at a predetermined point in the circuit. A fixed resistor is one the resistance of which is not designed to be changed. Its resistance will not vary except for changes in resistivity due to variations in temperature. A rheostat is a variable resistor.
c. A fuse is a device designed to open an electric circuit whenever the amount of current flowing becomes abnormally great, due either to a “short circuit” or an oversupply of voltage. It is purposely made to have as little resistance as possible, yet it is the heat developed due to its resistance which causes it to melt when the current reaches a dangerous value. A good fuse material must, therefore, melt at a low temperature. The vapor formed during melting must also be a poor conductor so as not to produce an electric arc between the fuse terminals. Alloys made of lead and tin satisfy these conditions. Fuses are usually made with glass or composition containers as a safety precaution to enclose the arc, if one should occur. Fuses are made in the form of plugs or cartridges to facilitate replacement.
13. Electric circuit.—a. An electric circuit is a closed path through which an electric current can flow. Electrical circuits usually include a source of e.m.f. and one or more devices which convert electrical energy into some other form of energy, such as heat, light, work, or sound. In order to understand how aircraft electrical equipment functions, it is necessary to be able to recognize the several simple types of electrical circuits described in section II and to be able to perform simple calculations in connection with these circuits. Conventional symbols commonly employed to represent electrical units on drawings are shown in figure 3.
b.	A load circuit is any circuit which is attached to a source of e.m.f. for the purpose of drawing energy from it. A resistive load ismne which does not itself contain a source of e.m.f., and in which virtually all of the inflowing energy is converted into heat.
c.	When a circuit is broken so that current cannot flow, it is said to be “open.” A switch is connected in a circuit so that it can be
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opened at will. More often, the term “open circuW1 refers to an accidental break in the wires or connections.
d.	A short circuit is usually an accidental path of low resistance which allows an abnormally large current to flow from the source
/ A'c GENERATOR OR MOTOR	*—|||||||p  BATTERY	•— SWITCH. S.P ST.
ft?) ft?) D< an6ERAT°R	CONDENSER	—-•	SWITCH, SPOT.
A A (	) ( a 1	BRUSH CONNECTIONS <	) k ojJ TO COMMUTATOR	—	 RESISTOR, FIXED	SWITCH, DPS T
?^?)	MOTOR GENERATOR	RHEOSTAT	SWITCH, D.P.D.T.
RECTIFIER TUBE \/\J	half wave	A A A A A POTENTIOMETER	I VV.VVV	 OR VOLTAGE	1	SWITCH, PLUNGER |	DIVIDER	—• •—	OPERATED
—A Pl  RECTIFIES TUBE \Z\J	FULL WAVE n	( )	OHM		 z\	WIRES, —	 	'	NOT JOINED
—ammeter	COIL	■	।	—1—WIRES, JOINED
—» VOLTMETER	t-		 IRON CORE	—FUSE
—( G 1— GALVANOMETER	(fT^)	LAMP	■■■ ।	GROUND
Figure 3.—Conventional electrical symbols.
of electrical energy. If there is a fuse in the circuit it usually burns out or “blows” and opens the circuit, but if there is no fuse in the circuit the wires may become hot enough to cause a fire. For instance, if the positive terminal of a battery or generator becomes grounded
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to the frame of the aircraft by any piece of metal such as a tool or wire, or if the lead from the battery or generator is grounded as a result of worn insulation, a short circuit results. There usually are no fuses in these parts of the circuit close to the battery or generator and therefore a short circuit on the battery or generator side of the fuse is especially dangerous. If a wire leading to some low-current unit such as a navigation light becomes grounded to the metal framework of the aircraft, the only result will probably be a blown fuse.
Section II
DIRECT CURRENT CIRCUITS
Paragraph
Aircraft electrical circuits_____________________________________________________ 14
Ohm’s law________________________________________________________________________ 15
Units in parallel________________________________________________________________ 16
Units in series__________________________________________________________________ 17
Units in series-parallel-________________________________________________________ 18
Power in d-c circuit------------------------------------------------------------- 19
Solution of simple problems______________________________________________________ 20
14. Aircraft electrical circuits.—a. Most of the electrical equipment on modern aircraft is designed for 12- or 24-volt direct current operation. A typical aircraft d-c circuit is shown schematically, by use of symbols, in figure 4. The engine-driven generator is repre
METALLIC FRAMEWORK OF AIRPLANE
Figure 4.—Typical aircraft electrical circuit.
sented by G, H represents the electrical heating element in a Pitot tube. The energy received by the generator from the engine is conveyed by means of the electrical circuit to the Pitot tube, where the energy is converted into heat. For a simple picture of the operation of this system, one may imagine the electrical circuit to be completely filled throughout with positively charged particles. The wires are
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covered with insulation. When the pathway is completed by closing the switch S, the electrical force supplied by the generator pushes the particles around the circuit in the direction indicated by the arrows. The metal framework of the airplane is a part of the circuit, as it provides a return path for the particles to the negative side of the generator. An ammeter, represented by A, is inserted in the circuit so that the moving charges must pass through it. It measures the rate of flow in amperes.
Z>. The particles lose energy as they move around the circuit. When the system is operating, voltmeter Fx, located in the cockpit and electrically attached to the terminals of the generator, measures, in volts, the amount of potential energy imparted to the particles in the generator. We may imagine another voltmeter, V2, to be attached to the ends of the heating element to measure the loss of potential, or loss of energy, of the particles in their passage through the heating element, in which they give up energy in the form of heat. A voltmeter so attached to any load unit will always show a slightly lower reading that the voltmeter it the cockpit, which voltmeter is attached almost directly to the energy source. This is due to loss of energy along the connecting wires. Suppose, for instance, the voltmeter V2 reads only 12 volts when voltmeter Ft reads 14. This indicates the existence of a 2-volt drop in potential, or energy level, along the wires which are conveying the particles to the load unit. The drop in potential along the return path of metallic framework is negligible. The loss of energy along the wires, known as line loss, is due to the resistance to movement which is encountered by the particles. They must use up some of their energy in overcoming this resistance. In the case cited, %4 of the energy given to the particles by the generator is being wasted “along the line” and only 12/14 is reaching the Pitot tube. To minimize such loss, the resistance of connecting wires should be as low as possible.
15. Ohm’s law.—a. The voltage difference between the terminals of a load unit is designated by the letter E; the current in the unit, measured in amperes, is designated by the letter Z; and the resistance of the unit, measured in ohms, is designated by the letter R. The relationship between the three is known as OEmls law and may be expressed by the following formulas:
E
(1)	Amperes—voltsohms; or, I~r'
(2)	Volts = amperesXohms; or, E=1R
E
(3)	Ohms=volts-^amperes; or, 2?=j-
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AIRCRAFT ELECTRICAL SYSTEMS
b.	If the voltage difference between the terminals of a load unit and the resistance of the unit are known, then by use of formula (1) the flow of charge in amperes through the unit may be calculated. If the current and the resistance are known, formula (2) will give the voltage drop to be expected within the unit. Finally, if the voltage drop and the current are known, the resistance of the unit may be calculated by use of formula (3).
c.	Ohm’s law, in addition to its use in connection with individual load units, can also be used to solve for unknown values of an entire
/ E\
/ I R \
Figure 5.—Chart of Ohm’s law.
circuit. In this case E is the net e.m.f. in the circuit: I is the current which flows through the source of supply; and R is the net resistance in the circuit which the net e.m.f. must overcome. As previously stated, Ohm’s law can be used to find any one of the three values if the other two are known.
d.	The relationship between E. I, and R may be remembered more easily by use of the device shown in figure 5. If one places his finger on the symbol which represents the unknown value, the necessary operation to be performed with the other two values is shown by the direction of the line between them (the vertical line signifies multiplication and the horizontal line signifies division).
449399°—41
•2
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16. Units in parallel.—a. The majority of the electrical devices on aircraft are operated in paiallel, being connected together in the manner shown in figure 6Q. This figure schematically illustrates an aircraft battery and three load devices. One side of each device is grounded (connected to the metallic framework of the aircraft). The other end of each device is connected to the positive end of the battery through switches and meters, which have been omitted from the diagram for simplicity. The battery draws charge from, and each load unit returns charge to, the metallic framework.
b.	When two or more electrical units are thus connected, with one terminal of each attached to a common point or to the same conductor, and their other terminals similarly attached to another point or conductor, they are said to be in parallel. A parallel circuit is one in which there are two or more load units connected in parallel. The full e.m.f. of the energy source is therefore applied separately to each load unit; the same voltage drop is created in each load unit. The charge which flows out of the source of energy is divided among the several units and is reassembled in the return path to the battery. Therefore, the total current is equal to the sum of the currents in the individual load units.
c.	If the leads of a voltmeter are placed in contact with the terminals of any load unit designed for parallel operation with the other units, the meter will show almost the same deflection in every case. The values will always be slightly lower than the reading of the voltmeter in the cockpit, because of the line losses previously mentioned.
d.	In order to solve problems which may arise, it is sometimes necessary to calculate the net value of resistance from the individual values of resistance of units connected in parallel. The net resistance may be calculated by use of the formula—
1 . 1 . 1 , p । 7? । p । etc.
Iti £12	£^3
When the load units in parallel have equal resistance, this formula may be simplified to the form,
resistance of one unit ________________________ the number of units
For example, the net resistance of eight 5-ohm units in parallel is % ohm. It follows, therefore, that when two or more load units are connected in parallel, the effective resistance of the group is always less than that of the unit having the least resistance.
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17. Units in series.—a. Some aircraft electrical devices are- used in a series circuit, illustrated by figure 6®. A battery and three load units are shown connected in series. Meters and switches have been omitted for simplicity.
5. A series circuit has only one closed path. There are no points along it where the current may branch off. It consists of electrical units which are connected in series, so that the charge must flow through each in succession. The outstanding feature of a series circuit is
© LOAD UNITS CONNECTED IN PARALLEL
(2) LOAD UNITS CONNECTED IN SERIES
Figure 6.—Types of circuits.
(3) LOAD UNITS IN SERIES - PARALLEL
that the current, in amperes, must necessarily have the same value in every unit in the circuit.
c. As the charge progresses from the positive side of the generator or battery to the ground, it loses energy in each load unit. It follows that the total voltage drop must be divided between the several load units. For instance, if, when the circuit is closed, the potential difference across the battery is 12 volts, then the sum of the values of voltage drop across R2, and Rz should be 12 volts. If, by use of
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17-19	ARMY AIR FORCES
a voltmeter, the voltage drops across the three load units were found to be 2. 6, and 4 volts, respectively, this would indicate that no mistakes in observation had been made, for these numbers add up to 12. It would also indicate that the units possess unequal values of resistance. Their resistances in ohms can be calculated by use of the formula
E .
R= j, if the current in the circuit is known. The three resistances will be found to be in the same ratio to each other as the observed voltage drops.
d. When two or more units are connected in series, their resistance effects are additive. The resistance of the group is the sum of the individual resistances. Thus, if the three units in the above example have resistances of 3. 9, and 6 ohms, respectively, their total resistance (the net resistance in the circuit) is 18 ohms.
18. Units in series-parallel.—In a few instances aircraft load units are connected in series-parallel. Figure 6@ is an illustration of a series-parallel circuit. The current first flows through to point B. where it is divided among the four units in parallel. It is reassembled at C and returns to the battery. Once the resistance of the parallel group has been found, the circuit and all load unit values may be found by use of Ohm’s law and by use of the load unit relationships given in paragraph 16 and 17.
19. Power in d-c circuit.—a. The time rate at which energy is developed or used is called power. For instance, an aircraft engine is rated in horsepower. The number of foot pounds of energy which it can develop in 1 second, divided by 550, is its horsepower rating. As stated previously, the metric unit of energy of work is the joule. The power of an electrical device is measured in joules per second. When energy is conveyed or converted, or when work is done, at the rate of 1 joule per second, this rate is called one watt. A 240-watt electric light bulb obtains 240 joules of energy per second from the circuit to which it is attached. A 3-kilowatt (3.000-watt) motor draws energy from the line at the rate of 3,000 joules per second.
b. The power demand or the power delivery of an electrical unit, in watts, is designated by the letter P and may be calculated by use of either of the following formulas:
(1) Watts = voltsXamperes; or, P=E1.
(2) Watts = amperesXamperesXohms; or, P=I2R.
If two or more load units are operated simultaneously from the same source of power, the sum of the individual power demands of the units, plus the demand represented by the line losses, equals the power delivery of the source, regardless of the method of connection
20
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AIRCRAFT ELECTRICAL SYSTEMS
of the units in the circuit. Usually the line loss is small enough to be ignored in making calculations.
20. Solution of simple problems.—a. Table II is a summary of the relationships between E, I, R, and P and methods for their measurement. The formulas apply, as given, to direct current circuits. Modifications of these formulas usually must be made for application to alternating current circuits.
b. Problems involving a single load unit, or two or more units in parallel, series, or series-parallel, may be solved by use of the appropriate procedure of those given below.
Table II.—Tabulation of electrical fundamentals
Electrical term	Nonelectrical language	Unit	Symbol	Formula	Measured by—	How meter is connected in circuit
Current	_	Rate of flow...	Ampere..	1		Ammeter. .	Series.
Electronic-	Motive		Volt		e. m. f.	R	Voltmeter	Parallel. Parallel.
tive force. Potential	Difference in	Volt		E	E=IXR	Voltmeter. ..	
difference. Resistance.-.	energy level. Opposition		Ohm		R	R^	Ammeter and	Series and parallel
Power _ _	Time rate of	Watt		p	P=E%I	voltmeter. Wattmeter or am-	Series and parallel.
	work.			P=I*XR	meter and voltmeter.	
(1)	Single load unit.—(a) Problem.—A typical 12-volt aircraft landing light bulb is rated to draw a current of 20 amperes. What is its resistance when hot? What is its wattage?
(&) Solution.—Tabulate the known and unknown values as follows: A=12 volts.	R— 1 ohms.
7=20 amperes.	7*= ? watts.
Choose the appropriate form of Ohm’s law to determine the unknown E
value. In this case, by use of the formula R— y 5
12
7?=Qn=0.60 ohms.
To determine the wattage, use either power formula given in paragraph 19. Thus, by use of the formula P- El,
P— 12X20 = 240 watts.
By use of the formula P=I2R,
P=20 X 20 X 0.60 = 240 watts.
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(2)	Load units in parallel.—{a} Problem.—Six 4-watt navigation lamp bulbs are operated in parallel on a particular aircraft which uses a 12-volt d-c system. How much current does each bulb draw? What is the resistance of each when hot? What is the total current drawn? What is the resistance of the entire circuit? What is the total power demand ?
(&) Solution.—Draw a diagram and indicate the known values at the appropriate places. In this case the diagram would appear as in figure 6(T), except that there would be six instead of three load units. Tabulate the known and unknown values for each load unit and for the entire circuit as follows:
Each lamp bulb	Entire circuit
£’=12 volts	£’=12 volts
I = ? amperes	I = ? amperes
R= 1 ohms	R— 1 ohms
P=4 watts	£*= ? watts
First, attempt to apply Ohm’s law to any of the units or to the entire circuit. If two of the three terms E, I and R pertaining to any one of the units or to the entire circuit are known, Ohm’s law may be applied in the manner shown in (1) above. In the event that only one of these three terms is known, then if P is known, the problem can be solved by use of the appropriate power formula. Thus, in the present example, it is seen that Ohm’s law cannot be applied immediately. But the demand of each lamp is given as 4 watts. Its current, Z, may be found by using the formula P=EI:
4=12XZ; or,
, 4 1
= 3 ampere.
The remaining unknown value for each bulb, R, can now be deter-
mined by use of Ohm’s law.
R=
12
¥3
£* Applying the formula R=-j-, = 12X3=36 ohms.
Since there are 6 lamp bulbs each drawing l/3 of an ampere, it is seen that the total current will be 2 amperes (par. 16). The resistance (R) of the entire load circuit may now be determined by use of the Z?=^ =6 ohms.
E
formula R=—’ as follows:
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AIRCRAFT ELECTRICAL SYSTEMS	20
The same result would be obtained by dividing the resistance of one lamp (36 ohms) by the number of lamps (6) since the lamps have equal resistance (par. 16). The power demand of the entire load can now be determined by use of either of the power formulas, as follows:
P=PZ= 12X2=24 watts;
or, P=Z2P=2X2X6=24 watts.
It should be noted that the same total power could be obtained by multiplying the 4-watt demand of each lamp by the total number of lamps (6). The procedure outlined above would work equally well if the load units were to possess unequal resistances.
(3)	Load units in series.— (a) Problem.—A small 3-volt cockpit instrument lamp, similar to a tiny flashlight bulb, is operated successfully on the 12-volt d-c circuit of an airplane by use of a 68-ohm series resistor. Calculate all of the unspecified values in this arrangement.
E = 12 VOLTS
R= 68 OHMS
Figure 7.—Instrument light circuit.
E=3 VOLTS
(Z>) Solution.—Draw a diagram and indicate the known values at the appropriate places. In this case the diagram would appear as in figure 7. Tabulate the known and unknown values for each load unit and the entire circuit, as follows:
Lamp
E—3 volts I=? amperes R=? ohms P— ? watts
Resistor
E=? volts I=? amperes P=68 ohms P=f watts
Whole circuit
E—12 volts I=? amperes R-? ohms P~? watts
No set procedure of solution is recommended inasmuch as the procedure must be designed to fit each case, as will now be shown. First, it is to be noted that the formulas will not apply in the case of either of the load units, or the entire circuit, inasmuch as only one of the terms E3 Z, R, and P is known in each case. Therefore, it is necessary
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to make use of known relationships between the load units and the entire circuit. The sum of E for the resistor and E for the lamp should be equal to E for the entire circuit (par. 17). Therefore, E for the resistor must be 9 volts. This having been determined, I and P for the resistor can be determined as follows:
1=
R 68
=0.132 amperes.
P=£7=9X0.132 = 1.19 watts.
Inasmuch as the current I must be the same throughout the series circuit, it follows that I in the lamp and in the entire circuit is 0.132 amperes. R and P for the lamp may now be determined as follows:
2?=y=^-^=22.7 ohms.
1 U. 1 (jZi
P=EI=3X0.132 = 0.396 watts.
For the entire circuit:
E_ 12
I ~0.132~
90.9 ohms.
P=KZ=12X0.132 = 1.58 watts.
The sum of the wattages of the lamp and the resistor should be equal to the wattage for the entire circuit. Also, the sum of the resistances of the load units should equal the total resistance of the circuit (par. 17). These conditions are satisfied within allowable limits by the results of the above solutions, and the accuracy of the calculations is thus verified.
(4)	Load units in series-parallel.—(a) Problem.—Three resistors are connected to a 12-volt battery in such manner that current flows first through Rly then through R2 and Rs in parallel, and back to the battery. The values of resistance of Rr, R2 and R3 are 2%, 2, and 4 ohms, respectively. Calculate the rate at which charge will flow out of the battery.
(b~) Solution.—Draw a diagram and indicate the known values at the appropriate places. Such a diagram for this problem will be similar to figure 6® with R4 and R6 omitted. The net resistance of R., and R3 is found as follows:
R = J. _L_ = F~I= 0.50 + 0.25 = 075 = 1 ’333 = 1 % °hmS'
R2 R3 2 + 4
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TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	20-21
The addition of this resistance to that of Rr (2% ohms) gives a total of 4 ohms for the net resistance of the circuit. The current drawn from the battery will then be—
T E 12 o
Z=^=-^- = 3 amperes.
Section III
MAGNETISM AND MAGNETIC DEVICES
Paragraph
Fundamental magnetic phenomena---------------------------------------- 21
Magnetic fields_______________________________________________________ 22
Electromagnetic field------------------------------------------------- 23
Cause of magnetism---------------------------------------------------  24
Magnetic induction---------------------------------------------------- 25
Retention of magnetism------------------------------------------------ 26
Magnetic circuit------------------------------------------------------ 27
Electromagnetic fields of. coils______________________________________ 28
Solenoid and electromagnet____________________________________________ 29
Force on current-carrying conductor----------------------------------- 30
Torque on current-carrying coil--------------------------------------- 31
Weston meter movement------------------------------------------------- 32
The d-c motor— ---------------------------------------------------,---- 33
21.	Fundamental magnetic phenomena.—a. The following facts may be observed with respect to the behavior of magnets:
(1)	The ends of a magnet are more active (magnetically) than its central part. If a magnet is dipped into a box of iron filings, the filings cling more abundantly to the ends than to the middle. The end portions of a magnet are said to be magnetic poles.
(2)	If a magnet is suspended at its midpoint by a fine thread, it will always come to rest with one particular pole to the north. This pole is called the magnet’s north pole; the other pole is its south pole.
(3)	When magnets are suspended near each other, their like poles repel each other, whereas their unlike poles attract. This behavior is similar to that of electrically charged bodies (par. 3). But if a charged body is brought near a magnet, interaction forces are not observed. A charged body does not respond to the presence of the magnetic poles and vice versa.
(4)	The magnetic force of a pole is very powerful in the areas immediately surrounding the pole but becomes rapidly weaker at increased distance therefrom. Considerable force is required to detach an iron nail from a magnet; but once the nail has been removed a short distance (perhaps an eighth of an inch) from the magnet it may be continued to be removed with relative ease.
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b.	The needle of every magnetic compass is in reality a magnet. It has a north and a south pole. It is mounted on a suspension having very little friction, so that it may turn easily under the magnetic influence of the earth. The end of the compass needle which points northward is a north magnetic pole (by definition).
22.	Magnetic fields.—a. A magnet is surrounded by a magnetic field. A magnetic field is an area in which a small compass needle will be noticeably affected.
b.	If a small compass is placed at numerous locations about a magnet, -and the direction in which the north end of the needle points at each of these locations is indicated on paper by means of lines and arrowheads, the result will be a graphic representation of the magnetic field. The lines are called lines of force. They indicate (by their arrowheads) the direction in which a magnetic north pole would be urged if placed in the field at the various locations. The lines do not exist as lines in any magnetic field, nor do they indicate movement or flow. They merely represent, or map out, the magnetic field.
c.	Tests for the direction of a magnetic field are performed with a compass needle. The north end of the needle is attracted in the direction of the field, while the south end is pulled the other way. The two forces cause the needle to turn until it becomes parallel with, or tangent to, the lines of force, with the north end pointing in the direction of the field.
d.	The magnetic fields about straight and U-shaped magnets are illustrated in figures 8® and (2), respectively, by means of lines of force. Small compasses have been placed in the field, to show how their needles would “set.” It should be noted that the lines of force emerge from the north pole of the magnet and go into its south pole, the compass needle indicating the direction of the lines.
e.	A compass behaves as it does in its normal mode of use because the earth is a huge magnet. One of the earth’s poles is in northern Canada and the other is below Australia, near the geographical south pole. The pole in northern Canada is a south magnetic pole, since the north end of the compass needle points that way. The pole at the southern part of the earth is a north magnetic pole.
23.	Electromagnetic field.—a. A wire which carries current is surrounded by a magnetic field. Because this field is caused by an electric current, it is called an electromagnetic field. A small compass needle may be used to determine the direction of this field. It is found that the lines of force in the field about a straight wire are concentric circles, as shown in figure 9. A compass placed in the field always
26
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23
AIRCRAFT ELECTRICAL SYSTEMS
sets tangent to one of these circles. The field, in this case, has no poles. The force of the field is strongest close to the wire and falls off rapidly with increased distance from the wire. It is also proportional in magnitude to the current in the wire. There are no interaction forces
y|y^ © MAGNETIC FIELD ABOUT A BAR MAGNET (f) MAGNETIC FIELD ABOUT A U-SHAPED. MAGNET Figure 8.—Magnetic fields.
between this field and electrically charged bodies. The behavior of the field is similar in every respect to that of the field about a magnet.
b.	The direction of the magnetic field about a wire may be determined by use of the right-hand rule; if one imagines the wire to be grasped with the right hand in such a manner that the thumb points in
27
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the direction of the flow of conventional current, then the fingers of the hand point in the direction of the field. In figure 9® the field is shown about a straight wire which is perpendicular to the paper,
® CONVENTIONAL CURRENT FLOWING (2)CONVENTIONAL CURRENT FLOWING FROM OBSERVER.	TOWARD OBSERVER.
Figure 9.	—Electromagnetic field about current-carrying straight wire.
with current directed away from the observer. The field shown in figure 9® would be caused by a current coming out of the paper toward the observer. The direction of these lines of force may be verified by the right-hand rule.
CONVENTIONAL CHARGE/	f I J \
IN MOTION -----'	I	J
Figure 10.	—Electromagnetic field of a current-carrying loop.
c.	Figure 10 shows a length of wire coiled into a loop, with conventional current flowing in the direction indicated. If the observer imagines himself to grasp the wire with his right hand, the thumb
28
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AIRCRAFT ELECTRICAL SYSTEMS
pointing in accordance with the right-hand rule, and to slide his hand all the way around the loop, it becomes apparent that the fingers always point leftward when inside the loop, and to the right when outside. The magnetic field, when mapped, has the direction shown.
d.	The magnetic field associated with a loop of wire is much the same as the field of an imaginary short, thick bar magnet placed in the position shown by the dotted lines in figure 10. The poles of this magnet would be in the positions indicated, with the lines emerging from the north pole and entering the south pole. The field of the loop is, therefore, of a dipole nature; it may be imagined to possess two magnetic poles, similar to those of a magnet. Whatever is imagined to happen to such a magnet with respect to torque or motion, will actually happen to the loop of which the magnet is the equivalent.
24. Cause of magnetism.—a. Magnetism and magnetic fields are the result of the motion of electrically charged particles. When a thin stream of positively charged particles is projected through a vacuum,
ELECTRON
(5) MAGNETIZED BODY.
(t) MAGNETIC ROTATING
EFFECT OF ELECTRON (g) UNMAGNETIZED BODY. IN ITS ORBIT.
Figure 11.—Magnetic dipoles.
it is found that the stream of particles is surrounded by a magnetic field similar to that about a wire, and the direction of the field conforms to the right-hand rule. If a stream of electrons is projected through a vacuum, it is surrounded by a field identical with that produced by the same number of positively charged particles projected in the opposite direction.
b. The above facts and others mentioned earlier in this section give us a strong hint as to the cause of magnetism in iron and other magnetic materials. The planetary electrons of atoms circulate around their nuclei; therefore each creates a tiny magnetic dipole, as illustrated in figure 11®. In most cases, the electronic dipoles are arranged at random so that their magnetic fields cancel each other. But it appears that in certain substances, groups of atoms, perhaps several hundred in a cluster, contain a number of electrons which apparently
29
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ARMY AIR FORCES
are rotating in the same, or parallel, planes. The effects of their fields are cumulative, and the cluster of atoms therefore constitutes a small magnetic dipole. The current of circulating electrons within the cluster produces a dipole in the same manner that electrons circulating in a loop of wire produce a much larger dipole. A huge number of these molecular magnetic dipoles exist in a small piece of iron. They are normally arranged in a haphazard manner, as illustrated in figure 11(2). When thus arranged, the fields of the dipoles cancel each other, and the iron is unmagnetized. But when the dipoles are alined, as shown in figure 11(3), their effects are cumulative and the result is a huge dipole, the magnet itself.
25.	Magnetic induction.—a. When an unmagnetized piece of iron whose magnetic dipoles are arranged at random (fig. 11®) is situated in a magnetic field, the field tends to aline the dipoles parallel with or tangent to its lines of force, and thus temporarily magnetizes the iron. The temporary magnetization of a magnetic material is known as magnetic induction. A magnetic material is one which possesses magnetic dipoles which may be rotated and alined. Iron, steel, cobalt, nickel and combinations of these metals are magnetic materials.
6.	If, for instance, the south pole of a bar magnet is brought near to an iron nail, the north end of each tiny dipole within the nail will be rotated to some extent toward the magnet. This operation also rotates the south ends of the magnetic dipoles toward the far end of the nail. A north pole is therefore induced on the near end of the nail, and a south pole on the far end. The nail is temporarily magnetized. Its north pole is attracted by the adjacent south pole of the bar magnet and they are drawn together. Regardless of which pole of the bar magnet is used to induce magnetism in the nail, the pole induced in the near end of the nail will always be of opposite polarity, and attraction is always the result. Iron filings cling to a magnet because the filings become magnetized by induction. Brass or copper objects do not respond to magnetic effects because they cannot be magnetized.
c.	When magnetization occurs, the nuclei of the atoms retain their fixed positions in the substance. What apparently happens is that the planes of rotation of the swirling electrons are rotated. This is possible because there is an abundance of open space within each atom and between them. The axes of the magnetic dipoles can therefore be rotated without moving the nuclei.
26.	Retention of magnetism.—a. Permanent magnets possess magnetic dipoles which remain alined over a long period of time. The retention of magnetism depends upon the chemical and physical nature of a substance and the treatment to which it is subjected. Certain
30
AIRCRAFT ELECTRICAL SYSTEMS
TM 1-406
26-27
steels and alloys, notably cobalt steel and alnico (aluminum, nickel, and cobalt), may be powerfully magnetized and will remain magnetized for a long period of time if not abused. Such substances are used for permanent magnets. On the other hand, the magnetic dipoles of soft iron not only are easily alined, but also readily return to the random array as soon as the magnetizing influence is removed. However, there is usually just a little magnetic effect left in a piece of iron which has been temporarily magnetized by induction; the magnetic-dipoles do not return to a complete random array.- The magnetism which remains is called residual magnetism. Aircraft generators depend upon residual magnetism in their pole pieces to start the build-up of voltage when the rotation of the armature is started.
b. Residual magnetism in steel parts of aircraft, temporary magnetization of such parts by induction in the earth’s field, and stray fields from current-carrying circuits in aircraft all contribute to errors in compass bearings.
27.	Magnetic circuit, a. An unmagnetized piece of soft iron will become temporarily but powerfully magnetized by induction if placed across the ends of a U-shaped magnet, as shown in figure 12. The polarity of the iron will be as indicated. If the area surrounding the magnet and iron is explored with a small compass, it is found that the field is weak. This is due to the concentration of the field within the iron and magnet. They constitute a magnetic circuit. The field within the circuit may be represented by lines which are given the usual directions with respect to the poles in the circuit.
b.	Magnetomotive force, or m.m.f., is the name applied to the magnetizing influence which a magnet or coil of current-carrying wire can exert in a magnetic circuit. It corresponds to e.m.f. in electrical circuits. A magnet will possess a certain amount of m.m.f., in the same manner as a battery possesses e.m.f. Likewise, a coil which carries a current applies a definite amount of m.m.f. to a magnetic circuit with which it is linked. M.m.f. may be measured in ampere turns, for example, a 90-turn field coil carrying a current of 3 amperes, supplies 270 ampere turns of magnetomotive force to the magnetic circuits with which it is linked.
c.	Flux in the magnetic circuit is the equivalent of current in the electrical circuit. Flux may be represented by lines of force which are imagined to exist in the circuit. The stronger the magnetization of the circuit, the greater will be the flux.
d.	Reluctance is the magnetic counterpart of electrical resistance. It is the property of a magnetic circuit which determines how much flux will be established when a given amount of m.m.f. is applied to
31
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ARMY AIR FORCES
the circuit. The greater the reluctance, the fewer will be the flux lines which a given m.m.f. can establish in a magnetic circuit. The shorter the length and thicker the cross section of parts of the magnetic circuit, the smaller will be its reluctance. An air gap does not “open” a magnetic circuit. However, it does introduce a great amount of reluctance into the circuit. As small a clearance as possible is
Figure 12.—Magnetic circuit formed by permanent magnet and piece of soft iron.
provided between the rotating member and pole pieces of a motor, generator, or magneto to minimize the reluctance in their magnetic circuits.
e. Magnetomotive force, flux, and reluctance are interrelated in a manner similar to the Ohm’s law relationship between the corresponding quantities in the electrical circuit. The amount of flux established in a circuit is directly proportional to the m.m.f. and inversely proportional to the reluctance.
28. Electromagnetic fields of coils.—a. If a current is passed through a coil of wire consisting of 8 turns close against each other, and an equal current is passed through a single turn loop of the same
32
U-SHAPED MAGNET
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	28-29
diameter as the coil, the magnetic fields will be found to be almost identical in direction at every point. Each field when mapped will have the appearance shown in figure 10. However, the magnetic field strength of the 8-turn coil will be approximately 8 times that of the single turn loop. This is because the fields of the 8 turns are virtually parallel with each other at every point, and their effects are therefore cumulative at every point.
b. If, however, the 8 turns are spread out into a helical coil or solenoid, the magnetic field will be as shown in figure 13. The field is very weak between the turns, because the fields of adjacent turns
Figure 13.—Electromagnetic field of a current-carrying helical coil.
are opposite in direction and tend to cancel each other. But inside the solenoid, the fields are cumulative for the most part, and the net result is a strong field of fairly uniform intensity, represented by nearly straight lines of force.
c. Each of the above-mentioned coils will have a north magnetic pole at one end and a south pole at the other. The direction of the field depends upon the direction of current flow and may be determined by use of the right-hand rule.
29. Solenoid and electromagnet.—a. A solenoid may be defined as a coil of wire, wound in the form of a helix, and used to produce a magnetic field. In aircraft equipment, the term “solenoid” is used to designate an electromagnet whose core, or part of its core, is free to move. The field of the coil tends to pull the core (or the movable part thereof) into the coil when the current is turned on. Many mechanical devices may thus be actuated by remote control. Figure 14 shows a solenoid in schematic form. The core contains two steel cylinders. Cylinder A is fastened to the coil assembly, whereas cylinder B is free to move back and forth. The shaft attached to
449399°—42----3
33
TM 1-406
29
ARMY AIR FORCES
cylinder B moves freely through cylinder A and transmits the motion of cylinder B to the point of application. When the current is turned on, both cylinders become magnetized by induction. If the rules previously described concerning magnetic induction are applied, it will be seen that regardless of which way the current may flow in the coil, the cylinder ends which face each other will acquire unlike magnetic polarity. Consequently, the two cylinders are pulled together with considerable force. A spring is placed on the shaft to push the cylinders apart when the current is turned off.
b. If wire is permanently wound around a fixed soft iron core, the resulting assembly is called an electromagnet. The electromagnetic field of the current strongly magnetizes the iron, so that the total magnetic effect is that of the current in the coil plus that induced in the iron. By this means a very powerful magnetic force can be obtained. The strength of an electromagnet depends upon 3 factors: the number of turns of wire, the amount and kind of metal employed, and the value of the applied current.
c. The electromagnet is frequently employed as the essential part of an actuating movement, such as is found in the landing light relay or the generator control panel. The principle of operation of the electromagnetic actuating movement will be discussed with the aid of the elementary relay shown in figure 15. A small piece of soft iron is fastened to a strip of nonconducting material which is pivoted at one end. The iron is held away from the core of the electro-
34
Figure 14.—Elementary solenoid.
CYLIN DER-B------
—CYLINDER-A
TM 1-406
29-30
AIRCRAFT ELECTRICAL SYSTEMS
magnet by a spring. When the current is turned on, the iron is pulled toward the core. In some cases it is restrained by means of a stop pin from actually making contact with the core. At the other end of the nonconducting strip there are contact points, arranged (depending upon the type of relay) to close or open one or more circuits when the relay is actuated. The relay illustrated has four terminals, two for its coil, and two which are attached to the contact
flexible wire
/	rnon-conducting	\
/	NX \ MATERIAL	Y
~~~~	x
i■
Figure 23.—Wheatstone bridge.
40	. Thermocouple thermometer.—a. Figure 24© shows a circuit of two kinds of wire, namely, iron and constantin (an alloy of copper and nickel). This constitutes a thermoelectric (or thermocouple) circuit. At the thermo junctions A and B, some of the surface electrons of the iron atoms are more strongly attracted by the constantin nuclei than by the iron nuclei (par. 7). Hence, at both junctions, there is a slight shift of electrons from the iron to the constantin. This gives the constantin part of the circuit a negative charge with respect to the iron part: or it might be stated equally well that the iron becomes positively charged due to its shortage of electrons (those given up to the constantin). The contact difference of potential thus created between dissimilar metals is very small (a few thousandths of a volt); yet this difference of potential may be utilized, as will now be shown.
h.	Because of their contact difference of potential, the junctions take the part of battery cells and supply e.m.f. to the thermoelectric circuit. Their relative positions in the circuit, as illustrated in (2), are such that the e.m.f. of one opposes that of the other. When the two junctions
48
AIRCRAFT ELECTRICAL SYSTEMS
TM 1-406
40
are at the same temperature, their e.m.f.’s are equal; therefore, the net e.m.f. in the circuit is zero (par. 9). Consequently, no charge flows in the circuit when the junctions are at the same temperature.
c.	However, if junction A is made warmer than 5, its e.m.f. becomes greater than that of B and, within the range of temperature measured, increases in direct proportion to the temperature. The net e.m.f. (the difference between the two e.m.f.’s) now causes a flow of (conventional) charge in the direction shown by the arrows in @. The
electrical situation is illustrated in (4). If A and B were storage cells, A would be discharging, while B would be “on charge,” since current is being forced through it in the reverse of the direction of flow on discharge. Cell A would be losing energy and cell B would be gaining it.
d.	Suppose another metal, such as copper, is inserted in the circuit so as to form two new junctions, such as C and D in ©. As long as both of these junctions are kept at the cold temperature, the net e.m.f. in the circuit is the same as with one cold junction. Therefore, a galvanometer movement may be used in the circuit, as shown in ©, without disturbance to the e. m. f. of the circuit. Inasmuch as the resistance of the meter constitutes more than 99 percent of the resistance of the circuit, the resistance of the circuit as a whole
449399°—42-
49
A Rri-JUNCTIONS AT EQUAL TEMPERATURE-^ H B	A ____ ---------EQUAL E.M.E’S------► g
^-CONSTANTIN =J	T_______________________________T
© ©
z^-
. lt^HOT	COLD-^f-i	A_____	.	+1
A ml JUNCTION	JUNCTION B	A ___ -•------UNEQUAL E.M.F S-----»- B
^-CONSTANTIN	QJ	—p	j
■ 4.^-^-HOT	COLD—\ ^COPPER	a ■-t- '"HOT	I
A R^^juncton	junction \	a R-junction	( G )
^-CONSTANTIN	D	^-CONSTANTIN
© © Figure 24.—Thermoelectric circuits.
TM 1-406
40-43
ARMY AIR FORCES
is independent of temperature variations. Therefore, the current in the circuit follows Ohm’s law; the current is in direct proportion to the net e.m.f. The deflection of the needle of a Weston movement is in direct proportion to the current in its coil. Therefore, it follows, from the chain of relationships just outlined, that the needle of the galvanometer moves in direct proportion to the rise in temperature of the hot junction. The divisions of the thermocouple thermometer scale are therefore equally spaced.
e.	Thermoelectric circuits could be made with many combinations of metals other than that of iron and constantin. Each pair of metals would have its own set of values of thermoelectric potential differences. Iron and constantin serve best in measuring aircraft cylinder temperatures.
Section V
CONDENSER
Paragraph
General__________________________________________:____________ 41
Construction______________________________________________    42
Operation____________________________________________________  43
Capacity____________________________________________________   44
Break-down____________________________________________________ 45
41. General.—The condenser is a device for temporary accumula-
tion of electrical charge. It is generally used on aircraft as a voltage stabilizer, to suppress or retard change in voltage. Its specific applications in connection with ignition and generator systems are described in section VI.
42.	Construction.—a. A condenser consists of two plates, or sets of plates, or strips of metallic foil, separated from each other by a nonconducting substance known as the dielectric of the condenser. Mica or waxed paper are the most common dielectrics in aircraft condensers other than in radio condensers in which air, oil, or an electrolytic film are also used as the dielectric.
b. Opposite kinds of electrical charge may be stored on adjacent plate surfaces of condensers. The electrons on the negatively charged surfaces cannot flow through the dielectric, since it is a nonconductor. If they could, they would neutralize the electron deficiency of the positively charged surfaces and thus discharge the condenser.
43.	Operation.—a. A condenser will receive charge from a circuit and will return most of this charge when the circuit calls for it; consequently, it will receive energy from an electrical circuit and will return part of this energy on demand. Figure 25 shows a con-
50
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	43
denser (0), wired (for illustration of operation) so that it may be charged and then discharged. Before the switch (xS) is closed, both plates of the condenser are electrically neutral (par. 6). If the switch is thrown to the left, electrons will flow from the upper plate of the condenser into the electron-hungry positive end of the battery. Simultaneously, an equal quantity of electrons flows from the negative end of the battery to the lower plate of the condenser. The two simultaneous movements of electrons render the upper plate positively charged, and the lower one negatively charged. The flow of
Figure 25.—Circuit for demonstration of charge and discharge of a condenser.
electrons (the charging current) ceases as soon as the plates of the condenser have become charged to the same potential difference as that of the battery.
b. If the switch is now thrown to the right, the battery is thus disconnected and a load resistor substituted across the terminals of the condenser. The electrons on the negatively charged plate quickly flow into the resistor, and at the same time electrons flow from the resistor into the positively charged plate of the condenser. Usually, both plates become electrically neutral in a fraction of a second. The less the resistance of the load unit, the more speedily the condenser will discharge. The discharge current flows until the voltage difference between the plates has been reduced to zero.
c. The charging of the condenser by the battery represents the transfer of energy from the battery to the condenser. If the switch
51
: c	r ,

~=-	R’2A C±Q
--------------------------1 X_ COIL -»------------------------1*4 AMPERES------------------TERMINAL
(3) CIRCUIT VALUES AT TIME OF .001 SECOND IN CURVE B'
Figure 26.—Effect of inductance upon rise of current.
Ohm’s law, has been established, the magnetic fields of the coil will rapidly collapse, for the agency which sustains the current (the e.m.f. of the battery) is thus removed from the circuit. The fields will try to unload their energy by pushing the free electrons of the coil for-
55
TM 1-406
47-48
ARMY AIR FORCES
ward, in the same direction as that of their motion before the circuit was opened. This induced e.m.f. tends to sustain the flow of electrons. But in view of the opening of the circuit, continued circulation of electrons is made impossible. The heretofore positive end of the coil is therefore made highly negative, for a moment, by a sudden accumulation of electrons which cannot find a way to leave. Likewise, the other end of the coil is made equally positive by a shortage of electrons. As a result, the potential difference between the ends of the coil rises, for a moment, to a value of several hundred or several thousand volts. When the collapsing magnetic fields have spent their energy to the extent that they are no longer able to add more electrons to the accumulation at one end of the coil, the electrons spread out and flow back through the coil, so as to produce neutralization at all points along the wire.
e. The value of the self-induced electromotive force existing at any instant in a conductor depends directly upon the time rate which the magnetism associated with the conductor is changing at that instant. The collapse of the magnetic fields of a coil which occurs when the circuit is opened takes place in much less time than that which was required for their creation; therefore, the e.m.f. induced on the “break” of the circuit is much greater than that induced on the “make.”
/. Each step in energizing and de-energizing (magnetizing and demagnetizing) an inductive load unit involves a transfer or transformation of energy. Energy is transferred by electrons from the battery to the coil and its core, where the energy is stored. The evidence of the storage process, a counterforce on the electrons, disappears when current value (in accordance with Ohm’s law) has been reached. When the circuit is opened, the magnetic fields transfer their energy to the free electrons of the wire. The energy received by the electrons may be utilized by attaching a load unit, which is normally internally open-circuited, to the ends of the coil; if this unit becomes closed at the moment when the electrons are being crowded toward one end of the coil, the electrons will flow through this attached load unit and therein give up energy as they return to the other end of the coil. This is achieved in the electrical ignition system, discussed in paragraph 54.
48. Induction of alternating1 e.m.f. by variation in flux linkage.—a. If the amount of magnetic flux associated with a coil is varied by any process whatsoever, an e.m.f. is induced in the coil. A simple method for constant variation of flux in a magnetic circuit is illustrated in figure 27. A permanent magnet is mounted within the gap of an unmagnetized iron yoke so that the magnet may be rotated. The m.m.f. of the magnet is capable of producing flux in the magnetic
56
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	48
circuit composed of the magnet and the yoke (par. 27). With the magnet in the position shown, the flux is a maximum, for the reluctance of the circuit is a minimum. As the magnet is turned through 90°, the reluctance increases and the m.m.f. applied to the circuit decreases; both of these changes reduce the flux. At the 90° position the flux decreases to zero value, for in this position, the poles of the magnet are equally distant from each pole piece of the yoke, and the effect of one magnetic pole on the circuit is canceled by that of the other. As
O’*--ALTERNATING E. M. E---
(INDUCED)
/ \ \
\	I
\
Figure 27.—Induction of alternating e.m.f. by rotation of a magnet.
rotation is continued, the yoke circuit becomes re-magnetized with the flux lines reversed in direction. The flux attains a maximum value at 180° of rotation of the magnet from the original position. It again decreases to zero value at the 270° position, and is reestablished to the original value and direction when the original position is reached. The flux therefore reverses direction twice per revolution (at the 90° and 270° positions).
b. If a coil is mounted on the yoke in the manner shown, it is considered linked with the magnetic circuit. Whenever a coil is linked with a magnetic circuit in which the flux rises, falls, and changes di
57
TM 1-406
48-49
ARMY AIR FORCES
rection periodically, an alternating e.m.f. is induced in the coil. A graphical representation of an alternating e.m.f. is shown in figure 28. The primary feature to be noted is that the induced e. m. f. reaches a maximum, not when the flux linked with the coil is a maximum, but when the flux is at zero value and just reversing its direction. The two maximum e.m.f.’s occur at the 90° and 270° positions of the rotating magnet.
c. The magnitude of the induced alternating e.m.f. depends directly upon a number of factors, the more important ones being—
(1) Number of turns in coil.—Regardless of the number of turns, the same value of e.m.f. is induced in each turn; and the turns are in series. Hence, the total e. m. f. is in proportion to the number of turns.
E.M.E I---------------------------Y-------------------------1
I O	l\ o	I o	/ o
90	18©	270	3ro0
Figure 28.—E.m.f. induced at successive positions of magnet rotating at uniform speed.
(2)	Speed of rotation.—The force delivered to the free electrons in the wire depends upon the rapidity of change in flux. The faster the magnet is rotated, the more quickly the flux disappears and appears.
(3)	Strength of magnet.—The stronger the magnet, the greater will be the values of flux. Consequently the rate of change of flux will be greater.
(4)	Clearance between magnet and pole pieces.—The smaller the air gaps, the less reluctance there will be in the magnetic circuit; hence, the greater will be the values of flux established by the magnet.
49. Effect of load on terminal voltage.—Whenever alternating e. m. f. is induced in a coil, the values of the maxima, or peaks, of potential difference which successively occur between the terminals of the coil in alternate directions depend upon the nature of the load, if one
58
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	49-50
is attached to the coil. When no load is attached, the potential difference depends almost entirely upon the amount of induced alternating e.m.f., and the potential difference may rise to peak values of hundreds or thousands of volts. If a heavy load is applied by bridging (shorting) the terminals with a short length of copper wire, the opposite extreme condition is produced. The wire will drain away the electrons almost as quickly as they are pushed to either end of the coil by the induced e.m.f., and will conduct them to the other end of the coil. An alternating current, perhaps in the order of several amperes, will flow in the circuit thus formed. The successive concentrations of the electrons at alternate ends of the coil therefore cannot become as great as when there is no load circuit; and the potential difference created between the terminals can reach peak values which are only small fractions of the peak values obtained with no load.
50. Generation of e.m.f.—a. General.—A generator is a machine which converts rotational mechanical energy into electrical energy by means of electromagnetic induction. An a-c generator produces alternating potential differences at its terminals, while a d-c generator (like a battery) creates a more or less constant potential difference in one direction between its terminals. Every generator, regardless of type, functions through induction of alternating e.m.f. in coils, produced by means of periodic changes in the amount and direction of magnetic flux threading through (linked with) these coils. The variations in flux linkage may be accomplished by any one of a variety of methods, several of which will be discussed.
b.	Elementary a-c generator.—The arrangement shown in figure 27 and discussed in paragraph 48 illustrates a process of a-c generation in which the coil is stationary and the magnet is rotated to produce periodic changes in the amount and direction of flux linked with the coil. Another method of a-c generation is to use a magnet which remains stationary, producing periodic changes in the amount and direction of flux linked with the coil by rotating the coil itself. An elementary a-c generator of this type is illustrated in figure 29. In (T) is shown a coil wound on a rotatable soft iron cylinder located midway between the pole pieces of a magnetized yoke. The flux in the magnetic circuit threads through (is linked with) the coil when the latter is in the position shown. Magnetically, the situation is the equivalent of that shown in figure 27. If the coil is rotated from this position, the amount of flux threading through it is decreased. When the coil is at right angles to the position shown, none of the flux of the magnetic circuit threads through the coil. When the coil is rotated at uniform speed, e.m.f. will be induced in it which will vary approximately as indicated
59
TM 1-406
50-51	ARMY AIR FORCES
in figure 28. The e.m.f. will reach peak value twice per revolution, at the two positions in which no flux threads through the coil. At these positions, the flux is reversing in direction and the rate of change is then greatest. The soft iron cylinder is used within the coil for reasons given in paragraph 33d. The values of induced e.m.f. will depend, for the same reasons given in paragraph 48c, upon four primary factors listed below:
(1)	Number of turns in coil.
(2)	Speed of rotation.
(3)	Strength of magnet.
(4)	Closeness of clearance between soft iron cylinder and pole pieces.
Figure 29.—Fundamentals of a-c generator operation.
The generated e. m. f. is transmitted to the terminals of the machine and to the load circuit by means of two slip rings and brushes, as shown in figure 29®.
c.	Elementary d-c generator.—The successive pulsations of electromotive force which alternate in direction in a coil, as it is rotated, may be applied in one direction to an external circuit by use of a commutator and brushes, such as are used in a d-c motor (fig. 19). The function of the commutator and brushes is to reverse the connections of the coil to the load circuit every half-revolution of the coil. The e. m. f. applied to the load circuit as the coil is rotated at constant speed will then be in one direction and will vary approximately as shown in figure 30®. The e.m.f. reaches a maximum value twice per revolution.
51.	Practical d-c generator.—a. The terminal voltage of practical d-c generators is much more constant than the very unsteady e.m.f. shown in figure 30®. This condition is accomplished by use of an armature consisting of a slotted soft iron cylinder and a number
60
TM 1-406
51
AIRCRAFT ELECTRICAL SYSTEMS
of uniformly spaced armature coils. The number of magnetic circuits, magnetized pole pieces and commutator brushes is increased. The manufacturer may wire the armature coils together in any one of several ways and connect them to a commutator of many segments.
uj - /	\	\
L__________I__________X__________I__________J-
0'	90’	180’	270’	360
ANGLE OF ROTATION
(T) E.M.F. OF A SINGLE COIL
D.C. GENERATOR
y----WITHOUT CONDENSER
uj	\
< “ \
q _	---WITH CONDENSER
____I___I____I____1...J___I I I
0’	90’	180’	270’	360*
ANGLE OF ROTATION
(2) TERMINAL VOLTAGE OF A FOUR COIL DC GENERATOR UNDER LOAD
Figure 29.—Fundamentals of a-c generator operation.
The principles of operation, however, remain the same. There will still be a slight tremor in the value of the terminal voltage of the machine, known as commutator ripple, which may cause electrical interference in a nearby raidio receiver and be objectionably audible therein.
61
TM 1-406
51
ARMY AIR FORCES
The interference may be reduced or eliminated by stabilizing the terminal voltage with a filter condenser attached to the terminals of the generator. The condenser alternately absorbs the peaks of e. m. f. and makes up the deficiencies, smoothing out the terminal voltage to a steady value. Unfiltered and filtered terminal voltages are illustrated graphically in (2).
b. The fields in which the armature coils move are greatly strengthened, and consequently the induced e.m.f. is greatly increased, by use of field coils wound upon (linked with) the magnetic circuits of the generator and energized by e.m.f. developed by the generator itself. All aircraft d-c generators possess at least one shunt field coil, shunted directly across the brushes (fig. 31©). The m.m.f. of a
SHUNT FIELD COIL	SHUNT COIL SERIES COIL
m.mf______	mmt	m.m.f
I LLULL L |	LLLLL ----—
O	I M I
1	r	J	!—
+	-	*	-
® SHUNT WOUND GENERATOR	@ COMPOUND WOUND GENERATOR
Figure 31. -Elementary d-c generators.
shunt field coil is therefore directly proportional to the voltage difference between the brushes, unless limited by a voltage regulator (par. 68). A generator which contains only this type of field coil is a shunt wound generator. The terminal voltage of the shunt wound generator (when used without a voltage regulator) will be reduced as the load is increased. Figure 32© illustrates graphically the change in terminal voltage which occurs as the load upon a shunt wound generator is increased (that is, the resistance of the load decreased with the generator speed held constant). At no-load the terminal voltage is high and the load current zero. As the load increases, the load current increases and the terminal voltage decreases as shown by the solid portion of the curve. The point on the curve where the voltage decreases very rapidly with a slight further increase in load is known
62
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	51
as the critical point of the d-c generator. If the resistance of the load circuit is decreased still more, not only the terminal voltage but also the current will decrease as shown by the dashed portion of the curve; and, with a short circuit across the terminals, the voltage will be zero but the current about half of the rated full load current.
c.	A generator may be compounded in order to stabilize its terminal voltage. A compound wound generator contains, in addition to its shunt field coils, at least one series field coil, connected in series with the load circuit (fig. 31®). A series field coil is wound on the machine in such manner that its m.m.f. (which is proportional to the current in the load circuit) aids the m.m.f. of the shunt coils. Therefore, when the load current increases, the flux in the machine is increased and a greater e.m.f. is developed by the armature. The
--------------------r
UJ______________________________
<	FULL LOAD-XiK
§ CRITICAL POINT-1---------------------.J
I
ARMATURE LEADS
£ SHORT CIRCUITED
LOAD CURRENT -------•—
OVER COMPOUNDED—^
—* FLAT COMPOUNDED
UNDER COMPOUNDED^^^^r FULL LOAD
w	l\
2?	i	X
□	i
§	I
?	।
I	;
LOAD CURRENT ---------—
© TERMINAL VOLTAGE OF SHUNT GENERATOR
Figure 32.—Characteristic
< E
— MAKE AND BREAK _____________________I C
CONTACTS \
“ZT	X ’-C
j’b^SF^RK PLUG
s J L
Figure 34.—Elements of an electrical ignition system.
Z>. When the switch is closed, current rises in the primary coil and circuit, reaching Ohm’s law value in perhaps a few thousandths of a second (par. 476). During this time, energy is stored in the primary and in the core (par. 47c). If the points are now pushed open, this act breaks or opens the primary circuit. The rapid collapse of magnetism which results induces e.m.f. (par. 47rZ) in both primary and secondary, the total e.m.f. induced in each coil being proportional in magnitude to its number of turns.
c. Under the influence of the induced e.m.f., the free electrons in the primary tend to rush forward and rapidly accumulate on the first contact point reached. The other contact point will be drained of electrons. If the condenser were not present, the potential difference between the points would rise so rapidly that the points would be unable to separate without the occurrence of an arc between
67
TM 1-406
54	ARMY AIR FORCES
them. This arc would burn the surfaces of the points and, by its conductivity, would hinder the opening of, and the removal of battery e.m.f. from, the circuit. The condenser receives the electrons sent to the one contact point and supplies the electrons demanded of the other point; a considerable quantity of electrons must flow before any considerable voltage difference between the terminals of the condenser can be established. Hence, the rise of potential difference between the points is retarded and suppressed. (In this respect, the condenser produces the same result as the coil-shorting wire mentioned in par. 49). As soon as the value of the induced e.m.f. in the primary has decreased to, and falls below, the voltage to which the condenser has risen, the latter discharges back into the primary. The condenser thus returns to the coil some of the energy received from the coil. The condenser’s discharge current reverses the m. m. f. of the primary and this reversed m. m. f. tends to remove the core’s residual magnetism. The total change of magnetic flux is thus made greater. The primary condenser therefore accomplishes three desirable results: it lengthens the life of the points; it hastens the collapse of the magnetic fields; and it increases the total change in magnetism. The latter two of these results materially increase the value of e.m.f. induced in the secondary. The value of the capacity (par. 44) of the condenser is important. If too large, the condenser’s counter-voltage does not build up in time to be fully effective. If too small, the voltage rises to too high a value and arcing at the points results. The design engineer selects an intermediate value of capacity which fits well with the other constants of the circuit and results in best performance. A shorted condenser will prevent the opening of the primary circuit and thus render the apparatus inoperative. An open condenser results in arcing points and a low voltage output from the apparatus.
<7 ...The fdrce on the free electrons in the secondary caused by the collapsing fields is many times greater than on those in the primary because of the greater number of turns. The electrons are forced either toward the center electrode of the spark plug, or toward its outer electrodes, in accordance with the direction in which the secondary is wound on the core. The voltage difference might reach 30 to 50 thousand volts, except for the fact that the gap in the plug sparks over (when in the engine cylinder), at approximately 20,000 volts. The spark is a conductor; it closes the secondary circuit, and further rise in voltage is thus checked. Current flows in the secondary circuit as long as the collapsing fields have sufficient energy to maintain the spark discharge. When the voltage between the electrodes
68
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	54-55
falls below the minimum value necessary to sustain the electrical discharge, the spark “goes out” and the flow of charge ceases. All of this happens in a very small fraction of a second.
e. The two coils and core previously discussed are referred to collectively as an Ignition coil. When such a coil includes a mechanical vibrator for interruption (make and break) of the primary circuit, the coil is known as an induction coil, or in the Army Air Forces, as a booster coil. The booster coil is described in section IX. The magneto, the theory of which is discussed in paragraph 55, also contains a primary, secondary, and core. Magnetic, self, and mutual induction take essential parts in sources of high/voltage for aircraft electrical ignition. In each device energy is stored in a primary and core and is transferred to a secondary. In each device the energy is delivered at a very high voltage because of the high turns ratio between secondary and primary, and because of the rapidity of the collapse of magnetic flux.
55. Theory of magneto.—a. The magneto is a special type of electrical generator which produces pulses of high voltage. Flux is produced in the magneto by magnets and motion, rather than by means of a battery.
b.	The essential parts of a simple model of magneto are shown schematically in figure 35®. A 4-pole permanent magnet is arranged for rotation between the pole pieces of an unmagnetized soft iron yoke. The pole pieces of the yoke are so spaced that they are simultaneously adjacent to two magnetically opposite poles of the rotating magnet. A primary coil (Z5) having, perhaps, 150 turns of wire, and a secondary, (N) of many thousands of turns, are wound upon the yoke. (These coils are shown on other side of the yoke, in order to simplify the illustration.) One end of the primary is permanently grounded. The other end is connected to ground through a pair of contact points (2?) known as “breaker” contact points, which are normally held together by spring tension. The primary is therefore shorted when the breaker points are closed. The breaker points are shunted with a primary condenser ((7). One end of the secondary is grounded; for simplicity in manufacture, this ground connection is made to the terminal which is common to the primary, condenser, and one breaker point; when the breaker points are open, the secondary current easily reaches ground through the primary coil because of the low resistance of the latter. The other end of the secondary ultimately reaches the spark plugs through a distributor.
c.	Because of features of similarity between the magneto and the generator, ignition coil, and transformer, the operation of the magneto
69
TM 1-406
55
ARMY AIR FORCES
TO
DISTRIBUTOR
■■nil
p
Xb	A%Z
-==—	<(s/\iy>	•=&■
® ELECTRICAL AND MAGNETIC CRCUITS
(g MAGNETIC FLUX AT THREE POSITIONS OF MAGNETS.
Figure 35.—Circuit diagrams of simple magneto.
may be explained in several ways. A complete and detailed explanation is difficult in any event because the circuits within the magneto are so closely linked with each other, electrically or magnetically, that an action in one is reflected (by induction) in the others. The following is a simple explanation. When the poles of the magnet are
70
FLUX TO RIGHT	NO FLUX	FLUX TO LEFT
FF FF
ill Ui	ut iU
I IF Fl I L -J 11 4 H11
) X (	Is L I s	J F (
\ S 7~\ N /	) \	\ N / s /
xy yz	/n \	Y/Z
cr	45’	®0*
TM 1-406
55
AIRCRAFT ELECTRICAL SYSTEMS
in the position shown in ®, the m.m.f. applied to the yoke is a maximum. The m.m.f. is in such direction as to produce flux, directed to the right in the core of the coils. When the magnet structure is rotated, starting from the position shown, the magnetomotive force applied to the magnetic circuit (yoke and magnets) decreases to zero and reverses, as shown in (2). The changes are shown graphically in figure 36(1). The m.m.f. decreases to zero, increases to a maximum in the other direction, again falls to zero, and so on. There are four positions per revolution of the magnet in which the m.m.f. is zero, and four in which it is maximum (two in each direction). This varying m.m.f. creates varying flux in the yoke and induces alternating e.m.f. in both the primary and secondary (par. 485). The alternating e.m.f. induced in the secondary by this flux change is too weak to break down the gaps in the spark plugs. The secondary circuit therefore remains open and no ’effective energy leaves the magneto as a direct result of the rotation of the magnets. The breaker points are periodically opened and closed by a cam which is on the shaft of the rotating magnet. The cam is adjusted to close and open the primary circuit at certain positions of the rotating permanent magnet. These positions are indicated in figure 36®. The breaker points and the primary circuit becomfe closed shortly after each peak of applied m.m.f., when the latter is beginning to decrease in magnitude. While the circuit is closed, a rising surge of current is induced in the primary. This happens four times per revolution. The surges alternate in direction. Each time the breaker points are opened, the primary current rapidly falls to zero and, due to the action of the primary condenser (par. 54c), momentarily flows in the other direction. The m.m.f. which is applied to the magnetic circuit by these surges of induced primary current is shown in (2). Two m.m.f.’s applied simultaneously to a magnetic circuit are additive when in the same direction and subtractive when in opposite directions. Their net sum at any instant is the amount of m.m.f. impressed upon the magnetic circuit, and’ the amount of flux will be in proportion to the net m.m.f. The sum of the two m.m.f.’s applied to the yoke of the magneto at every instant during one complete rotation of the permanent magnet is shown graphically in @. (Curve @ may be verified by adding curves (T) and @ graphically, with due regard to the directions of the m.m.f.’s.) The total m.m.f. abruptly reverses direction four times per revolution. The reversals of m.m.f. are centered approximately about the zero m.m.f. line. The reversals of m.m.f. occur when the magnets are not far beyond the 45°, 135°, 225°, and 315° positions, in which positions the reluctance of the magnetic circuit (par. 27<7) is a maximum.
71
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ARMY AIR FORCES
Figure 36.—Graphical representations of m.m.f. and e.m.f. of a four-pole magneto.
72
6*__45*_90*_135*_180*_225*_270*_315*_380*
I I I T I I I
OPEN ^^SEO OPEN ^^SED OPEN ^CLOSED OPEN
| \ \ /
J —A-----------------A--------L—
’! \i \ A
w	y.	x.	/
0 MAGNETOMOTIVE FORCE DUE TO PERMANENT MAGNETS.
I 1 A
। —~,.— —~~ -,... -.
© MAGNETOMOTIVE FORCE DUE TO INDUCED CURRENT IN PRIMARY COIL.
5 "'^Ssss^s'
----------------------------------
(5) TOTAL M.MF. BEING APPLIED TO MAGNETIC CIRCUIT.
XhHlH
@ INDUCED EMF IN SECONDARY COIL.
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	55-56
The abrupt changes in m.m.f., which occur when the points are opened, with the circuit nearly at the condition of maximum reluctance, result in great and extremely fast changes of flux. Therefore, at each reversal of flux a great force is exerted on the free electrons in the secondary. The secondary voltage builds up until the secondary circuit is closed by a spark discharge (d below). The e.m.f. both appears and disappears very quickly, as indicated by the sharp peaks in
d.	Reverberatory surges of current may and frequently do occur in the high voltage distribution system which connects the spark plugs to the magneto. These surges may reflect energy back into the magneto. “Transient” variations in voltage, current, and magnetism result within the various interlinked circuits of the magneto. Since these transients are usually minor in magnitude, and unimportant, they have not been indicated in the curves of figure 36. However, in magnetos the rotors of which possess a large number of poles (as in an 8-pole magneto), a surge may be reflected at such time and be in such direction as to partially cancel and weaken a following surge of output voltage. This effect may be eliminated by use of a secondary condenser, which is connected in series with the secondary circuit. The effect of secondary condenser is to lengthen the electrical reverberation time of the secondary circuit. The condenser receives and stores the energy which would otherwise be reflected back into the magneto, and applies this energy to the next discharge. The energy is thus not only kept from being reflected back into the magneto, but is used to strengthen the next spark. This operation is possible because the pulses of output voltage of the magneto, unlike those of the ignition coil or induction (booster) coil, alternate in direction. When the secondary condenser tends to “unload,” its discharge current is in the reverse direction to that of the current which charged it; and since the next output surge caused by the magneto is in this new direction, the two currents aid each other and are additive.
56. Transformer.—a. The transformer is used to couple two a-c circuits (which usually operate at different voltages) so as to transfer energy from one circuit to the other. The voltage is raised by a “step-up” transformer, or is lowered by a “step-down” transformer. A transformer consists fundamentally of two coils (primary and secondary) wound upon and electrically insulated from a closed magnetic core (fig. 37). The coils are thus linked with the same magnetic circuit. The secondary or output coil of a step-up transformer has
73
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56	ARMY AIR FORCES
more turns than the primary. The secondary of a step-down transformer has fewer turns than the primary.
6. The core consists usually of laminations (layers) of soft iron If solid iron were used, electrical eddy currents would be set up in the core during operation of the transformer. Eddy currents create heat which impairs the magnetic qualities of the iron, lowering the efficiency of the transformer. Oxide films or nonconducting coatings on the laminations tend to prevent or reduce eddy currents without interfering with the magnetic operation of the core.
c. When the primary of a transformer is connected to a source of alternating e.m.f., the resultant pulsations of alternating current tend to magnetize the core first in one direction, then in the other.
LAMINATED IRON CORE	/	-----
• 1 :
-----....-	/ / J X 1' <__________........;____,. - - —
SECONDARY	/ PRIMARY
Figure 37.—Elementary transformer.
Energy from the a-c source is stored in the core during each establishment of flux. When the secondary is without a load, the e.m.f. induced in the primary coil by the rising and falling flux is virtually as great as the e.m.f. of the energy source, so that the net e.m.f. is almost zero. Consequently, the alternating current in the primary is almost zero. In effect, energy is returned to the supply line almost as fast as it is received. Transformers may therefore be left connected to the high voltage feeders at all times without appreciable waste of energy. As long as the secondary circuit is open, the a-c voltage induced between the secondary terminals does not do any work and, as far as the magnetic core is concerned, the secondary is virtually nonexistent. But when the secondary circuit is closed (as, for example, by switching on the lights in the circuit), the resultant flow of alternating current in the secondary circuit periodically removes some of the energy periodically placed in the magnetic circuit by the primary. Therefore, the back e.m.f. of the
74
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AIRCRAFT ELECTRICAL SYSTEMS	56-57
primary is reduced; the net e.m.f. in the primary circuit is increased; and the primary current rises. A well-designed transformer, therefore, draws current from the line in proportion to the demand of the connected load.
d. The most common reason for raising voltage is to transmit electrical energy over a considerable distance with minimum loss and at minimum expense for equipment. The higher the transmission voltage, the less current is required to transport energy at a given rate. The lower the current, the less the heat loss, and the smaller (and less expensive) need be the transmission wires. The voltage is stepped down at its destination to simplify insulation problems and to render the electrical energy less dangerous. Transformers will operate only with alternating current equipment. This fact is one reason for the widespread use of a-c rather than d-c in commercial power systems.
e. It has been found more satisfactory to use d-c equipment on aircraft for a number of reasons, the primary reason being that a-c electrical energy cannot be stored as such. However, where certain devices require a-c, a special source of a-c power must be provided, with which the transformer finds application. In larger aircraft wherein there is sufficient need for an electrical system operated by its own power pi ant and therefore independent of the aircraft engines, the use of a-c on a large scale may be found practicable. In such event, the aircraft may be relieved of considerable weight in copper by use of transformers and the transmission of energy at high voltages over small wires.
Section VII
AIRCRAFT STORAGE BATTERIES
Paragraph
General____________________________________________________________________ 57
Principles of lead-acid cell_______________________________________________ 58
Aircraft storage battery construction----------------------------,--------- 59
Ratings of storage batteries_______________________________________________ 60
Battery deterioration______________________________________________________ 61
Testing methods____________________________________________________________ 62
Battery charging----------------------------------------------------------- 63
Maintenance________________________________________________________________ 64
57.	General.—a. The purpose of the aircraft storage battery is to provide a reserve source of electrical energy for operating the various units of aircraft electrical equipment. During normal engine operation the engine-driven generator serves as the primary source of electrical energy and, in addition, maintains the battery in a charged state. The battery serves as the source of electrical energy at starting,
75
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during low engine speeds, and in the event of generator or engine failure.
b.	A storage battery is composed of a number of cells wherein chemical energy is transformed into electrical energy. The electrical principles and connections of chemical cells have been previously explained (pars. 4 and 9).
c.	The composition of the chemicals in a dry cell is progressively altered as the current flow is maintained. Eventually, the cell loses its ability to maintain a useful flow of current, and must then be replaced.
d.	In a storage cell, the means of achieving chemical separation of charge (par. 6'Y \ ^<55SSa=^//
rt- THUMB SCREWS FOR	Lurr.Tiur tc-ol.ma,	/
TERMINAL BOX COVER----.	NEGATIVE) TERMINAL	/
METAL CONTAINER-----/
(pueBER Lire©	/
Figure 40.—Typical aircraft storage battery.
which moves past a particular point in a circuit when an unvarying current of 1 ampere is maintained for 1 hour. Theoretically, a 100-ampere hour cell or battery will furnish 100 amperes for 1 hour, or 50 amperes for 2 hours, or 5 amperes for 20 hours, etc. Actually, the ampere hour output of a particular battery is dependent upon the rate at which it is discharged. Heavy discharge currents create heat and excessive internal losses within the battery. Hence, the efficiency and the ampere hour output of a battery are greatly reduced when the battery is discharged at a high rate. For aircraft batteries, a period of 5 hours has been established as the discharge time in the rating of battery capacity. This time of 5 hours is not to be construed as the length of time during which the battery is expected to furnish charge. Under
80
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60
AIRCRAFT ELECTRICAL SYSTEMS
actual service conditions the battery could be completely discharged within a few minutes or might never be discharged by virtue of constant charging by the generator, or periodic bench charging.
(2) Figure 41 shows a discharge curve based on values obtained during tests made on a 68-ampere hour battery. The load current was held at 13.6 amperes for 6 hours (except during momentary opening of the circuit at the end of each hour to obtain the open circuit voltage). It is to be observed that the open circuit voltage remained substan-
CO
O -----------o---------------------------------
> 2 < •------o-------e__	\
cr	\
Id	\
5 1-	\
J	TERMINAL VOLTAGE OF 68 AMPERE HOUR	6
p	CELL DISCHARGED AT RATE OF 13.6	\
Z	AMPERES FOR 6 HOURS
Id I-	-------------OPEN CIRCUIT VOLTAGE
2	------------- CLOSED CIRCUIT VOLTAGE
0-t-------------------------+--—4-----------♦-----♦---------
1	2	3	4	5	6
TIME. IN HOURS
Figure 41.—Discharge characteristics of typical aircraft storage battery.
tially constant during the first 5 hours of discharge, whereas the voltage while under load decreased gradually. After the rated capacity of the battery (68, or 13.6 X 5, ampere hours) had been exceeded, both voltages decreased rapidly as the discharge was continued.
c. The more common models of aircraft storage battery are rated as follows:
Nominal e.m.f. (volts)
12
12
24
Capacity (ampere hours)
68
34
17
449309°—42---6
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ARMY AIR FORCES
The ampere hour capacity of a cell depends upon the total effective plate area. Cells connected in parallel have a net capacity equal to the sum of the individual cell capacities. A battery of identical cells connected in series has the same ampere capacity as one individual cell. In multi-engine aircraft, where more than one battery is used, the batteries are connected in parallel; the net e.m.f. is thus equal to that of one battery but the ampere hour capacity is increased; the total capacity is the sum of the ampere-hour ratings for the individual batteries.
61. Battery deterioration.—a. Various factors promote deterioration of a battery and tend to render it useless for service. Sulfation (of plates) of a permanent nature (as contrasted with that occuring normally during discharge) causes a permanent increase in the resistance of a cell. Increased internal resistance transforms the electrical energy of a battery into useless and harmful heat within the battery, and energy is thus diverted from use in the load units; internal resistance also results in an increase in temperature within the cell while the cell is being charged. Excessive temperatures result in expansion and consequent buckling of plates, which in turn hastens shedding of active material. During the normal life of a battery approximately 20 percent of its active material may be shedded. The pores of material not shedded may become clogged with sulfate or other impurities, or the material may corrode. In consequence, the capacity of the battery is decreased. In addition, accumulation of shedded material may cause shorting of the plates and result in internal discharge. Internal discharge may also result by reason of “local action” between portions of the active material and metallic impurities on the same plate.
b. The state of charge of a storage battery should not be permitted to fall below a definite value. A battery may be permanently injured if permitted to remain in a low state of charge.
62. Testing methods.—a. The state of charge of a storage cell depends upon the condition of its active materials, primarily the plates. It is difficult or impractical, however, to ascertain the plate condition or degree of sulfation by visual inspection. Therefore, an indirect method (hydrometer test) is used to determine the state of charge of a lead-acid cell of a storage battery. In order to ascertain the fitness for use of a cell, which condition cannot be ascertained by hydrometer test, the high rate discharge test is performed.
b. The specific gravity or hydrometer test is performed periodically on a storage battery installed in aircraft, as a part of established inspection procedure, to determine if the state of charge is
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AIRCRAFT ELECTRICAL SYSTEMS
adequate. The hydrometer test determines the state of charge of a cell by measuring the degree oi concentration of electrolyte.
(1)	Pure sulfuric acid is 1.835 times as heavy as an equal volume of pure water. A new, fully charged, aircraft storage battery cell is filled with a mixture of approximately 30 percent acid and 70 percent water (by volume). This mixture or solution is 1.300 times as heavy as pure water. During discharge the solution (electrolyte) becomes less dense. Its “specific gravity” (weight as compared with water) drops below 1.300. Thus, by determining the specific gravity of the electrolyte, a reliable indication of the state of charge of the cell is obtained. As heretofore stated, the concentration of the electrolyte is not the cause of the particular state of charge of a cell; it is rather a normal incidental effect. Thus, if a cell has a low specific gravity reading (indicative of a low state of discharge), the mere addition of some pure sulfuric acid will increase the concentration
Figure 42.—Storage battery hydrometer and syringe.
and accordingly raise the specific gravity of the electrolyte, but the state of charge of the cell will remain as before, inasmuch as the sulfation of the plates remains the same. Even if the electrolyte were replaced with fresh electrolyte having a specific gravity of 1.300, the condition of the plates, and hence the state of charge, would remain the same. Therefore, the specific gravity test of a cell is reliable only if nothing has been added to the electrolyte except occasional small amounts of distilled water to replace the quantity lost as a result of normal evaporation.
(2)	The hydrometer commonly used to determine the specific gravity of battery electrolyte is shown in figure 42; it consists of a small sealed glass tube weighted at its lower end so as to cause it to float upright. A paper scale is enclosed within the narrow stem of the tube. The scale is marked from 1.100 to 1.300 with intermediate graduations. The hydrometer is generally enclosed (for convenience) in a syringe. With this arrangement sufficient electrolyte may be withdrawn from a cell, by suction, to float the hydrometer. The more dense the electrolyte, the higher the hydrometer will float. The higher number (1.300) will, therefore, be at the lower end of the hydrometer scale. The scale value indicated at the level of the electrolyte is the specific gravity.
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(3)	Aircraft batteries are generally of small capacity but are subject to heavy loads. The values specified for state of charge are therefore rather high. An aircraft battery in a so-called “low” state of charge may have an appreciable amount (perhaps 50 percent) of charge remaining, but the charge is nevertheless considered low in anticipation of heavy demands which would soon exhaust it. A battery in such state of charge is considered in need of immediate
HIGH
Figure 43.—Specific gravity indications of hydrometer.
ti
a o i t
i
1
i
1 i I i
recharging. In accordance with present aircraft storage battery specifications, a specific gravity reading (fig. 43) between 1.275 and 1.300 indicates a high state of charge; of 1.250, a medium state of charge; and of 1.200 or below, a low state of charge.
(4)	Hydrometer readings should not be taken immediately after, but before adding distilled water to electrolyte. This precaution is necessary for the reason that appreciable time is required for the water to mix thoroughly with the electrolyte; the hydrometer syringe might thus suck up a sample of electrolyte which is not representative of the electrolyte as a whole.
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AIRCRAFT ELECTRICAL SYSTEMS	62-63
(5)	Following testing, the sample of electrolyte should be returned to the particular cell from which it was drawn.
(6)	Care should be exercised during the hydrometer test of a lead-acid cell, inasmuch as sulfuric acid has a pronounced burning effect on clothing and skin. First-aid treatment consists of copius flushing with water. Bicarbonate of soda may be applied, but only after thorough washing with water.
c.	(1) The high' rate discharge tester is used in bench testing to measure the terminal voltage of a cell while it is under appreciable load, which is the essential indication of its usefulness. The tester (fig. 44) consists of two heavy prongs (with handle) which are bridged by a thick nichrome shunt of low resistance. A voltmeter, mounted on the tester, is connected to the ends of the shunt so as to measure any potential difference created between them. The voltmeter is a zero-at-center instrument with a range of 2.5 volts in either direction, so that either tester prong may be placed on either terminal post. When the ends of the prongs are pushed into the terminal posts of a cell, the resultant heavy current which passes through the nichrome shunt simulates a load current such as the cell might experience in actual service. The prongs are held in firm contact with the cell terminals for approximately 15 seconds. During this time the observed voltage should remain substantially constant.
> (2) It must be emphasized that the high rate discharge test is strictly a comparative test. Since the voltage reading is affected by both the resistance of the shunt and the internal resistance of a cell, the test consists solely of a comparison between the performances of cells of equal ampere hour capacity when checked with the same tester. In order that the test may accurately indicate the condition of a cell, the latter must be fully charged before the test is made: and the observed voltage should then be compared with the average of the readings of a number of cells in good condition, fully charged, and each equal in capacity to the cell under test. If any cell of a battery shows a voltage which is more than approximately 0.2 volt below the average of the readings of the good cells of similar capacity, the cell may be considered to be internally deteriorated. In a border-line case, the general physical condition and history of the battery should be considered in deciding whether or not the battery should be discarded. For this reason the high rate discharge test is usually performed only by experienced personnel.
63. Battery charging.—a. A storage battery may be restored from a low to high state of charge by the process of charging (par. 57d).
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ARMY AIR FORCES
b.	(1) The e.m.f. of the external charging source must be greater than the e.m.f. of the battery. The e.m.f/of a 6-cell lead-acid battery is approximately 13.2 volts. When, however, a charging current flows through the battery, its voltage soon rises to a value of approximately
VOLTMETER
Figure 44.—High rate discharge tester.
METAL_ PRONG
INSULATED
HANDLE
LOW RESISTANCE SHUNT
14.2 volts, known as the “gassing voltage.” The explanation for this , rise of 1 volt need not be given here, but its existence must be recognized.
Therefore, to charge a battery normally rated at 12 volts, an external e.m.f. of more than 14.2 volts is required.
86
CELL
POSTS
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	63
(2)	The value of the charging current depends on the net voltage in the circuit, which voltage is the amount by which the external e.m.f. exceeds the battery e.m.f. If the two were equal, no current would flow. To maintain a heavy charging current, the value of the external e.m.f. need be only a fraction of a volt higher than the e.m.f. of the battery because of the low internal resistance of the charging circuit.
(3)	The charging current must always flow in the same direction. Hence, only d-c, and not a-c, may be used in the charging process. The respective terminals of the charging source must be connected to the like terminals pf the battery. If incorrectly connected (or if the value of the external e.m.f. is below that of the battery) the battery would discharge through the intended charging source, possibly at a rate which would hastily deplete the charge.
(4)	The charging current, in amperes, should be suited to the ampere hour capacity of the battery and to its state of charge. A battery with large plates may be charged, without risk of damage to its plates, with a higher current than that which could safely be used with a battery having small plates. A battery in a low state of charge may be charged at a high initial rate, and, as it approaches a fully charged state, the charging rate should be reduced. If a battery in a low state of charge is charged at a slow rate, merely time is wasted. After a battery has reached a high state of charge, the continued application of a high charging current merely serves to heat the cells (which is detrimental), and, further, causes the liberation of oxygen and hydrogen. (These gases form an explosive mixture.) During charging, a small amount of “gassing” normally occurs. When the gassing has the appearance of violent boiling, it is a sign of charging at an excessive rate, or completion of the process. During charging, the temperature of the electrolyte should not be permitted to exceed 110° F.
(5)	A battery is permitted to remain on charge until the specific gravity of electrolyte attains normal value (1.275 to 1.300) and no increase in specific gravity is noted at three successive readings taken at 30-minute intervals.
c.	Batteries are charged in battery rooms by either of two methods. By use of a motor generator set (constant voltage), it is possible to start the process with a high current which automatically tapers off, reaching a value of approximately 1 ampere when a state of full charge is reached. When a tungar rectifier outfit (constant current) is employed, the battery usually cannot be charged at a rate higher than 5 to 7 amperes. The current remains substantially constant during the entire charging process. The constant voltage method
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ARMT AIR FORCES
requires less time, and does not require much supervision. The constant current method requires a longer time to charge a battery fully, and toward the end of the process care must be exercised to avoid overcharging.
(1) The constant voltage method employs a generator driven by an electric motor. The output leads of the generator are connected to large copper bus bars, across which the batteries to be charged are shunted. The batteries are therefore connected to each other in parallel, and must have the same nominal voltage rating. The potential difference between the bus bars is adjusted by means of a field rheostat to a value of 14.25 volts or slightly higher. Each battery automatically draws a current suited to its own ampere hour capacity and in accordance with its state of charge. A battery of high ampere hour capacity has a lower resistance than that of a battery of low capacity; the battery of high capacity draws a heavier charging current when it and a low capacity battery in an equal state of charge are subjected to the same charging e.m.f. As the state of charge of any battery increases, its e.m.f. increases; the net e.m.f. decreases, and the charging rate of the battery decreases. The initial charging current furnished to a battery in a low state of charge may be 30 to 50 amperes. The value of the charging current of any battery may be ascertained readily at any time by switching an ammeter, provided for the purpose, into the battery circuit. When a battery reaches a fully charged state it may be permitted to remain connected to the bus bars without harm, for the charging current is then approximately 1 ampere.
(2) When the available electrical source is a-c, a tungar rectifier outfit may be used for battery charging. This equipment includes a transformer to step down the line voltage, and a tungar rectifier bulb which converts the a-c into d-c, which is required for battery charging. The rectifier outfit may include two independent charging circuits, each separately controlled by transformer tap switches. Only batteries of equal ampere hour capacity should be charged in one circuit at one time; whether they are 12- or 24-volt batteries is immaterial. As many batteries as desired may be connected in series, plus to minus, up to a total nominal e.m.f. not in excess of 84 volts (with most charging outfits), in one charging circuit. The charging rate (5 to 7 amperes) does not decrease to any great extent as the charging process goes on. The resistance in the charging apparatus is much greater than that of the batteries under charge. The e.m.f. applied to the charging circuit must therefore be, from
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TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	63-64
the start, some few volts above the e.m.f. of the batteries in order to overcome this resistance. Therefore, the small increase in battery e.m.f. as the batteries become charged produces little change in the net e.m.f. in the circuit; and likewise, the decrease in internal resistance of the batteries produces little change in the total resistance of the circuit. Therefore, the charging current remains substantially constant. The specific gravity of the batteries must be checked frequently, for some batteries will reach full charge sooner than others, and may be damaged as a result of overcharging if left in the circuit.
d. A storage battery in service in aircraft is charged by a constant potential system, the voltage of the engine-driven generator being held constant by use of a voltage regulator. The generator voltage is predetermined and adjusted in accordance with the voltage of the particular storage battery to be used; the varying charging requirements of the battery are automatically satisfied.
64. Maintenance.—a. Aircraft storage batteries are of relatively low capacities (to conserve weight). It is impractical to repair an aircraft battery, and therefore it is merely replaced when it becomes unfit for further service. Maintenance consists chiefly of periodic external inspections, and testing of electrolyte.
b.	(1) Check the vent tubing and note the condition of the metal tubing used for protection over the rubber tubing, especially at bends, where there is danger of pinching. The downwind tubing outlet should be located where there is no danger of electrolyte drainage coming in contact with the parts of the aircraft. Check the outlet to see that the slipstream has not whipped and damaged the protruding tube end. The vent tubing must be clear of obstructions, otherwise dangerous accumulations of spilled electrolyte or battery gases might occur. Battery cell vent plug holes should be unobstructed.
(2)	Check the condition of asphalt varnish coating where it has been applied to protect aircraft or engine surfaces near the battery from spilled electrolyte.
(3)	In the event any electrolyte has been spilled on external parts of the battery or on nearby surfaces, corrosive effects may be neutralized by applying baking soda mixed to the consistency of a thin cream. Fresh applications should be repeated until all bubbling action has ceased. This should be followed by washing with water and drying. The neutralizing solution must not be permitted to enter any of the cells.
c.	Check the security of terminal connections. The plain washer should be under the wing nut, and the lock washer under the plain washer.
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ARMY AIR FORCES
d.	At specified periods, check the state of charge of the battery with a hydrometer. Test each cell; if the specific gravity is 1.200 or below, and all the cells are approximately the same, a normal discharged condition is indicated and the battery should be replaced. Regardless of state of charge, if the specific gravity of any cell differs appreciably from that of the remaining cells of the battery, a defective condition is indicated which requires battery replacement.
(1)	After the hydrometer test, check the level of the electrolyte in each cell and add distilled water if necessary. The electrolyte level should be maintained from i/8 to % inch above the top of the plates. With a battery having a baffle installed just above the plates, the electrolyte should be maintained level with the baffle.
(2)	All cells in a battery should require substantially the same quantity of water; if one cell requires appreciably more than the others, examine the battery for leakage.
(3)	Acid is never added to a cell to increase the specific gravity. If leakage or spilling of electrolyte has occurred, the battery should be replaced. Leakage may occur around the base of a vent plug which is not tight, or may be the result of addition of too much water.
(4)	Regardless of hydrometer readings, if a battery does not hold its charge under normal service conditions, or otherwise fails to function normally, it should be replaced.
e.	(1) Where low temperatures are encountered, it is especially important that batteries be kept well charged or the electrolyte may freeze and thus render the battery unfit for further use. Temperatures at which mixtures of sulfuric acid and water will freeze are as follows;
Freezing point
Specific	gravity of electrolyte	(Fahrenheit)
1.000 (pure water)____________________________________ +32°
1.050_________________________________________________ +26°
1.100_________________________________________________ +19°
1.150__________________________________________________ +5°
1.200 ______________________________________________ -16°
1.250_________________________________________________ -62°
1.275_________________________________________________ -85°
1.300_________________________________________________ -96°
(2) In tropical climates the normal service period of a battery is considerably lengthened by reducing the proportion of acid to water. Battery manufacturers furnish instructions for reducing the concentration of electrolyte.
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AIRCRAFT ELECTRICAL SYSTEMS
f. When an aircraft is to remain idle for more than 1 week, the battery should be removed and returned to the battery room for proper maintenance.
Section VIII
GENERATOR AND REGULATOR SYSTEMS
Paragraph
General___________________________________________________________   65
Construction features of aircraft d-c generator_____________________ 66
Electrical features of aircraft d-c generator_______________________ 67
Regulation of generator voltage_____________________________________ 68
Reverse current cut-out relay_____________________________.--------- 69
Two-element control panel_____________________._____________________ 70
Vibrator type current limiter_______________________________________ 71
Twin-engine generator control circuit_______________________________ 72
Plug-in models of voltage regulator________________________________  73
Generator current control switch relay______________________________ 74
Armature testing____________________________________________________ 75
Maintenance--------------------------------------------------------- 76
65. General.—The engine-driven direct current generator is used in modern aircraft. The purpose of the generator is to supply the energy required to operate the various units in the electrical system. The generator also maintains the storage battery in a charged state. An alternating current generator may be incorporated in aircraft as auxiliary equipment for use with those devices requiring a-c.
66. Construction features of aircraft d-c generator.—Figure 45 shows (in part) a sectional view of an aircraft d-c generator. Each feature of construction is discussed in detail in the following paragraphs.
a. Armature assembly.—The armature consists of a steel shaft, a laminated soft iron core, armature coils or windings, and a commutator.
(1)	The armature core (fig. 46®) is constructed of soft iron stampings. The core is laminated (par 565) to reduce eddy currents, which produce heat. The slots of the armature core are lined with fish paper (insulation paper).
(2)	The armature windings, which are of copper wire, may be insulated with a varnish baked on the conductor, varnish-impregnated cotton, or spun glass. The latter will withstand much higher temperatures than will the cotton or varnish insulation. The windings are secured in the core slots by wedges. Bands of steel wire aid in holding the windings in place.
(3)	The commutator consists of copper segments (fig. 46@) insulated from each other with mica and held firmly in place by wedge
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ELECTRICAL TERMINALS-7 r—FIELD WINDING	END BELL —
j jjl	BRUSH / SPRING DRIVE COUPLING-j
/	/	i—POLE PIECE	/
.X\(\ J Ij	l/ Hi
(rolfe° I 1i| MbJI/U
' I 1 / v= I	~ j ^gg I I |p--.-1
iTffT
COMMUTATOR—'	/ ARMATURE-^ \
FIELD WINDING '	'—CENTER FRAME
Figure 45.—Sectional view of typical aircraft d-c generator.
,SLOT FOR CONNECTING CONDUCTORS
P.	.. u
—2______s—
Q LAMINATED IRON CORE
(2) COPPER COMMUTATOR SEGMENT
M,CA
WEDGE RING-----
I—।
SLEEVE ------
(3) TYPICAL COMMUTATOR CROSS SECTION
Figure 46.—Armature and commutator construction.
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rings (fig. 46®). The leads from the armature coils are soldered into slots or risers located at the inside ends of the segments.
(4) The generator armature is connected to the engine by means of a suitable coupling. The coupling protects the armature from violent twisting motions caused by starting or sudden acceleration or deceleration of the engine.
b.	Field frame assembly.— (1) The circular steel or cast-iron center frame or yoke serves as a mechanical support for the generator and also forms a part of the magnetic circuit connecting the poles.
Figure 47.—Magnetic circuits of a 4-pole d-c generator.
(2)	The pole pieces are rectangular in cross section and are fastened to the center frame. Figure 47 shows the magnetic circuits of a 4-pole generator. The center frame, pole pieces, and soft iron armature core are all parts of the magnetic circuits.
(3)	Generators used on aircraft are either shunt wound or compound wound. Shunt field windings (which usually carry a small current) are relatively thin, varnish-insulated copper wire with many turns wound on each pole. In the compound wound generator, the series coils consist of only a few turns of thick wire (which must carry the entire generated current) wound around each pole.
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(4)	Interpoles, which are used in some recent models of generators, are small poles located between the main poles. The windings on these poles are connected in series with the armature circuit and must carry the entire generator current. The purpose of interpoles will be discussed later.
c.	Brush assembly.— (1) Generator brushes are usually small blocks of graphitic carbon which will cause minimum wear of the commutator segments and yet be sufficiently hard to give long service. Each brush is provided with a flexible lead for good electrical connection.
(2)	The brush slides freely in its holder so as to follow any irregularity in the surface of the commutator. A spring provides adjustable pressure on the brush. The brush bears upon the commutator with a pressure of approximately 1.5 pounds per square inch of contact area.
d.	End bell assembly.—(1) The supporting frame at either end of the center frame is commonly called an end bell (fig. 45). The end bells support the bearings, and therefore the armature. The end bell on the commutator end of the generator also acts as a support for the brush holders and the generator terminals. On the drive shaft end, the end bell is flanged and bolted to the generator supportingmount on the engine and must withstand any strain due to torque or manipulations of the aircraft.
(2)	The end bells are held in place with through bolts extending through the center frame, or by studs screwed into the center frame. These bolts must be tightened securely or the bearings will be thrown out of alinement, cramping the armature shaft.
(3)	Ball bearings are used to permit the generator to operate satisfactorily with the shaft in any position. The bearings must also withstand severe thrusts caused by the inertia of the armature as the airplane changes its direction of travel or speed. These bearings are lubricated at the factory and require very little attention.
(4)	The generator is provided with an oil seal located between the generator flange and the supporting-mount on the engine. Inasmuch as the driving gears operate under a pressure lubricating system, this oil seal must be tight.
67. Electrical features of aircraft d-c generator.—The general principles of induction, residual magnetism, voltage build-up, and compounding of generators have been discussed in section VI. Several additional electrical features of aircraft d-c generators follow.
a.	Rating of generators.—Rise in temperature is an important limiting factor in determining the rating of an electrical machine or
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device. A generator is usually given a continuous rating of a definite number of volt-amperes. That is, the generator will operate continuously at the rated quantity of volts times amperes (watts) with not more than a specified temperature rise. Factors which govern temperature rise, and thus the rated capacity of a generator at a constant speed, are, among others, the number of conductors connected in series between the brushes, the size of the conductors, and the strength of the magnetic field.
6.	Types of armature winding.— (1) The armature windings of almost all modern generators are closed circuits. In a lap wound armature, the two ends of a given armature coil are attached to adjacent commutator segments (fig. 48©). The coils are therefore connected in series, in one continuous closed circuit. In a ware wov/nd armature, the coils are also connected in series, forming a closed circuit, but the ends of each coil are connected to commutator segments which are approximately two pole spaces apart. A simple 5-coil, 5-segment, wave wound armature is shown in figure 49.
(2)	Although at first glance the lap and wave systems of winding appear to be quite different, they really represent equivalent methods of connecting in series the armature conductors which are at a given •moment under induction, and of positioning under the brushes the segments between which there is no e.m.f., so that no “live” coils are shorted by the brushes. Therefore, one description will serve to show the basic principle of both types of winding. Imagine an armature having a 10-slot core with two 3-turn coils wound in each pair of oppositely located slots. The e.m.f. induced in each coil when it is rotated in a magnetic field will rise, fall, and reverse (par. 48). When maximum flux threads through the coil, the e.m.f. is zero; when the flux is zero and reversing its direction through the coil, the e.m.f. is maximum. Therefore, the induced e.m.f. in coils which are in the positions a and a' as shown in figure 48© (and which coils are wound in the same slots) is zero because a maximum amount of field flux threads through them. There will be some induced e.m.f. in coils in the positions 6, 6', e, and e' (perhaps 2 volts in each coil). The most induced e.m.f. will be in coils in the positions \\ WnJ/ /S-ZV
4 V	4 *	"	~*
„______APT/ / ll\ ______________ 2* /I______________J\ 2v.	-L
b'^ /IO\ c	/ Ov \	T - k_______________J
-4-l2VOLTS^+	-4-12 VOLTS-k+	-l_l2 VOLT$_X +
E.M.F,	E.M.F	E.M.F
Q TEN ARMATURE COILS	@ BATTERY ANALOGY OF THE	@ EQUIVALENT CIRCUIT
LAP WOUND	TOTAL E.M.F GENERATED IN
ARMATURE COILS
Figure 48.—Wiring diagram and electrical operation of a lap wound generator armature.
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Figure 49. Induction of e.m.f. in a wave wound armature.
449399 —42-----7
S7
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the electrical circuit is the equivalent of two 12-volt batteries in parallel. Such a generator would have 10 small ripples per revolution in its developed e.m.f. (par. 51a).
(3)	In the wave wound armature, there are only two parallel electrical paths between the brushes, regardless of the number of brushes or poles on the machine. In a lap wound armature, there are as many parallel paths as there are poles on the machine. Therefore, a 2-pole machine, whether wave or lap wound, will have two brushes spaced 180° apart, as shown in figure 50®. A wave wound 4-pole machine needs only two brushes, spaced 90° apart, as shown in (2). However, two more brushes may be used, making four in all, as shown in (i). The current is thus divided up equally between the brushes, and results in less sparking at the rubbing contacts. The majority of aircraft d-c generators are of the 4-pole, 4-brush, wave wound type.
(4)	Figure 51 shows, by battery analogy, the electrical features of the brush connections to the closed armature circuits of 4-pole machines. In each closed armature circuit, the net e. m. f. at every moment is zero. Therefore, current does not circulate within the circuit, and the armature coils do not “short circuit” each other. If any brush in a wave wound machine ®, should fail to make contact with the armature, the full load is placed on the other brush of the same polarity, but both halves of the armature continue to function. In the case of the lap wound machine @, if any brush loses contact, only half of the armature remains effectively connected and must bear the full load.
c. Field windings.—(1) In the compound wound aircraft generator (fig. 52®), there are three terminal posts, marked A +, A —, and F+, wherein the letter A stands for “armature,” and F for “field.” A heavy wire conducts the generated current from the A— terminal to the negative brushes; the current passes through the armature and emerges at the positive brushes. From there it passes through four series field coils, and emerges from the machine at the positive terminal (A + ). The shunt field terminal (F+) obtains its current from A + by a connection not shown; the current passes through the four shunt field coils (which are in series) and finally reaches the negative brushes. The two coils wound on each pole piece carry current in the same direction and aid each other, producing the magnetic polarity indicated.
(2) A noncompounded aircraft generator is always shunt wound. Its field connections would appear as in ®, except that the heavy wire shown attached to A + would lead directly to the positive brushes (the series field coils being omitted).
(3) In some generators, series field coils are mounted on separate field cores called interpoles (fig. 52®), for reasons given below.
98
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jzl+ \	o	s' JZ1+ "'"x
i80°	x^'' o *	\	—*
(T) TWO POLE GENERATOR	FOUR POLE TWO	(3) FOUR POLE FOUR
(JJ IVYU rULt ULNlhA I UK	BRUSH GENERATOR	BRUSH GENERATOR
Figure 50.—Brush arrangements on d-c generators.
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n
BRUSH^	^BRUSH	—	—
+□ +□	z z
-H	1+	—	~
—	~~~	--- ---------- /—BRUSH
1+	BRUSH^
"^~zn	5	2=:
BRUShX	-BRUSH	~	—
+C_______k +F
J
----------------------	BRUSH- 0 WAVE WOUND ARMATURE-® LAP WOUND ARMATURE
Figure 51.-—Battery analogy of armature connections in 4-pole generators.
BRUSH
100
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Q COMPOUND WOUND GENERATOR
(2) GENERATOR WITH INTERPOLES
Figure 52.—Field winding connections of 4-pole d-c generators.
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d. Distortion of magnetic field.— (1) When the generator is functioning its output current flows through its armature, since the load and armature circuits are in series. The armature conductors therefore are surrounded by electromagnetic fields whose strength varies as the load current. These added fields tend to distort the magnetic field furnished by the main poles (fig. 53). The distortion becomes greater as the armature current increases.
(2) As a result of this field distortion, the brushes must be set at such a position that the plane of the coils which are shorted by the brushes is perpendicular to the distorted magnetic field. If there
DIRECTION OF ROTATION
WEAK POLE TIP 1 k	.— STRONG POLE TIP
____p p p ppp — -------------~ \-------------—
—-—' __________—-------------- -------------------------
JUT	vJ	wni—
Cneutral plane	|
WITH LOAD-1
1-WITHOUT LOAD
Figure 53.—Magnetic field distortion and rotation of neutral plane.
were induced e.m.f. in the coils shorted by the brushes, sparking between the brushes and commutator segments, pitting of the commutator, and excessive brush wear would result. The output of the generator would thus be weakened. Therefore, the brushes are moved forward in the direction of rotation. This is referred to as shifting the neutral plane, or plane of commutation. On some generators, the brushes may be shifted manually, as desired. On nonadjustable generators the manufacturer sets the brushes for minimum sparking at two-thirds to three-fourths of full load.
(3) Interpoles may be used to counteract field distortion, for shifting the brushes is inconvenient and unsatisfactory, especially when the speed and load of the generator continually vary. The magnetic strength (and therefore the corrective influence) of the interpoles varies with the load on the generator. This is ideal, because the field distortion varies with the load. The interpole magnetically
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strengthens the weak main-pole tip (fig. 53) and weakens the strong main-pole tip. In a generator the polarity of the interpole should be the same as that of the next main pole in the direction of rotation (fig. 52). In motors this rule is reversed. By use of interpoles, the efficiency, output,, and service life of brushes and armature are all appreciably improved.
68. Regulation of generator voltage.—a. General.—Since most electrical circuits are designed to operate within a definite range of voltage, it is necessary to maintain the voltage within those limits,. In aircraft, the generator is driven at variable speed and under
A +. 
$HUNT FIELD—7
C	p pp	RHEOSTAT	>
X	—MAM	<
]js	n	10X0 >
I
A-
Figure 54.—Regulation of generator voltage by field rheostat.
wide ranges of load, and in order to maintain a stable voltage under these conditions, automatic control is essential.
b. Principle of field regulation.—(1) The e.m.f. of a generator depends upon three factors, namely, the number of armature conductors connected in series, the rotating speed of the armature, and the strength of the magnetic field. The strength of the magnetic field is the only one of these factors which may be varied at will. The strength of the magnetic field depends upon the number of ampere turns of the field windings, and since the number of turns of wire is a fixed quantity, the current sent through the windings must be used as the control factor.
(2) Figure 54 is a simplified diagram which illustrates the principle of voltage regulation by variation in shunt field current. This is the basis of operation of modern systems used for terminal voltage regulation of both shunt and compound machines. If the field circuit
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resistance is increased (as by a rheostat), the field current decreases, which in turn decreases the field strength and the generated e.m.f.
c. Vibrator type voltage regulator.— (1) A simplified vibrator type voltage regulator (fig. 55) consists of a coil of fine wire (F) wound around a soft iron core; two contacts (<7), one mounted on a movable soft iron arm and the other stationary; a helical spring (/?); and a resistor (2?). The voltage winding (F) is connected across (in parallel with) the generator terminals, and the resistor is connected across the contact points.
(2) When the generator is not operating, the spring holds the contact points closed. As the generator comes up to speed, current flows directly from the A+ terminal to the F+ terminal through the
a+-------------------- ---------------------
is a SHUNT FIELD	(_____	$ _______
. r	c"— •	>
A-
Figure 55.—Generator voltage regulation by simple vibrator type regulator.
closed contact points. As the voltage rises, the current through F increases, and its iron core becomes more strongly magnetized. When the magnetic attraction on the movable arm becomes strong enough to overcome the spring tension, the contact points thus are separated. The field current must now flow through the resistor, and because of the resistance thus added to the field circuit, the current in the field circuit decreases. The magnetic field of the machine is therefore weakened and the generated e.m.f. decreases. As the generated voltage decreases, the current through F and the pull of the magnetized core decrease, and ultimately the spring closes the contacts. As a result, the field strength and the generated e.m.f. rise again. This cycle of events occurs over and over again, the points opening and closing many times per second. The terminal voltage of the generator rapidly
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varies above and below an average value determined by the tension of the spring, which may be adjusted.
69. Reverse current cut-out relay.—a. Purpose.—The purpose of the reverse current cut-out relay is to disconnect automatically the generator from the aircraft battery when the generator is inoperative or its terminal voltage is less than that of the battery, to prevent flow of current from the battery into the generator under such condition. This flow of current would tend to operate the generator as a motor. The current drawn from the battery would be very large and would probably damage the generator, or in any event, discharge the battery.
6. Construction.—In figure 56 a reverse current cut-out relay is shown as a part of a simplified generator-battery system. There are
—=—(=(7}^-^, A [~l___________________	MAIN /
0	-.-•	-~	-Mrj.	LINE7
X	> I —I- OB0	SWITCH	+
s i	—
ppp	f	>cw
*	[ ®	S
----£+.
A~
Figure 56.—Generator-battery system with reverse current cut-out.
two windings on the soft iron core. The current winding (CW), which is in series with the line, and must carry the entire generated current, consists of a few turns of heavy wire. The voltage winding (VW) consists of a larger number of turns of relatively fine wire, and is shunted across the generator leads. The two coils are wound on the core in the same direction. The movable arm carries a pair of contact points (C) which normally are held open by a spring (/S').
c. Operation.—When the generator is not operating, the contact points are open so that the battery does not discharge into the generator even if the main line switch is left closed. As the generator voltage builds up, the voltage winding of the cut-out relay magnetizes the iron core. When the generated voltage produces sufficient magnetism in the iron core, it attracts the movable arm, thus closing the contacts (<7), and the generator commences to charge the battery. The coil
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spring is so adjusted that the voltage winding will not close the contact points until the voltage of the generator is slightly above that of the battery. The charging current passing through the current winding of the cut-out relay aids the voltage winding in holding the points tightly closed. When the generator slows down, or for any other cause the generator voltage decreases below that of the battery, the current flowing from the battery remagnetizes the current winding in the other direction, so that it now tends to counteract the magnetizing influence of the voltage winding, reducing the magnetism of the core. Therefore, the contacts are opened by the spring, and the circuit between the battery and the generator is broken. Under normal operation, the contact points do not open until the reverse current has reached a value of 5 to 10 amperes.
70. Two-element control panel.—a. Generator control panels are usually referred to as two-element or three-element control panels. A two-element control panel consists of a voltage regulator and a reverse current cut-out mounted on the same panel. Figure 57® is a top view of a two-element control panel. A simplified diagram of a single-engine generator Circuit with two-element control panel is shown in (2). A simplified wiring diagram is shown in ©, for use in circuit analysis.
b. A reverse winding (RW) is shown on the iron core of the voltage regulator in the two-element control panel. This winding is connected in series with the field current-limiting resistor (2?) and is wound on the core in the reverse of the direction of the main voltage winding (P). The purpose of the reverse winding is to speed up the operation of the contact points (<7). When the generated voltage becomes sufficiently high to cause coil V to open the contact points, the current which then flows through RW and R tends to demagnetize the core;, therefore, the contact points close much more quickly than they would if coil RW were absent. The increased frequency of operation of the points results in a steadier terminal voltage; the difference between the peak and minimum values of voltage during each cycle of contact operation is materially reduced. In all other respects, the units on this panel operate as described in paragraphs 68 and 69.
71. Vibrator type current limiter.—a. Purpose.—If the circuit should be too heavily loaded it is possible that burn-out of the generator may result. A fuse, or a current limiter (limitator') will protect the generator. The current limiter is designed to automatically limit the current of a generator to values within safe operating limits, to protect the generator and other devices in the circuit.
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-------------SPRING TENSION ADJUSTOR —7 m jf
VOLTAGE	I	\	REVERSE	\ \
0+ REGULATOR	-K---.\	/ CURRENT \
FT r~V rx \ // lx U/n
I—I	W I
@1
-lb-a. z' 'x / /	\\	Ab-
"	& VJ
'/ RESISTOR
® TOP VIEW OF TWO-ELEMENT CONTROL PANEL
I VOLTAGE REGULATOR ' REVERSE CURRENT y ______________________
A-
X X
(g> FEATURES OF TWO-ELEMENT CONTROL PANEL.
I 2- T
/	if	5cw i n
/s'	। I"' Si
\C/	^C*2V2VW	_t
/	"T”
*/---------Zt I.-J	—
<3 DIAGRAM FOR STUDY OF CIRCUIT ANALYSIS
Figure 57.—Generator-battery system with two-element control panel.
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b. Construction.—The construction of the current limiter is similar to that of the voltage regulator except that the actuating coil is in series with the main line. Thus, the amount of current flowing in the line is the determining factor in the operation of the limiter, instead of the voltage across the line, as in the case of the voltage regulator. The coil consists of several turns of large wire, since it carries the entire line current.
c. Operation.—In figure 58, a vibrator type current limiter is shown as part of a simplified wiring diagram of a three-element control panel. The spring S2 holds the contact points C2 together until the current through the main line and series winding SW becomes excessive, magnetizing its iron core to such an extent that the magnetic pull on the movable arm overcomes the tension of the spring, and opens the contact points. This inserts resistor R2 into the field circuit of the generator, decreasing the field current, the strength of the magnetic field, and the generated e.m.f. With decreased e.m.f., the generator current is reduced. The iron core is therefore damagnet-ized, and the spring closes the contact points. This causes the generator voltage and current to rise, until the current reaches a value sufficient to start the cycle again. The spring may be adjusted to limit the current to any desired value within the rating of the limiter. In other respects, the three-element control panel is similar to the two-element panel.
72. Twin-engine generator control circuit.—a. On twin-engine aircraft, a generator may be mounted on each engine. Each generator, controlled by a separate 2-element panel, furnishes electrical power to the same or an interconnected bus bar, as indicated in figure 59. The batteries are thus effectively connected in parallel. Either generator serves to charge both batteries. It is desirable to use both generators so that the total load may be as nearly equally divided between them as is practicable. To accomplish this, both voltage regulators should be adjusted to maintain the voltage at the same specified value. By use of a two-position voltmeter switch, the voltage of each generator system may, in turn, be observed on the voltmeter. It is advisable to follow such procedure with a more accurate check by the use of a test voltmeter connected as shown in the figure by the dashed lines. When zero indication is observed, the two voltages are equal.
b. One ammeter may indicate a greater generator current than the other ammeter indicates, by reason of slight variations in line conditions ; such difference, unless pronounced, may be considered a normal operating feature.
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73. Plug-in models of voltage regulator.—a. General.—In recently designed aircraft, the voltage regulator and generator current control are installed as separate units. The voltage regulator plugs into a standard panel socket, so that it may readily be removed for inspection or replacement. The plug-in models are designed to control the voltage of generators rated to generate several hundred amperes at 24 volts, and to provide regulation over a wide range of generator speed and load.
b. Vibrator type.—The vibrator type plug-in regulator provides field current regulation in three stages, by successive use of three sets of vibrating contacts. A diagram of this unit is shown in figure 60.
A+	_______ TaF s'
(------------------*A+ B+»-—(A) —
i	>--- ______	J
(v)	—©—~~|	y?
I I
A+	_______ _
L------------------»A+ B+>. (A)—•< •—-----I +
U-F i
Figure 58.—Generator-battery system with three-element control panel.
As long as the generator voltage is of low value, the spring $ holds contact points A and B closed, and points C open. The generator field terminal is then directly connected to the generator armature terminal, and the regulator then does not limit the field current. When the generated voltage rises to a predetermined value, contact points A begin to operate under the influence of magnetism of the voltage coil G. The operation of these points intermittently cuts resistor P into the generator field circuit. When the generator speed is increased or the load current decreased, the operation of points A is inadequate to control the voltage; the increased magnetism of G then causes points A to remain open and causes points B to be set in operation. In this second stage of regulation, the field current alternately passes through re-
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VOLTAGE	CURRENT	REVERSE CURRENT
REGULATOR	LIMITER	CUT-OUT
----- A+ !g+ ' --------------------- !
-------- ------:  1.  i~~I —i. ITT ■______Tx—I I (_________________ I (________>	; A_n	.	____ ' MAIN//
i ,v(	5 । n ; sw t	>	i 5r~.- । line	.
H—<=d L itfe 5W,TCH 0 shunt field	; < > *,g I   f 2 i f ’ <— "cw I	-
Il ft e;T '• [<■ j * ■	•	I	■
!	1	1	1
; _________। _______ ।	;
!	[	!	;
।	■	।	•
।	।	।	•
■	ii	■
1 1 1 „ 1 A-	F+	!F+	■	B-,
Figure 59.—Twin-engine generator control system.
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AIRCRAFT ELECTRICAL SYSTEMS	73
sistor P and resistors P and Q, in series. With a further rise in generator speed or further drop in load current, the operation of points B is unable to control the voltage; coil G becomes so strongly magnetized during the voltage peaks that points G are closed intermittently; points A and B remain open. When points G are closed, the generator field terminal is directly grounded, causing the shunt fields to collapse. Resistor R and coils H and I speed up and stabilize the regulator armature vibrations. Condenser K reduces arcing at contact points G. The operating voltage of the regulator is adjusted by change in tension of spring S. The entire mechanism is mounted on a rectangular plate, shown at the top of the diagram, from which four pins project on the side away from the observer. Pins B and G are insulated from the plate, whereas pins A and G are not. The heavy dashed line joining pins A and G to the relay mechanism indicates that these units are in electrical contact through the frame. Therefore, the plate, and in fact the entire framework of the regulator, are always at generator field potential. The two dotted circles in the plate represent the positions which would be occupied by the fifth and sixth pins, if this were a complete six-pin standardized voltage regulator plug.
c. Field-rheostat type.—The automatic field-rheostat type of plug-in voltage regulator does not require the use of vibrating contacts. It incorporates provision for automatic equalization of load when two or more generators are operated in parallel. The regulator contains two banks of resistors, of which as many as are required are automatically cut into the generator field circuit. A schematic diagram of the unit is shown in figure 61. Any current flowing in the voltage winding V or the equalizer coil EG tends to magnetize the iron yoke and causes it to attract the soft iron armature, through a distance roughly proportional to the current in either coil. The movement of this armature causes (by a mechanism not shown) the progressive separation of a number of silver-plated reed-like contacts from two silver-plated bars, Ni and S2, against which they normally rest by their own tension. The reeds are electrically connected to two resistor banks, Rr and R2. Therefore, until current flows in one or the other of the coils, contact B is directly connected to contacts A and G. The operating voltage of the regulator is adjusted by change in tension of spring S. The value of resistor Rs is set at time of manufacture or at the depot. In the rear of the unit is a standardized 6-prong plug, which fits into a corresponding panel socket. The regulator may be used to control a variety of types of generator, as indicated in the Technical Order for the regulator. Socket connections to a compound generator having two shunt field circuits are shown in the figure. Socket terminal K is attached to the
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corresponding terminal of all other generator regulator sockets on the aircraft, the interconnecting wire being referred to as an equalizer bus. This interconnects the equalizer coils of the various regulators. When
I--------------------------1
1 TO	T0
ARMATURE	GROUND I
i '-x	A	i
----h™LD	O '
—	I	®-------©------------1—I
T	i	I
B F	I
( r I_____2 LQUNting. _p - ate_I
i
■y------------— |Q i
। | j ...y
I	Ga aaa £ &
11	al
1 W>,\	0
*—®	__	H
□ J '	।
K
Figure 60. -Diagram of vibrator type plug-in voltage regulator.
the load is distributed among the generators in the proper proportion, no current flows in the equalizer coils. If for any reason one generator begins to carry more than its proper share of the load, current starts to
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AIRCRAFT ELECTRICAL SYSTEMS
Fl I Fl I------------------
o ol	3	3	I—I	V n
GENERATOR CURRENT	n_<>	n__<	1 p p p p r-
CONTROL SWITCH RELAY	11	">	11	>
3 O	n f S | J J III
Q.. > R,	0—-> Rj	ppp.. HWW^1 1
0___>	n •>	S
TO EQUALIZER BUS	n«>	Z *>	'ill IL L
1	u-	Sz
-----------♦---(A)	©-—J	(K)	@----------
/---------WWWVWW--------*
r3
l.jjj	jg) ©---	© ©d------------—---------
------1--- J©	\©t^---------------------------------
J	yD -------- N—Z
)O /	FRONT VIEW	VIEW FROM
}O /	OF SOCKET	REAR OF
>O /	REGULATOR
j/E_________
Figure 61.—Diagram of field- rheostat type plug-in voltage regulator.
449399°—42——8
113
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ARMY AIR FORCES
flow in its equalizer coil and the voltage winding is automatically aided, cutting more resistance into the field circuit of that generator.
74. Generator current control switch relay.—This relay performs the same duty as the previously-described reverse current cutout, and in addition serves as a remotely controlled generator main line switch. Two models of this device are represented in figure 62. In the simpler model, illustrated in ®, the voltage winding of the cut-out
FROM FT	J"	TO BATTERY |+
GEN.+
/ \ T FF c
\ *■>
\	( cw
\______ )
VW (--\
\
R \
(nm	(sw)
y--COCKPIT CONTROL SWITCH
(D
Figure 62.—Diagrams of generator
relay carries no current until the cockpit switch is closed. Other than this one feature, the unit operates as described in paragraph 69. In (2) the voltage winding of the cut-out relay is permanently connected across the line and has in series with it a rectifier which permits current to pass only in one direction (indicated by the arrow). Therefore, when the e.m.f. built up by the generator is in the proper direction, the voltage winding automatically becomes energized and closes contacts A. If the cockpit control switch is then closed, the contractor coil closes a
114
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AIRCRAFT ELECTRICAL SYSTEMS
multi-contact switch. The switch contains two pairs of contacts (C and /S'), one of copper and the other of silver. The copper contacts are the first to close and the last to open, so that the corrosive effects of unavoidable arcing occur at the copper contacts instead of at the silver contacts.
75. Armature testing.—The methods described below’ for testing armatures can be used for either lap or wave windings.
FROM RZ	T0
GEN.~7^	e	W----*
/ \ B------------------------
\j >cw —6
\----/	—^CONTACTOR
L_ ~	> COIL
> vw	S ----1
> \ ,______\
E	3=^-?	\
R rectifier -------------COCKPIT CONTROL SWITCH
current control switch relays.
a. To test an armature for grounds, an ohmmeter or other continuity tester is used to check the resistance between each commutator segment and the shaft, as illustrated in figure 63©. The diagram also shows how to test the armature for open or shorted coils. For this test the brushes should be lifted, if the armature has not been removed from the frame. To perform the open and short test, the ohmmeter leads are touched to each pair of adjacent segments. When so touched the ohmmeter is shunted across one coil, and also all the remaining coils,
115
TM 1-406
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ARMY AIR FORCES
—•------------ OHMMETER -------------•—
OPEN AND SHORT TEST
® TESTING PROCEDURE WITH OHMMETER
-------/VWW---------------
CURRENT LIMITING RESISTOR
® TESTING WITH BATTERY CURRENT AND VOLTMETER
Figure 63.—Testing armature for
if no open or short exists, as shown in (2). If, for example, there are 8 coils and each coil has a resistance of 0.5 ohm. the net resistance (between ohmmeter leads) may be found by applying the formula—
1.1,1 p t p 1 p , etc. ri-i /12	-U3
or
p __	1	1	17,
T~ F~ TTY=16=I6 ohm-
0.5 '7X0.5 0.5 '3.5 7
116
TM 1-406
75
AIRCRAFT ELECTRICAL SYSTEMS
.5 A
------------------VA---------------------
5 A	.5 A .5 A	.5 A .5 A .5 A	.5 A
v\a/ /vv—'AW—yx/V'—v\aa—aa/—
--------------------Jz-4--------------------
Z J —-OHMMETER
(a) equivalent circuit during open and short test
// )	/—ARMATURE under test
---------------------LAMINATED CORE --------------------- -COIL
-MOV^^^PLUG^	 '
(4) ARMATURE testing by use of growler
opens, shorts, and grounds.
Regardless of which two adjacent segments are being tested, the ohmmeter should indicate the same value. If zero resistance is indicated between any two adjacent segments, then that coil is shorted. On the other hand, if the ohmmeter reading between any two adjacent segments is abnormally high, an open coil is indicated. In the present example, an open coil should cause a reading of 7X0.5=3.5 ohms, for then there would be 7 coils in series without the one coil in parallel with them. It is not necessary to know the resistance or to make the above calculation; a comparison is all that is necessary.
117
TM 1-406
75-76	ARMY AIR FORCES
b. The same results may be obtained by the use of an applied voltage and a low range voltmeter as shown in (3). In this case, the same voltage should be obtained between any two adjacent segments. Zero voltage indicates a short circuit, whereas a higher voltage indicates an open circuit.
c. A device which is used in repair and rewinding shops for quickly localizing trouble in armatures is called the growler. The armature under test is placed in a V-shaped slot in the core of an a-c electromagnet; the relatively loud 60-cycle hum and chatter which result give the growler its name. Figure 63® is an end view of an armature resting on the growler. The armature completes the magnetic circuit; its windings act as the secondary of a transformer, with the growler coil forming the primary. Charred insulation is indicated immediately by smoke. Tests for short and open circuits are made by turning the armature and exploring the armature core with a thin strip of iron or steel, such as a hacksaw blade. Since the armature winding is symmetrical, the hacksaw blade will be attracted with the same force by a good armature regardless of its position. On the other hand, a shorted armature coil will cause a place of stronger attraction whereas an open coil will cause a place of weaker attraction for the hacksaw blade.
76. Maintenance.—a. Generator.— (1) Check the terminals for secure attachment of external leads.
(2)	Check for worn or loose brushes; binding brushes in brush holders; inadequate brush spring tension; and dirty or rough commutator. (Improper brush functioning usually results in a low generator voltage output, and a rough or dirty commutator causes excessive arcing at the brushes.)
(a)	When replacing a brush, the new brush must be seated to the commutator by inserting a strip of No. 000 sandpaper between the brush and the commutator, with the sanded side next to the brush. The sandpaper is then pulled in the direction of generator rotation until the brush is properly seated.
(b)	Binding brushes are wiped clean with a lint-free cloth moistened with unleaded gasoline.
(c)	Brush springs are adjustable on most types of generators; however, if proper spring tension cannot be obtained the generator should be replaced.
(4Wfl|K»8IRWR...J»'-!Tt>a
^•^1.	ICiL -
 
COOLING FINS	WcONNECTOR
' i STRAP
;«WF-
Figure 67.	—Cutaway view of type SF9LN—4 magneto.
2'	D fS K T Y PE
ROTOR
COMPENSATED >
CAM
■■_ .>■■■..	W
#rWSB*K^W' .
Figure 68.	—Cutaway view of type SF14L-3 magneto.
125
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ARMY AIR FORCES
per revolution of the magnet is therefore equal to the number of its poles. The number of cylinder firings per complete revolution of the engine is equal to one-half the number of engine cylinders. Therefore, the ratio of the magneto shaft speed to that of the engine crankshaft is numerically equal to the number of cylinders divided by twice
FL— ‘	FL" ' -=F
LJ	J J
X/ \s	4v>Fz
® TWO POLE	@ FOUR POLE	@ EIGHT POLE
Figure 69.	—Arrangement of magneto magnet poles.
ELECTRODE-i	/V™ SPARK PLUG
/	ZZfrom booster
"*	//Lf ABL0CK
/ /	‘Z-— \---- LEADING FINGER -Fi'Hx!.!' F
[ /	|| z&'F trailing finger ---hr . ©NW |
//o	M o \	—rtf Oil
L	r-	A in
o	L	B Wp
\ \ n	C\ /	'////	x©' booster
\ \ (J	(J /	--- 'WZ -COLLECTOR
\ \	/	V///	Ring
■111
Figure 70.-—Sectional view of disk type distributor.
the number of poles on the rotating magnet. In earlier models (lever type breaker), the movable contact point is mounted on a pivot arm actuated by a cam follower. In the modern, pivotless type breaker assembly, the movable point is mounted on a spring (fig. 71). The points are made of a platinum-iridium alloy; a frosty appearance of the points indicates that they are in good working condition. The cam follower is lubricated by means of a felt pad which is moistened with oil.
126
TM 1-406
79
AIRCRAFT ELECTRICAL SYSTEMS
(2) Magnetos used with radial engines may be equipped with a compensated cam, because on radial engines the time intervals between cylinder firings may not be uniformly equal. The compensated cam is made to rotate at the speed of the distributor rotor; one lobe on the compensated cam always fires the same cylinder, and the lobes are so spaced that each cylinder is fired at the right moment.
® TWO-LOBE CAM
(2) FOUR-LOBE CAM
(3) EIGHT-LOBE CAM
(4) COMPENSATED 14-LOBE CAM
Figure 71.—Pivotless type breaker assemblies.
s. Condensers.—The primary condenser may be either in rolled or flat form (fig. 72). One side of the condenser is grounded and the other side is connected to the movable breaker contact point and to the coil assembly primary terminal. When a secondary condenser is used, as in the 8-pole magneto, it is found in the distributor rotor, in series with the leading distributor finger.
/. Shielding and coil cover.—The magneto has a metallic cover, the joints of which are tightly fitted. A plug in the cover has a screened hole for equalization of air pressure between the inside and outside of the magneto. The magneto cover may have a number of fins to facilitate heat radiation when the magneto becomes warm. Air, ob
127
TM 1-406
79-80
ARMY AIR FORCES
tained by means of a small scoop located on the front of the engine, may be directed by a blast cover against these fins to cool the magneto. The air does not enter the magneto.
80. Types of magnetos.—a. Magnetos are built in single and double types. The double type (fig. 73) consists of two magnetos (without distributors) which utilize one rotating magnet in common.
Figure 72.—Magneto primary condensers.
rhe double type magneto contains two sets of breaker points; consequently. four sparks are produced by each coil assembly per revolution of the magneto drive shaft. The high voltage is distributed by two distributor heads mounted elsewhere on the engine. The double type magneto is commonly used on “in-line” engines, whereas two single magnetos are used on most radial engines.
b. A base mounted magneto is attached to a bracket on the engine by means of cap screws which pass through the bracket and into
128
TM 1-406
80
AIRCRAFT ELECTRICAL SYSTEMS
p— ..... —	..	............
r---—COIL ASSEMBLY-------j
\ ^-MOUNTING FLANGE ['
COOLING fl
FINSS	CONNECTION
W8K»BE mIF*	x
W	" CONN ECT ION
C ONDENSERS
:; -' ■	BREAKER
X _'	CO NT AC T
POINTS
© CUTAWAY VIEW
/	//	\ /-DISTRIBUTOR-a /* /I *\
\ -Li HEAD Yr)
TO IGNITION SWITCH;
—।	——
T JT
Xto ignition switch
@ WIRING DIAGRAM
Figure 73.— Double type magneto.
449399 —42-----9
129
TM 1-406
80-82
ARMY AIR FORCES
tapped holes in the base of the magneto. A flange mounted magneto is attached to the engine by means of a flange on the magneto. The holes in the flange are slots which permit a slight adjustment, by rotation, in timing the magneto with the engine. The single type magneto may be either base or flange mounted; the double type is flange mounted.
c. The various types and models of standard aircraft magnetos are designated by letters and numbers which describe the magneto as follows:
Symbol	Description
S_____________________________________________________Single type.
D-----------------------------------------------------Double type.
B_________________________________________________Base mounted.
F-------------------------------------------------Flange mounted.
R_______________________Clockwise rotation (from drive shaft end).
L________________Counterclockwise rotation (from drive shaft end).
9, 12, 14, etc___________________Number of distributor electrodes.
Example: Type SF14L designates a single type magneto, flange mounted, for 14-cylinder engine, and counterclockwise direction of rotation. An additional number or letter in the type designation, such as SF14D-la, denotes that some change or new feature has been incorporated in the magneto.
81. Internal timing of magneto.—The manufacturer determines, for each model of magneto, how many degrees beyond the neutral position (zero m.m.f., par. 55), a pole of the rotor magnet should be in order to result in the strongest spark at the instant of breaker point separation. This angular displacement from the neutral position, known as the E gap angle (fig. 64(g)), varies from 5° to 14°, depending upon the model. The rotating magnet will be in the E gap position as many times per revolution as there are poles. A step is cut on the end of the breaker cam for internal timing of the magneto. When a straightedge is placed on the face of this step and the shaft is turned until the straightedge coincides with timing marks on the rim of the breaker housing (fig. 74), the magnet rotor is then in the E gap position, and the breaker contact points should be just separated. The distance between the contact points at maximum separation is of no consequence in the pivotless type magneto.
82. Ignition boosters.—a. One type of ignition booster is an auxiliary booster magneto, driven by hand. A booster magneto is frequently built into a hand cranking gear assembly, if such is employed as a cranking device, so that the act of cranking also turns the booster magneto. The construction and principles of operation of the
130
TM 1-406
82
AIRCRAFT ELECTRICAL SYSTEMS
booster magneto are basically similar to those of the magnetos already described.
b. A booster coi] is.a small induction coil. The booster coil has three terminals which, in figure 75, are marked G, B, and HV. Terminal G is grounded, while B is connected through the starter switch to the ungrounded terminal of a battery; the B terminal connection may be routed through the magneto safety ignition switch, so that the booster coil will remain inoperative, even if the starter switch is closed, until the magnetos are turned on. The terminal HV is connected to
TIMING MARK-----t
//	\ /X	\\,-CONTACT POINTS
//	'	\\ (JUST opening)
// . \\
timing step
\\	—timing mark
'—STRAIGHT EDGE
Figure 74.—Timing contact points of pivotless type breaker assembly.
the trailing finger of one magneto distributor rotor. The breaker contact points of the booster coil are held together by spring tension. When battery voltage is applied to the coil, magnetism develops in the core until the magnetic force on the soft iron armature mounted on the vibrator exceeds and overcomes the spring tension, pulling the armature toward the core. This opens the contact points and the primary circuit, resulting in demagnetization of the core and reclosing of the circuit. The armature vibrates, rapidly breaking and making the primary circuit as long as the battery e.m.f. is applied to the booster coil. The booster coil generates high voltage by the same process as the ignition coil, described in detail in paragraph 54.
131
TM 1-406
83
ARMY AIR FORCES
83. Battery ignition.—In the type of electrical ignition referred to as battery ignition, an ignition coil energized by a battery or generator is employed as the source of high voltage. The other elements in the system are identical with those of the magneto ignition system except for the omission of a booster which is not required since the system does not depend upon rotation of the engine for its supply of energy. A representative battery ignition system is shown in figure 76. The ignition coil does not possess a mechanical vibrator. A cam, with several flat surfaces, is geared to the engine shaft. A cam follower opens a pair of contact points (and breaks the primary circuit) each
BOOSTER COIL—z
) BATTERY B X
P	E
VIBRATOR\|	n
CONTACT X _
POINTS Xk-I
TO DISTRIBUTOR ROTOR
HV
X—SOFT IRON ARMATURE
Figure 75.—Diagram of ignition booster coil.
time a spark is required. Either manual or automatic control of the breaker mechanism may be provided for advancing or retarding the spark. The process by which high voltage is induced in the secondary of the ignition coil each time the points are separated is described in paragraph 54. Since the e.m.f. which is applied to the primary by the battery always has the same direction, the polarity of the high voltage impulses is always the same. This is in marked contrast to the output of a magneto, the high voltage impulses of which alternate in direction. However, the heat of the spark created in the gap between the spark plug electrodes to ignite the fuel is independent of the polarity of the high voltage.
132
TM 1-406
84
AIRCRAFT ELECTRICAL SYSTEMS
IGNITION COIL—7	/q	q\ ______________________________
--------------1	|-------------------------|	DISTRIBUTOR S	\	/
) C-S	\4> |	5/—*
K	—1--------------*
)	K
<2X K	I
)	jo	MECHANICAL	] CYLINDER
	LINKAGE -L	<
1	|X PISTON	<1
p <=><	§ s	O	—
1°
—	'-------t"y-------------•-----------<0
-- xX~BATTERY	CONDENSER
-	BREAKER CONTACT POINTS
“	■■	M	■■	■■
Figube 76. -Diagram of battery ignition system.
84. Ignition switches.—a. General.—-Control of the ignition units, separately and in all necessary combinations, is provided at one point in the cockpit by the ignition switch. The ignition control switch for a magneto functions oppositely from a switch which
133
TM 1-406
84
ARMY AIR FORCES
(3) INTERNAL SWITCH POSITIONS (REAR VIEW)
Figure 77.— Single-engine ignition magneto switch.
134
R mag	safety bu
storage battery	(booster
Z/o^	/m~0 A
/	\\ Zn	aOA—1
1/ // [ |
\\	X Yx / /	\ GRO. Z~X /
\\ XLv	\ o	—।
NX	\	L.MAG/
If	L. MAG.
GRD.
® PRONT	(2) REAR
/ C\
I / BATT. /	\ B \ I	| /batt.	B* \ |
l///
A	4- L—
RIGHT MAG OFF LEFT MAG ON	RIGHT MAG. ON LEFT MAG. OFF
BATTERY CIRCUIT OPEN	BATTERY CIRCUIT OPEN
©/	"7t\
YT
BOTH MAGNETOS ON	BOTH MAGNETOS OFF
BATTERY CIRCUIT CLOSED	BATTERY CIRCUIT OPEN
1
TM 1-406
84
AIRCRAFT ELECTRICAL SYSTEMS
controls a battery-operated ignition unit. When a battery ignition unit is inoperative, the control switch is open. When the control switch is closed, the circuit is completed and the ignition unit is operative. The control switch for a magneto is connected in parallel with the breaker points. In the “off” position, the switch is closed thereby short-circuiting the breaker points. Thus the magneto is inoperative because no interruptions of primary current occur even though the breaker points are successively opened and closed. When the control switch is in an “on” position the switch is open-, the magneto is then operative because the primary current is interrupted by the action of the breaker points.
//f Tn
/ / \\ OFFt J	11 \\
A/
TO STORAGE
TO SAFETY BUS	BATTERY
(booster)
|	/Z®X\
/vaux. BAT\\
//9	S\\
R. MAG - // RO °x' Or \\ R MAG
II R	L U
L. MAG —Ur-0 0	0 0--------- L MAG
\\ L BO. BO L 7/
©REAR
®front
Figure 78.—Twin-engine magneto ignition switch.
b. A switch used in connection with a single-engine magneto ignition system has three “on” positions, marked “left,” “right,” and “both” (fig. 77®, indicating the control status of the two magnetos. Although indication is not made on the face of the switch, it also participates in the control of the booster and starter solenoid circuits. Battery voltage for energizing these units must pass through the ignition switch, and until the switch arm is placed in the “both” position, battery voltage is not supplied by it to the booster or starter solenoid switches. The ignition switch therefore acts as a safety device. When the switch is in any position other than the “both” position, thoughtless closing of either the booster or starter switch can do no harm; and when a crash landing is anticipated, turning off the magnetos also reduces the possibility of fire of electrical origin. The switch point connections are shown in figure 77®. The rotating member of the switch may be regarded as an electrically
135
TM 1-406
84-85	ARMY AIR FORCES
conductive triangle; when any one of its points does not rest upon one of the nine switch points, it rests upon insulation. The terminal connections at the rear of the switch are shown in figure 77®.
c. The twin-engine magneto ignition switch (fig. 78®) provides independent control of each magneto on each engine. In addition, it includes an emergency safety switch which, in the “off” position, grounds all magneto primaries and opens the battery circuit leading to the booster and starter switches. The switch must be placed in the “on” position before the engines can be started. The terminal connections at the rear of the switch are shown in ®.
85. Ignition system wiring.—The wires which interconnect the various ignition units are divided into two classes: high tension and low tension. High tension wire or cable is used to connect the
FLEXIBLE METAL ---
SHIELDING	SPRING CONTACT----\ \
ATTACHMENT FOR \ \ \ SHIELDED SPARK PLUG X/
Figure 79.—Flexible ignition conduit with couplings.
sources of high voltage to the spark plugs. High tension cable consists of strands of tinned copper covered with flexible insulating material, over which cotton yarn is closely braided and coated with a protective substance which is resistant to water, oil, and gasoline. The electrical properties and flexibility of high tension cable are not affected appreciably by extremes of temperature encountered in normal service. When the cable is pulled into the metallic harness, special care must be exercised not to scratch or injure the insulation. Once the insulating surface has been broken, progressive deterioration under normal conditions of use sets in. Insulation failure and consequent loss of ignition, due to previous mishandling of the high tension cable, may occur without warning during flight. Low tension cable is a thicker conductor consisting of tinned copper strands, with relatively thin insulation. High and low tension wires are separately shielded with rigid or flexible metallic conduit, equipped with suitable couplings at either end (fig. 79). High and low tension wires are never included in the same conduit.
136
TM 1-406
86
AIRCRAFT ELECTRICAL SYSTEMS
86.	Spark plugs.—11 4
SHORT	LONG >	K )	S
-REACH	REACH—->	U £	>
BASE	BASF - <	£ '	<
ELECTRODES	>	X /	5
-------------- OUTER -------\ Z	< --------------------------------------------- CENTER -< f”l /d>
©MICA INSULATION	©CERAMIC INSULATION
SHORT REACH	LONG REACH
Figure 80.—Typical shielded aircraft spark plugs.
to the grounded shielding at that point. The ceramic insulation does not absorb moisture, and does not leak electrically; it either provides virtually perfect insulation, or breaks down completely. The shell and barrel of the ceramic plug are assembled in one piece, and any attempt to disassemble the spark plug will render it unfit for use.
(3) As implied in the preceding paragraphs, the thermal conductivity of the spark plug is an important factor. A hot operating plug is employed in low horsepower engines, which do not operate at high
138
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	86-87
temperatures. The center electrode of the hot operating plug has a long nose, so that a considerable surface area of electrode insulation is exposed to the gases of combustion, and therefore remains at the cylinder temperatures. This feature tends to reduce oil fouling at low operating speeds of the engine. The center electrode of a cold operating plug, which is especially desirable for use in engines which operate at high temperatures, has a short nose. The heat absorbing area is thus reduced, and the thermal conducting path made shorter. As a result, pre-ignition, caused by too hot an electrode tip, is avoided.
(4) Non-shielded mica-insulated plugs are used in aircraft not equipped with radio apparatus.
c. Gap clearance.—Burning of the electrodes takes place in every spark plug and results in progressive widening of the gap. The wider the gap, the higher the voltage required to jump the gap and produce a spark, so that resetting of the gap may become necessary. Specifications with respect to gap clearance are covered in the appropriate Technical Order.
87.	Maintenance.—a. Magnetos.—(1) Check the breaker assembly by removing the breaker cover and inspecting for general cleanliness, damaged or worn cam follower, and proper felt lubrication. If major defects are found, replace the breaker assembly. In the pivotless type breaker, cam follower wear is indicated by a small depression where it lifts against the end of the main spring. The clearance between the cam follower and the main spring is checked with the breaker assembly removed. Felt lubrication is satisfactory if oil appears on the surface when the felt is squeezed with the fingers. If the felt is dry and requires lubrication, do not apply too much oil inasmuch as the excess oil may be thrown off during operation onto the contact points, causing them to burn and pit.
(2)	Check the main breaker spring for proper tension with an appropriate spring tension gage.
(3)	With the breaker assembly installed, check for worn or loose cams and cam bearings by turning the engine crankshaft and noting the opening of the contact points for each lobe of the breaker cam.
(4)	When checking the condition of the contact points of the pivotless type breaker, do not raise the main breaker spring beyond one-sixteenth of an inch clearance between the contacts. A further separation may weaken the spring, and result in faulty operation. If contact points are burned or pitted they must be replaced.
(5)	Check the internal timing of the breaker contact points. With the pivotless type, place a strip of clean shim stock of specified thickness between the points. As the crankshaft is turned, the shim
139
TM 1-406
87	ARMY AIR FORCES
stock should be released when the straightedge (par. 81) is lined up with the timing marks. Permissible service tolerances (maximum distances which may be permitted between the straightedge and the timing mark on the rim of the breaker housing) are given in the Technical Order for the particular type of magneto. If the adjustment is not satisfactory, the fastening screws A (fig. 74) are loosened, the eccentric pin B rotated until the proper adjustment is obtained, and then screws A retightened. The adjustment is rechecked. When checking the contact point clearance on the lever type breaker, rotate the engine crankshaft until the lever rests upon the peak of any lobe of the cam and adjust the contact point clearance in accordance with specifications for the magneto.
(6)	Check the distributor head and distributor finger for cracks and results of electrode arcing. Check the distributor finger for security of mounting. Check distributor block and rotor for cleanliness. Check for sticking or broken brush.
(7)	Magneto bull bearings and gears do not require lubrication between overhaul periods.
(8)	Check security of magneto mounting.
(9)	The magnetos are individually checked while the engine is operating at cruising manifold pressure. Operate the engine on one of the magnetos for 15 seconds and observe the tachometer to see that the decrease in r. p. m. does not exceed the amount specified in the Technical Order. The switch should be turned back to the “both” position to allow the engine to operate normally before the other magneto is checked. Magnetos which permit an excessive loss of r. p. m. should be replaced.
(10)	Ungrounding the magneto primary lead renders the magneto operative. Therefore, during maintenance, when it becomes necessary to disconnect the primary lead from the ignition switch, care must be exercised to ground this lead.
b. Spark plugs.— (1) Check the terminal connection of unshielded spark plug for condition and security.
(2)	Check the barrel core nut* (of the mica-insulated plug) for tightness, and if found loose the plug is removed for tightening with special tools. After tightening, the gap clearance is checked. Before reinstalling, check the threads of the spark plug for evidence of damage, and apply specified lubricant to the threads of the base.
(3)	Check the shielded spark plug elbow terminal and shielding nuts for condition and security. A snug but not too tight a fit is desirable.
(4)	Check shielded type spark plug terminal for mechanical or insulation failure and for accumulation of moisture.
140
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	87
(5)	When spark plugs are removed from an engine, they should be set out in a row for comparison, care being exercised to identify each plug with the cylinder from which taken.
(a	) If the plug is clean and the metal parts show signs of overheating, the cylinder has been running hot, indicating pre-ignition, detonation, or inadequate cooling.
(6	) If the plug is generally clean but is wet with gasoline or covered with a film of fresh oil, no conclusion should be drawn on this basis alone, since the appearance of the plug may be due to conditions which were present at the moment of stopping only.
( '	• s-v	........... ..
TORQUE OVERLOAD MOTOR DRIVING JAWy
^RELEASE CLUTCH	/
\	? f-elywheel /
starter\	.../
DRIVING \>	vM0T0R
vjaw Ji
> fe’S-iwfe IWOfiWSr * r
\ i gf iEW
Shew	;	°
SOCKET-	‘ '< |l *’ ?”*?<
FOP DR Ax* %*«*'*'	•
EXTENSION I •	■
I	-J
Figure 84.—Cutaway view of hand and electric inertia starter type C—21.
the slipping torque. The normal operation of the clutch is to slip for an instant after the starter and engine jaws become meshed, but while slipping, a torque is exerted on the crankshaft, until the initial resistance of the engine is overcome and the clutch is able to hold again. The specified values of the break-away (maximum holding) and slipping torque of the clutch are in accordance with the size of the engine to be cranked.
91. Combination hand and electric inertia starter.—a. The combination types of inertia starter employ a gear and clutch arrangement similar to that of the hand inertia starter. The starter shown in figure 84 is the hand-cranked type described in the preceding paragraph, with an electric motor attached. The flywheel may be ac-
449399°—42---10
145
TM 1-406
91
ARMY AIR FORCES
B	HELICALLY SPLINED
/—MOTOR SHAFT
//______
--MOTOR JAW
Figure 85.—Jaw-type starter motor engaging mechanism.
J	CRANK SOCKET
I.	TORQUE
t-J.L	OVERLOAD
mf % .	x VZ RELE A SE
MOTOR —.	T C H
lJBHv	:>■ taw
1 IMt a Mr	driving
JAW
W ; aj^^' flywheel
MESHING SOLENOID^jmiH'	|
Figube 86.—Cutaway view of hand and electric inertia starter type H-2.
146
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91
AIRCRAFT ELECTRICAL SYSTEMS
---STARTER DRIVING JAW	\	--
.XI®)//zx\
| [ o । ] । /©AXf	T)
\	jjy	\\\	///Z/	/ / X.____\J
i»—	\\\//FW/	\ .
\ 3S	\	\	//	MOTOR \
/FF S/	ARMATURE —1
p h FLYWEELX	/	—
TORQUE OVERLOAD \	-	ROLLErX^
RELEASE CLUTCH »	AY
0
Figure 87.—Diagram of inertia starter with roller-clutch motor engagement.
celerated by either a crank or the electric motor. When the starter is energized by cranking, the motor is mechanically disconnected and is inoperative. When the motor is employed, it is engaged directly to the inertia starter flywheel by means of a movable jaw on the helically splined shaft (fig. 85) of the motor. When the motor armature starts
147
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91
ARMY AIR FORCES
to rotate, the jaw will tend to remain at rest, resulting in its forward movement along the helix and engagement with the flywheel. A fault with this starter is that the jaw sometimes binds on the shaft so that it does not engage with the flywheel, and as a result the motor races. If the engine fails to start when cranked, it is necessary to wait until the flywheel of the starter comes to a complete stop before the electric motor may be re-energized, otherwise the teeth on the flywheel or on the motor jaw may be stripped.
MOTORr- FLYWHEEL
•	r- MESHING SOLENOID
TORQUE overload
R	retease clutch
f / / Jr dKBL
SOCKET FOR
CRANK EXTENSION^	STARTER
■ ”	'	■ D R1 v1N c J Aw
MOUNTING ' FLANGE^^^^C^
L	’	- S	....., 1
Figure 88.—Cutaway view of hand and electric inertia starter type JH-5.
b. In another type of inertia starter, shown in figure 86. the starter motor is engaged to and disengaged from the flywheel by means of a roller clutch, shown in figure 87, which fits snugly inside the flywheel. A slight rotation of the motor shaft forces the rollers out against the inner surface of the flywheel. Whenever the motor rotates more slowly than the flywheel, the rollers become free and the motor shaft is automatically disengaged. There is no need to wait for the flywheel to come to a stop before re-energizing the electric motor.
c. In still another type of inertia starter (fig. 88), the flywheel is mounted on the motor shaft. Thus the inertia of the armature is added to that of the flywheel. The brushes of the electric motor are lifted from the commutator by solenoid action (fig. 89). In
148
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91-83
AIRCRAFT ELECTRICAL SYSTEMS
some models, the motor is turned off (by raising the brushes) when the starter jaw is engaged with the engine; while in others, the motor is permitted to remain running and thus, by direct cranking action, the engine is rotated through several additional revolutions before the energy of the starter is exhausted.
—bm(~MEH,fd/~~~Wxaas8
—SOLENOID	-	Ini	SOLENOIC
rtzzi w fpi	o] M roi
/ I £§3 I \	/ I XM \
/	\	/ y/oXo-} \
if	a)
11 tJ// / -BRUSH \ \\\V .	I! ill.// / -BRUSH- \
iw /	\ w !iCZ>z /	\ vXh
•! (of i	i Uy:! !l ©T i	i
JJ	COMMUTATOR	J.	Ai	I COMMUTATOR I ----By
J	U	U	a
Q BRUSHES RAISED	(g) BRUSHES DOWN
Figure 89.—Solenoid-controlled motor brushes.
92. Inertia starter electric motor.—Series wound 12- or 24-volt d-c motors are employed in electric inertia starters. The constructional features of the motors are similar to those of the d-c generator (par. 66). The electrical resistance of the motor employed in an inertia starter is very small, so that when the starter switch is closed the initial value of the motor current is extremely large, resulting in a powerful starting torque. As the motor gains in speed, induced counterelectromotive force (par. 52) causes less current flow. An inertia starter motor which draws approximately 350 amperes at starting will draw approximately 75 amperes at high speed, the values depending upon the load. If the load is entirely removed, and if the internal friction of the motor is small, it will “race,” and the armature may “burst” from centrifugal stresses. For this reason it is not safe to test an inertia starter motor at full voltage unless there is a load upon it.
93. Inertia starter switches and engaging solenoids.—a. When the starter is located some distance from the control compartment, the inertia starter motor is usually operated by means of a solenoid-actuated switch controlled from the cockpit. The solenoid
149
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93
ARMY AIR FORCES
switch is usually mounted on the starter flange. Figure 90 illustrates the operation of a solenoid switch. When the cockpit control switch is closed, a current of low amperage energizes the solenoid, causing the solenoid plunger to advance, compressing a spring (par. 29). This action closes the solenoid switch contacts and completes the battery circuit through the starter motor. When the required flywheel speed of the starter is attained, the cockpit control switch is opened, the solenoid coil loses its magnetism, the spring returns the solenoid plunger to its original position, and the starter motor circuit is thus opened.
SOLID LINES-NO CURRENT IN COIL	Pl
SWITCH OPEN
DOTTED LINES-CURRENT in COIL	p f
SWITCH CLOSED	II I | ! j |\\\
/ I /	—SWITCH
/ II	CONTACTS
I	0
x. I / /	o
0 )
O	__
(f3 s'
Figure 90.	—Diagram of solenoid switch.
b.	When electrical control of the engagement of the inertia starter is desired, a meshing solenoid is employed. The mechanism of the meshing solenoid is similar to that of the solenoid switch, except that the plunger of the meshing solenoid is connected to the starter jaw engaging lever. Meshing solenoids are shown in figures 86 and 88.
c.	A diagram of a typical inertia starter electrical system is shown in figure 91. When the ignition switch is placed in the “both on” position, battery voltage reaches the cockpit control switch. If the latter is placed in the “energize” position, the solenoid switch is there
150
TM 1-406
93-94
AIRCRAFT ELECTRICAL SYSTEMS
by closed, and the inertia starter begins to speed up. When sufficient speed has been attained, the cockpit control switch is placed in the “engage” position. This actuates the meshing solenoid, and at the same time energizes a booster coil, which supplies high voltage to the trailing distributor finger of the right-hand magneto until the cockpit control switch is released.
[	) STARTER
r~r!____Lr||i’	X-L/
SOLENOID SWITCH
I MESHING
COCKPIT	| SOLENOID
i—CONTROL	Lt__f.
ENGAGE / SWITCH_______________ —1—
i	i	"	-
x_	*	t
---■■	ENERGIZE :
t[J* #|T0 RIGHT MAG
“	IGNITION	BOOSTER
SWITCH
Figure 91.	—Diagram of typical inertia starter electrical system.
94.	Direct cranking hand and electric starter.—a. An electric motor may be used to crank the engine directly, as in the automobile. This type of starter is not widely used on military aircraft engines because it requires a comparatively heavy current to develop sufficient torque. The type shown in figure 92 is a combination unit for hand or electrical operation; when energized electrically, the starter provides instantaneous and continuous cranking. The starter consists basically of an electric motor, ' reduction gears, and an automatic engaging and disengaging mechanism which operates through an adjustable torque overload release clutch. The engine is cranked directly by the starter; there is no preliminary storing of energy, as in the case of the inertia type starter.
b. The torque of the motor is transmitted through reduction gears to the adjustable torque overload release clutch. This, in turn, actuates a helically splined shaft which moves the starter jaw axially outward to engage with the engine cranking jaw before the starter jaw begins to rotate. Complete engagement is thus accomplished before cranking begins. When the engine starts, the starter automatically disengages.
151
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ARMY AIR FORCES
TORQUE OVERLOAD r~ RELEASE CLUTCH
/  I -------------- ------------- -------=	/	MOMENTARY	\
:._ —-X* . I ' '■ j-U-	ZZ=Z=ZZZC	-- 1—0	=1	y	F	CONTACT
X 1 X I =	.---	■■ s«l	\  5*lTCM	— GAOUtOED
""x; €feTO)
X'	•Wottc*0’* s*rtTv
\ Xx	-------®-------------11111111,111-i
FLEXIBLE SHIELDED-'	T “ ” f 7 IM	▼
CONDUIT	—___—_
WIRING DIAGRAM |	___________________
Figure 96.—Sectional diagram of cartridge-type starter.
opened at the end of the forward stroke, releasing the gas into an exhaust tube. A helical spring (located behind the piston) is compressed during the forward stroke, and returns the piston to its normal position, expelling the gas through the exhaust tube and
156
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	97-99
disengaging the starter jaw from the engine. At the completion of the return stroke, the piston automatically closes the exhaust valve. The starter is protected by a safety disk located in the fuel combustion chamber. In the event that the pressure in the starter exceeds the normal working pressure, the safety disk is ruptured and the gas is expelled into the exhaust tube. New safety disks may be readily installed.
c. After operation of the cartridge starter, care should be exercised to turn the breech lever upward through 45°, holding it in this position for several moments in order to release residual pressure in the system, before opening the breech.
d. In the event cartridges are not available or malfunctioning of the cartridge starter occurs, an emergency stretchable starting cord (bungee) and boot may be employed to start the engine. The boot is placed over the tip of the propeller blade so that the cord is tangent to the hub.
(1) The cord should just raise off the hub when it is stretched to within a distance of 20 feet from the unstretched position. If the blade is initially in a position which is too high the cord will not stretch sufficiently and the propeller will be turned too slowly. If the initial position of the blade is too low, the cord will not raise above center and the propeller will not turn.
(2) The cord may be stretched by means of a vehicle or by personnel. When it is necessary to use personnel, an additional length of rope should be tied (in the form of a V) to the rope provided with the emergency cord, so as to place such personnel out of the path of the boot when it is released.
98. Air injection type starter.—This type of starter system consists of a compact engine-driven air compressor, an engine-timed rotating distributor valve built integrally with the compressor, a storage tank, an automatic pressure-regulating valve, a pressure gage, and a fuel primer. Compressed air is stored at a pressure of approximately 450 pounds per square inch. The release of this compressed air is controlled from the cockpit and transmitted by means of the distributor valve to the cylinders in proper firing order. The air pressure dissipated in starting is replenished by the air compressor during engine operation. Provisions are made to introduce a priming charge of fuel into the distributor valve as the starter is being operated.
99. Maintenance.—a. General.—Starting equipment should be checked for security of mounting, tightness of bolts, and proper safetying. Check for breaks or cracks in housings and flanges. Equipment should be lubricated as required.
157
TM 1-406
99	ARMY AIR FORCES
b.	Inertia, starter.—(1) When installing a starter on the engine, remove the cover which is placed over the starter jaw for shipment or storage, and observe whether the engine jaw and starter jaw are of the same type and have correct direction of rotation.
(2)	To check the operation of an inertia starter, close the control switch to the “start” position for the time required to bring the flywheel up to speed, and then move the switch to the “mesh” position. When the engine starts, release the switch, which should return to its neutral position. If the engine fails to start, and the starter jaw remains in mesh with the engine jaw, it will be necessary to turn the propeller (with magnetos off) by hand a part of a revolution in the direction of rotation to release the starter jaw before operating the starter again.
(3)	To check the free turning of gears, the flywheel is energized and, without meshing to the engine, is allowed to “run down.” The flywheel should coast a minimum of 4 minutes.
(4)	Electric circuits should be examined for loose terminals.
(5)	If operating troubles are experienced with either the starting or meshing solenoids, use proper test instruments to determine the cause and, if necessary, replace faulty units.
c.	Motors.—Maintenance of a motor is concerned mainly with brushes and commutator.
(1)	Remove the motor brush strap and check for worn or binding brushes. Short brushes should be replaced. New brushes are seated with a fine grade of sandpaper (No. 000) inserted between the brush and commutator, and pulled in the direction of rotation. Test brush springs for tension.
(2)	Dirty commutators (and brushes) are cleaned with a lint-free cloth moistened with unleaded gasoline.
(3)	Commutators are smoothed in accordance with the Technical Order for the particular starter. If the commutator is badly scored, replace the motor (or the entire starter, if the motor is not detachable).
(4)	Motor jaws should be checked to see that they do not bind on the shaft.
(5)	If the motor is to be tested with no load, use approximately half the voltage normally required.
(6)	If the motor should fail to operate, check the electrical circuits before replacement of motor (or starter).
d.	Cartridge starter.—(1) Check the operation of a cartridge starter by attempting to start the engine. If the cartridge fails to fire, try the contact switch several times, but wait 5 minutes before
158
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	99-100
removing the unfired cartridge. Insert another cartridge, and if this fails to fire, check the electrical system.
(2)	Whenever a loud report and only a slight movement of the propeller are noted, it is probable that the safety disk has ruptured. If found ruptured, the disk should be replaced.’
(3)	The breech pin is an electrical contact and must be kept clean.
(4)	When necessary, disassemble the breech and clean with a cotton swab moistened with penetrating oil. Lubricate before reassembling.
(5)	When the combustion chamber is removed for cleaning, any residue should be cleaned with water, gasoline, or kerosene. Check the exhaust valve for setting and security. Inspect the safety disk.
(6)	Check the supply of cartridges.
(7)	Emergency starter cords (carried in the aircraft) should be inspected for condition and age.
Section XI
LIGHTING, LANDING GEAR, WARNING, AND RETRACTING SYSTEMS
Paragraph
Reversible motors-------------------------------------------------------100
Electrical retraction of landing gear_________________________________  101
Landing gear warning signal systems-------------------------------------102
External lighting system------------------------------------------------103
Instrument and cockpit lighting-----------------------------------------104
Maintenance------------------------------------------------------------ 105
100. Reversible motors.—Electric motors in which the armature may be caused to rotate in either direction may be used for retracting purposes or for positioning of flight control surfaces. Such motors are always series wound. Provision for reversal may be made in either of two ways:
a.	Reversal of armature or field.—If the direction of current flow through either the armature or field windings (but not both (par. 33/)) is reversed, the direction of rotation of the motor shaft is reversed. A double-pole double-throw switch may be used for this purpose. With the switch arrangement shown in figure 97© the current in field windings may be reversed.
b.	Use of split field.—Each field winding may be center-tapped, and the center of each coil connected to one brush (or set of brushes) as shown in figure 97@. The other brush is (or brushes are) attached to the frame of the machine and thereby grounded. The positive battery or generator potential is applied to either extremity
159
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ARMY AIR FORCES
of the field windings; therefore, only half of each winding is used at one time. When remote control is desired, appreciable weight in copper is saved by running one heavy positive wire to the motor and using solenoid switches, as indicated in the figure.
TO BATTERY +
I—ARMATURE
F,ELD "T-"A	j
D.P. D.T. SWITCH^
Q REVERSAL OF FIELD WITH D.P D.T. SWITCH
(2) USE OF SPLIT FIELD MOTOR
Figure 97. Methods of motor reversal.
101. Electrical retraction of landing gear.—One type of landing gear retracting motor is shown in figure 98. A train of gears is employed to reduce the speed and increase the torque. A torque overload release clutch is built into the device as a safety factor. Limiting switches, mechanically operated by the landing gear, open the circuit to the solenoid switches and thus prevent overtravel of the
160
TO BATTERY * x
^1	7---------------
SWITCH	/	\
cockpit	f	\	/
SWITCH-^	/	/
"	[p
AIRCRAFT ELECTRICAL SYSTEMS
TM 1-406
101-102
landing gear or damage to the motor. A magnetic engaging clutch may also be incorporated in the system, to engage the motor with the retracting mechanism only when the motor is energized. In the event of failure of the electrical system, the pilot may operate the retracting mechanism manually without the dead load of the gear train and motor.
Figure 98.—Cutaway view of reversible retracting motor.
102. Landing gear warning signal systems.—Landing gear warning signals are employed to warn the pilot, preparatory to landing, to lower the landing gear, and to notify him whether or not the landing gear is down and locked.
a. A diagram of a warning signal system is shown in figure 99. A switch is associated with each landing wheel; when the wheel is not down and locked, the switch is closed. The spring N in the relay normally holds contact points Cr closed. When the throttle is closed beyond a predetermined point, the throttle lever closes a switch mounted on the throttle control. If either wheel is not down and locked, the circuit to the horn is thus completed and the horn will sound. In the event that the pilot has no intention of landing, he momentarily closes the horn release relay switch, which energizes the relay, opens contacts silences the horn, and closes contacts 6L. The horn release relay switch is thus shunted, the relay remains energized,
449399 --4:
■11
16X
TORQUE OVERLOAD
RELEASE CLUTCH-
MAGNETIC ENGAGING CLUTCH---x
TM 1-406
102
ARMY ATR FORCES
and the horn is inoperative when the momentary contact switch is released. When the pilot reopens the throttle, the throttle switch is opened, and the relay is de-energized, permitting contacts Cr to reclose; when he again closes the throttle, the horn will again sound.
LEFT WHEEL	M
HOT DOWN AND LOCKED DOWN AND LOCKED	J
—.	HORN RELEASE^-
__________ .S	RELAY || S	HORN
THROTTLE	।	।	=7.
SWITCH	------ ■ ■ 2
<_________"
1 (_____________________________(
NOT DOWN AND LOCKED DOWN AND LOCKED i||ll—	'*
---	RIGHT WHEEL	H
HORN RELEASE RELAY SWITCH
Figure 99.—Landing gear warning horn system.
n n
r O rZJ TEST /	THROTTLE
/*S\	SWITCH /	SWITCHES
2	x
RED
I kJ	T	NOT
, I	DOWN	NOT
La/J	AND	°°WN
-	—	LOCKEDp	AND
»	DIMMER	LOCKED,
~^7	RESISTOR	LEFT WHEEL	V
*	ZZ7	SWITCH	[WHEEL
Y	H	L '	^^1
3=3	GREEN	DOWN	•
AND	DOWN
’ J	LOCKED	AND "==“
3V	LOCKED
Figure 100.—Horn and signal light landing gear warning system.
b. The signal system shown in figure 100 illustrates a combination horn and signal light landing gear warning system. The wheel switches are of a double-throw type and have no neutral position, consequently they are always in position, either up or down. If
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102-103
AIRCRAFT ELECTRICAL SYSTEMS
either wheel is not down and locked, the red lamps are lighted; in addition, the warning horn will operate when the throttle is closed beyond a predetermined point. (A horn release relay may be added to the system.) The green signal lamps are lighted only when both wheels are down and locked. The signal lamps may be dimmed, for night flying, by means of dimmer resistors. A test switch is incorporated to determine if the horn is operative.
103. External lighting system.—a. General.—The type of circuit and the number of external lighting units employed on aircraft depend upon the type, size, and military purpose of the aircraft. In addition to a set of standard navigation lights, the equipment may include landing and formation lights and a passing light. Figure 101 illustrates a typical external lighting system.
b. Navigation lights.—A set of navigation lamps is the minimum lighting equipment for aircraft operating at night, and consists of 1 red, 1 green, and 1 white unit. Each unit consists of two lamps streamlined into the aircraft surface to which they are attached. The green lamps are mounted at the extreme tip of the right wing,'and the red lamps are similarly located on the left wing. The white lamps are usually located on the vertical stabilizer in such position that they may be seen through a wide angle when viewed from the rear. The lamp units are connected in parallel and are controlled by a single switch in the pilot’s compartment. This switch has two “on” positions; it will remain in one “on” position, but must be held, against spring tension, in the other “on” position, which is used for signalling purposes.
c. Landing lights.—(1) Landing lights are extremely powerful and are directed at an angle so as to illuminate an area sufficiently large for landing in unlighted airports and fields. The lights may be located midway in the leading edge of each wing, streamlined into the airplane surface (fig. 94). The landing lights draw a high current. A relay, remotely controlled from the cockpit, may be used to control each lamp. Both the control circuits and the lamp circuits are fused for safety.
(2) In some installations, retractable landing lamps, set into the under surfaces of the wings, are employed. Icing of the lamp lenses is thus eliminated to a great extent. In one type of installation (fig. 102), these lamps may be lowered, retracted, or stopped in any intermediate position. The landing light motor has a split field winding. The center terminal of the field is connected to one of the brushes, the other brush being grounded through the coil of a solenoid which lifts a brake shoe which normally rests by spring tension against the
163
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103
ARMY AIR FORCES
motor shaft. The other two field terminals are connected to the outer terminals of the control switch aS through contact points C and I). When the landing light is retracted, points C are pushed open by the geared quadrant and points D are closed by spring tension. If switch aS is now placed in the “lower” position, the battery current
QQ -.______________________Q
9 ______Il	T
______/©y—l’l|l|l|l|lF—*
4-	" ———	, ZZ©
• I—fo <4—1	8
os	L-.......................... Xjr
rO^" __
--------------------------F—	 3	3-3-3
1.	Landing lights.	8. Rheostat, formation lights.
2.	Passing light.	9. Bus bar.
3.	Formation lights.	10. Switch, right landing light.
4.	Navigation lights, right.
5.	Navigation liglits, left.
6.	Navigation lights, rear.
7.	Battery.
11.	Switch, left landing light.
12.	Switch; passing light.
13.	Switch, navigation lights.
14.	Switch, formation lights.
Figure 101.—Wiring diagram of typical external lighting system.
which passes through the completed motor circuit energizes the brake solenoid, withdrawing the brake shoe so that the motor may turn, and the lamp will start to lower. After approximately 10° of movement, contact A touches and rides along copper bar B, lighting the lamp. When fully lowered, the projection on the end of the gear quadrant opens contacts stopping the motor and reengaging the brake. If
164
TM 1-406
103-104
AIRCRAFT ELECTRICAL SYSTEMS
the control switch is now placed in the “retract’’ position, the motor will operate in the other direction.
d. Formation lights.—Formation or position lights are used on certain military aircraft for night formation flying. This set of lights consists of 7 blue lamps, 3 of which are installed in a straight line running fore and aft along the upper surface of the fuselage; the remaining 4 are placed on the upper surface of the horizontal stabilizer (2 on each side of the vertical stabilizer). The formation lights are not visible from the ground, yet they provide illumination for safe group maneuvers with all other lights turned off. The current passing
]	"	RETRACT I
ir--------------------------------------—I I X—
_______________I LOWER J
Xm 'I' }
11 \\	//	z IT
I ' ‘	/	MAGNETIC BRAKE I L-J
; //	J®
I // / /	BRAKE SOLENOID
iTsid	w
Figure 102.—Diagram of retractable landing light.
through the formation lights is controlled by a rheostat so that the pilot may increase or decrease their intensity.
e. Passing light.—A passing light may be included in the lighting system, for use on or near civil airways at night. The lamp (generally controlled by a simple toggle switch) is provided with a red lens and is located in the leading edge of the left wing. It is used only as a precaution against collision when meeting other aircraft.
104. Instrument and cockpit lighting.—a. Indirect lighting of the instrument panel may be provided by lamps set in the panel, the light from these lamps being distributed over the entire instrument panel by a reflector panel which has openings for observing the various instruments.
Z>. Individual instrument lighting is provided by means of small 3-volt bulbs self-contained within the instrument case. The potential
165
TM 1-406
104
ARMY AIR FORCES
AUTOMATIC SWITCH
I	J E	FLUORESCENT LAMP
R’ i B> J-c 3 t <	<—’	I c»	"°s	/*=*	-===■
>	__>	DC	~
_	------■■	D C
•-----»■ rrai M	____________P p C S,
i*Y O’	AUTOMATIC SWITCH
= — cz	d	C	_____________________
k -=■	)	(
T	FLUORESCENT LAMP
°3	> S
।—ih“	r
k	------T _________DD________________
3v. INSTRUMENT LAMP
—Illlllllllll—1 Figukh 103.—Inverter system for fluorescent and instrument lights.
of the generator-battery system may he used to light these bulbs by inserting resistors in series with the lamps. The 3-volt e. m. f. necessary for the instrument bulbs may also be obtained from a winding on the inverter or the auxiliary box described in the following paragraph.
c. Flib&rescent lighting eliminates the glare encountered with use of other types of lighting. The lamp assembly consists of a shell which
166
TM 1-406
104
AIRCRAFT ELECTRICAL SYSTEMS
has a special lens which will pass only ultraviolet light, variable apertures for regulating the amount of light emitted, and an automatic starting switch (glow lamp). There is also an aperture for passing visible light. Instruments used with this type of lighting have the dial figures painted with a material which is sensitive to ultraviolet light. When the invisible ultraviolet light is directed on the instruments, the figures are outlined in a soft glow which enables them to be seen distinctly, although there is no visible light coming from the lamp. The voltage for the ultraviolet lamp may be obtained from either an inverter or a so-called auxiliary box.
AUTOMATIC SWITCH r.............................•}	-----a-—
I rrmiw,t !J £
Z!	FLUORESCENT LAMP I
CTOR-y	*	V
/	!	AUTOMATIC SWITCH
/	:	—-a---------
,x.t !  »	_____
\W/A.G SOURCE •	D C	!	k/
!	~	«	!	3* INSTRUMENT LAMP
3 fs	i
1 IL--................-----............J
Figure 104.—Auxiliary box system for fluorescent and instrument lights.
(1) The inverter changes the direct current of the generatorbattery system into 110-volt alternating e. m. f., with a frequency of approximately 400 cycles per second. The inverter is a special type of induction coil having a double-acting vibrator (fig. 103). The vibrating element sends pulses of current in successively alternate directions through the primary coil, producing alternating e. m. f. in the secondaries. One secondary operates the fluorescent lamps. Another secondary provides 3-volt a-c to operate the instrument lights. The resistors R^ R2 and condenser Ch are primary circuit Clements, which improve the operation of the inverter. The inverter is provided with a choke coil K and condensers C2 and <78 as a filter system; the input and output leads are shielded to eliminate radio interference.
167
TM 1-406
104-106	ARMY AIR FORCES
(2) When the aircraft is equipped with a centra] 110-volt 400-cycle a-c power supply, the fluorescent lamps may be operated by an auxiliary box. The auxiliary box (fig. 104) includes a reactor for each fluorescent lamp, and a step-down transformer for the 3-volt instrument bulbs.
105. Maintenance.—a. Check all visible wiring, including connections, terminals, fuses, and switches for condition and security.
5.	Keep lamp lenses and reflectors clean and highly polished. If a reflector is found to be cloudy, polish with a suitable compound such as a mixture of lampblack and alcohol. In cases of extreme cloudiness, reflectors may require relacquering, perhaps replating. Inasmuch as cloudy reflectors are usually due to an air leak around the lens, install a new gasket when the lamp is reassembled. Exercise care to insure proper focus and alinement.
c.	Malfunctioning of the lighting equipment may be located by systematically testing each lamp and lamp circuit with a continuity tester.
d.	Check operation of horn or vibrator by using the test switch. Check release relay for operation and security of mounting.
e.	(1) Check clutch of retracting motor for slipping torque; if clutch assembly does not function properly, it should be replaced.
(2) Other inspection requirements for a retracting motor are similar to those for the starter motor.
Section XII
WIRING SYSTEMS
Paragraph
General wiring features_____________________________________    106
Wiring diagrams______________________________________________   107
Circuit testing_________________________________________________108
106. General wiring features.—Various types of aircraft do not have identical electrical systems, but certain typical features are in general use. The interrelation of the more common features of a power distribution system are shown in figure 105.
a. From the battery and generator, feeder lines carry current to the main bus bar, which is considered the central point for the distribution of electrical energy to the various electrical devices. Both the battery and generator feeders are generally provided with master or main-line switches to enable all current flow to be cut off in one quick operation, as may be desirable in an emergency or when testing. These switches may be controlled remotely by means of relays.
168
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106
AIRCRAFT ELECTRICAL SYSTEMS
(1) The main bus bar is a copper bar located accessibly and enclosed within a metal junction box. Branch circuit fuses are usually attached to the screw terminals of the bus bar; from these fuses wires run to control switches in the cockpit, and then to various electrical units located throughout the aircraft.
AUXILIARY Q OR /I / SAFETY	----CZ o----- TO STARTER CONTROL
BUS BAR	crvz-------o----to	pitot heater
cZV^O-ty'''o----TO	FUEL PRESSURE
WARNING SIGNAL
TO	I
MAGNETO -*------HOG	\ SAFETY SWITCH
SYSTEM	\	,	O-/----
<-------X-b
0---TO RADIO SYSTEM
---ty^D----TO CABIN LIGHTS
MAIN cr^O---------ty'''0-—TO LANDING GEAR CONTROL
+	0---h-TO landing lights
BUS BAR
* O'V^O---oZo---► TO NAVIGATION LIGHTS
°'x-°—°---------------—IP1,
GENERATOR FEEDER^ c<~\^	PASSING LIGHT
BATTERY FEEDER ---------------------(yy
F° STARTER SOLENOID SWITCH
TWC POSITION A I I	__
MASTER SWITCH—V	—J 	A	II.
——X2 3 * * * * * * H(y—II11" ,1	|	STARTER MOTOR
"I—I — CONNECTOR PLUG
TO BATTERY CART
Figure 105.—Typical features of power distribution system.
(2) In some, installations, other bus bars, fed from the main bus
bar, are located in compartments occupied by the radio operator,
gunner, or other personnel, and serve as centers for the distribution
of electrical energy to a special group of devices.
b. When the battery master switch is closed, any device directly
connected to the main bus bar may be operated at any time by means
of its individual switch. Certain devices, on the other hand, are
connected to the main bus bar through a safety switch and auxiliary
169
TM 1-406
106	ARMY AIR FORCES
bus bar arrangement. The auxiliary bus bar is referred to frequently as the safety bus bar.
(1) Although the magnetos are entirely independent of the generator-battery system, the safety switch is often incorporated in the same assembly as the magneto switch. For example, in a single-engine safety ignition switch three of the terminals are for the magneto system, the remaining two (often designated “B” and “B. aux.” or “Bat.” and “Aux.”) serve to connect the main bus bar with the auxiliary bus bar. This connection is automatically established only when both magnetos are turned on.
(2) The Pitot tube heating element, the electric starter control, and other equipment which ordinarily should not be operated unless the aircraft is in flight or the engine is being cranked, are connected to the auxiliary bus bar. Thus, these devices cannot be inadvertently actuated by their individual control switches before the pilot has turned on the ignition preparatory to starting. Also, no damage or current waste can result because of failure of the pilot to turn off these devices at the end of a flight, provided he has shut off the ignition.
o. (1) The lead wire from the positive battery terminal proceeds first to one terminal of the motor starter switch. This is because the starter motor circuit draws the heaviest current and requires conductors of very low resistance. Therefore, the battery, starter motor, and starter switch are generally located as close to each other as practicable, to allow their heavy leads to be as short as possible. The line which connects the bus bar to the battery does not carry so heavy a current and may consequently be longer and thinner. It taps the positive battery wire where the latter is attached to the starter switch terminal.
(2) In some installations a two-position battery master switch is provided. In one position of the switch (as shown in the illustration), the starter and main bus bar are connected to the aircraft battery. In the other position of the switch, the starter and main bus bar are connected to a heavy connector plug accessibly located so as to permit connection to the batteries of a portable battery cart.
d. (1) Wires used in aircraft electrical systems carry an identifying number. This number may be prefixed with a code letter to identify the general circuit of which the wire is a part. When replacing a defective wire, no splices are permitted, and a complete new length of wire is installed, of a type, length, and size in accordance with specifications. Before the wire is installed, a band of surgical tape
170
TM 1-406
106-107
AIRCRAFT ELECTRICAL SYSTEMS
is wrapped around it near each of its terminals. The identifying number is written on the tape with india ink, and a protective coating of shellac applied. With this means of identification, tracing and testing of circuits is facilitated.
(2) A connector panel is a panel of insulating material provided with terminals which are thus insulated from qach other. Each terminal serves as a convenient junction for a wire to be joined to one or more lengths of wire.
(3) A connector plug and receptacle assembly is used to facilitate the removal and reinstallation of an electrical unit or group of units. For example, the receptacle may be permanently mounted on the engine side of the fire wall, and by disengaging the plug the electrical unit may be easily removed; thus, laborious breaking of connections is averted. When the unit is reinstalled in the aircraft, the electrical connection is quickly made by reengagement of the plug. After engagement, the plug and receptacle are fastened together with a threaded coupling. The electrical connection must necessarily be correct because the contact pins are so staggered as to fit into the sockets in only one way. Letters or numbers may be stamped at the pins and sockets for identification purposes when wires are replaced or new plugs or receptacles are installed.
107. Wiring diagrams.—a. The manufacturer of the particular aircraft furnishes a wiring diagram of the electrical system. This wiring diagram is in blueprint form, and is kept in the data compartment of the aircraft so as to be available at all times. The wiring diagram is a valuable aid to the mechanic when making inspections, testing circuits, and replacing electrical equipment. A small part of an aircraft wiring diagram is shown in figure 106.
b.	The type and serial number of the aircraft, and various explanatory notes to accompany the wiring diagram, are given on the blueprint. The wiring diagram is subdivided by dashed lines to indicate in a general way the main sections of the aircraft. Dashed lines may also denote a junction box or other enclosure and in some instances mechanical linkage.
c.	In addition to many of the common symbols shown in figure 3, various other symbols, as shown in figure 107, appear on aircraft wiring diagrams to designate items of electrical equipment. The serial number assigned to each separate item is printed (frequently underlined) near its symbol.
(1)	A table of equipment is given on the blueprint, and each item of equipment is listed numerically therein. Information for each
171
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107
ARMY AIR FORCES
I	NACELLE NO. 4.
/	M*—,	61	61	494
I	A An T? 'fl'
iUJ J	ru
5 02.	'3\	-A'	i	.V.	i !	>
I mm । -]_r_____h'4’"I	?®J__,	„ ^-*22 'X	*5-; !	■ J.-X;
M » Bi B t+f '<-< , *) *> *o •	। k k • । ** -	X •	r E £ £ ।
; j_u-l j	I Lbi P—Hb-—lFtFA
(	!	'if' 5 30	,±24.	1	-ter	(t‘<	I
\	i z r‘^4*'	4	/	•
s	: u :	£*	/	„	l
/	J	L T J	* I*
CLL-J”! 322. __	... _______________ £2________ *	__
i ?A '	‘T^Tl	' Tr ■	' Tr "*	I'T'T	' J	i'FXri
I ; SI	IS ?:	! ; 2 !	! “ s !	i - ? ?	!	&8"	;:J«s !
__________! Si 5°XuF.,________________________________________________499j_nnl,____ *'	___L _ 1 fli H iii xs4 i i iiii !
ll-ut— ........ ................U-----------^-,	!
L -_____________________________I I____________4
Figure 106.—Part of an aircraft wiring diagram.
172
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	107
item includes its serial number, name or description, quantity required, Army Air Force type, and perhaps other pertinent information. Tabulation of equipment shown in part in figure 108 includes some of the items shown symbolically in figure 106.
(2)	The table of equipment should be consulted to identify a symbol, or to insure that the correct type of equipment will be obtained when replacement is required.
d.	Wires which connect various units are indicated by solid lines running vertically or horizontally. For identification, the same serial number which appears on the wire in the aircraft is printed somewhere along the line which represents the wire on the diagram.
(1)	A wire table, shown in part in figure 108, is also given on the blueprint, and lists numerically the serial number of the wires. Information for each wire may include its length, size, and type of terminal lugs.
(2)	The wire table should be consulted when a wire in the aircraft is to be traced, or when a defective wire is to be replaced. When replacement is made, the serial number should be affixed to the new wire in the manner previously described.
e.	Conduit is generally represented on wiring diagrams by a single solid line, at each end of which a bracket is drawn to include all the separate wires entering or leaving the conduit. All wire lines entering a conduit line at one point must leave it at another point, but not necessarily in the order of entry. Care must be exercised not to confuse conduit and wire lines. A conduit line may or may not have an equipment number, but does not have a wire number.
/. (1) Occasionally, a unit (or group of units) appears in a separate sketch. Figure 109® illustrates a conventional method used to show the relation between the sketch and the main body of the wiring diagram.
(2) Some wiring diagrams use a complete “code” in order to avoid confusing multiplicity of wire crossings (fig. 109®). The full length of a wire is not drawn; only a small length extending from its terminals is shown. At each terminal of an item of equipment, various code numbers are given. The wire connected to a terminal is represented by a number (conduit, if used, also has a number). Another number adjacent to the wire number designates the item of equipment to which the wire connects. When tracing a wire on such a diagram, first the item of equipment so designated is located, and then the terminal to which the wire (which is being traced) connects.
173
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107
ARMY AIR FORCES
BUS BAR WITH FUSES

CONNECTOR PANEL
CONNECTOR PLUGS AND RECEPTACLES
CONDUIT - | RIGID
CONOUIT— | FLEXIBLE
EQUIPMENT ITEM
NUMBERS
PITOT HEATER
RESISTANCE BULBS
THERMOCOUPLE
WARNING HORN
SHIELDING
SHIELDING BRAID
switch
PRESSURE OPERATED
ISH'i
WIRE NUMBERS
-37——
$47---
RELAY SWITCHES
Figubm 107.—Conventional symbols
174
SWITCHES	0/0/ ° /
MECHANICALLY GANGED £~T ~T ~
TM 1-406
1Q7
AIRCRAFT ELECTRICAL SYSTEMS
O	IO	ZpO X
.----1	/ E DO\
o______I o	/	\
SOLENOID SWITCHES	GENERATOR-DOUBLE FIELD	CO
zw:	w
o	j	OB	KO
SOLENOIDS WITH	_	GENERATOR VOLTAGE ° A
MECHANICAL LINKAGE	Z~XZ'Y'"'Y-~X i REGULATOR
rro-v	x go
 ^^AT°R CURRENT
VOJU	|OH____Bo
hrn ’	r-r-]
WTHRMAGNE-nCTCLUTCH	\	7	BOOSTER COIL
uw 5	—
z^o~X / a+ X GENERATOR-SINGLE FIELD ( F+oj	MAGNETO
V(p_X	O O
OG+ B+O	/cT'X
GENERATOR CONTROL	IGNITION SAFETY SWITCH [OL R0|
PANEL-TWO ELEMENT	ONE ENGINE '	\	„ . /
or,b-o|
Zo n n n n	/ L 0 L \
l_l l_l L_l l—o	IGNITION safety switch oR G -A
SHUNT-AMMETER	--------- Two ENGINES	I
o	o	\	/
—	\BAT AUX/
o	o|
used on aircraft wiring diagrams.
175
TM 1-406
107
ARMY ATR FORCES
g. The service instructions in the Technical Order for a particular aircraft may contain diagrams of specific circuits assembled from the complete wiring diagram. Figures 110 and 111 are examples of such assembled diagrams, and refer to the aircraft wiring diagram and tabulations of which figures 106 and 108 are a part. An assembled diagram is especially helpful when the mechanic is concerned with particular circuits. The service instructions may also contain equipment and wire tabulations compiled from the blueprint.
(1) When an assembled diagram as contained in service instructions is not available, or when it is desired to further simplify the representation of a specific part of a circuit, this may be accomplished as shown in figure 112. Typical details of a wiring diagram are illustrated in (T). The essential features of the generator-battery (items 3 and 12) circuit may be simplified by following the respective connections from terminals A + and F + of item 3. For example, starting with wire number 8, the connection from A+ is found to lead to the 6r+ terminal of item 17 (control panel) ; one short line may be
176
।	|	I WIRE TABLE
\	__________________________________________________________ \	IGNITION
j	TABLE OF EQUIPMENT ____________________________ /	wire no! size I length
I N0[	DESCRIPTION	IreqI TYPE IPART NoJlNSf,DWC.II \	/ I 1	16 £64	\
I 61 MAGNETQ--BENDIX-SCINTILLA_________8 5F9LI 10-5377 155900 f	\ I 2	16	48	)
62 SHIELD- ENGINE PULL____________ 4	BAC	3-105634 2	6 8107-1	\	) I 21	18	312
65 PLUG- IGNITION PULL____________4 BREEZE 35A4Z73 15-5284	/	1 22	18	48
64 COIL - BOOSTER__________________4	A-l	2520 '----------  \	\ I 23	HT	6	\
6S SHIELD OUTDRD. IGNITION_________2	BAC	311256	6'7692	I	<124	“ht"	36	/
66 SHIELD BOOSTER 5 WITC H (OUTBPQ	2 BAC 21-9627	72 I
67 SWITCH-BOOSTER________________4 B~GB	AN30IG 9-2917	(	748 Tg 72~	]
(	68 SOCKET- HIGH TENSION___________ 4 BREEZE E III7-I-20 6-7692	L-l-----L. J-----J - I
U69 SOCKET- IGNITION___________________4	■■	35A4269 6~7692
70 SHIELD - l-NBRD IGNITION_______2_ BAC	3-11256 6-7690
71 SHfELD-STARTER CONNECTOR OUFBRD 2	"	5-1 1251 6~7692
^GENERATOR-ENGINE No,2-314______' 3 E'7	F-644GJ-2 I5-5 9OO	J	WIRE TABLE I (
PLUG____________________________4 BREEZE 35A4273 15-5204	\	1 DC POWER I
SOCKET__________________________4____“	35A4269 ----- (	wire n<] size, length \
CAPACITOR_______________________7. B-l 5F«<5OI3I-------/	( P 2 IB 60
SOCKET - GENERATOR_______________3	BREEZE 35B5IG8	3-11255	\	P 62	IB	372	I
PLUG-GENERATOR__________________3_	"	36A2203i	15-5284	/	P68	6	120	/
*--*--------------------------—----------------------------------*	P78	6	60	\
(	491	THERMOCOUPLE - UNIT	~4~	w.A.C,	62508	62102	J
\	494	RESISTANCE BULB-CARB. AIR TEMP 4	F-2	5P«cZ7823	15'5900	(
1	495 SHIELD-OUTBRD THERMOCOUPLE 2 BAC	3'1/377	\
\	496 SHIELD - INBRD.______»_________Z_ -_________••	311969	/	---------------
/	497 SHIELD-ENGINE PULL___________ 4	-________- G-8IO7 \	! WIRE TABLE
I	493 PLUG__________________________4 BREEZE 35A2533 15'5204	)	I	STARTER CONTROL	'
)	499 SOCKET_________________________4	-	35A253S 3'11969	\	(	wint no size length
/	501 SOLENOID-STARTER ME5HING_______4	A-1A C'53633	9~29I7	)	I	521	10	372
(	502 5TARTER________________________4	C'ZI F-42331	15'5900	1	/	5 23	16	288
1	503 SOCKET •______________________ 4	BREEZE 36AI88I	[	]	S 24	2	56
j	505 SOLENOID-STARTER_______________4	A-5A C-5354O-I	 ----- \	I	531	2	120
(	509 PLUG__________________________  4	BREEZE 36AI874	15-5284	(	\	-533	2	48
I	515 MOTOR-PROPELLER FEATHERING PUMP 4 PESCO 41-7770 15'4726	/	|	5 34 __2	48	)
j	516 SWI.TCH- , - PRESSURE CUT-OUT 4 HAMILTON 53574	I5?5900	[	I	5 35	10	64	\
Figure 108.—Tabulation of equipment and wire.
TM 1-406
107
AIRCRAFT ELECTRICAL SYSTEMS
PART OF MAIN BODY OF WIRING DIAGRAM
(C)	(B)
r.........___ r4i------------------'•■—I
(G >—।--5-^0-| < SEPARATE , 2 (	?
[_________^j^SKETCHES^M :
VITEM NO --------------------------CONDUIT NO. 142 n -------------------------- ( A	।- WIRE NO. |--------------------------------WIRE CONNECTION TO BE
I A p_________ C5-2I8	/~ UNDERSTOOD FROM CODE
C5*219	/	r—WIRE NO.
Z'~\	--------- /	’	C69
6 \ 0 C 6 9-413'*'	14 2-413---
/	\	„____r/m-QOK NO. OF ITEM TO WHICH-^
WIRE CONNECTS-------------------107
6 A O—Cl 6-205
Figure 109.—Wiring diagram code conventions.
449399 ° —42---12
177
TM 1-406
107
ARMY AIR FORCES
I	J	I	J
MO. 2 EN&.	I__|	I  I	MO. 3 ENG.
r-IIAloR'p"	/	1’7	\	U ««>)-WO-|
I X, o-|—Ilfa —«— T5S-O—154-4 W	«?♦	> *r	140—o—14\ -»-l42---b° /
/	71—L3B —<4-152 -o—I=M —&>«C—T3T -o-I3& ->>-15'5 --------------k . 7\
~ (°u <43	3- ^y X
___ 	P^o-na-to_—
<0*1 4	feT_
C	3
H	°
51	3
i> •l|— >—150-----------—-----------155 -y----------15fio — H'1
>----156 --O- 1 5-----R------------!'*»	----------*•«=>----- ’
55	3 S	p
w - J	;:
V	' £L
-4 p	3-^77
C4 f jX G»~l $	7	^~~7" —
3	- 4-R-'A	5
t. Zh *4—II*	I °)	1
/	3---150	—<4-175 —^-126 ------- -----------14 te -0-14*1 »-I4e>	—IPo R \	J
Lno -t°R _ ,..	-n-4-o y
MO v EUGr.	EMCr.
figure 110.—Ignition circuits assembled from complete wiring diagram.
178
TM 1-406
107
AIRCRAFT ELECTRICAL SYSTEMS
EMG MO l	o M U	HMCr b4O 4-
H H so.	A J 502.	*2 7_^Pyi	52?( -A--5OL
I I ,lhrr	Ti'i	I 1
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do	£ s s
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j, ,	-°-5»S -<*VD-—I	kr»—S14—O-S1*>  
61	Z»'
-----------SIS —o---- -----------------«»X| ________ o-*PPI»
9 ~ - D -—pl	,n- U r>
I MaL? S44— ---Slfe	ar\_p— svs-Q— szZ —• l~ijr I
1 l_ VXjuf	kA 11 LAV	uUJu^i » J
♦	525	“» -a -a	525 T T J
M	- p;? ;;
«1 f M > S - tl *rA H I I	A* T «
— WJH-----(u JSOZ.	if Ji	•£&(	1___I?—,?	521
0 -	r* S
EN&MO.Z ~3~ *■ u&flfij*	ImofI	EWG. MO.2»
EMGr.MQI j$H	EUO.MO.4-
EJN6r. HO 2 BJMCr.MO. %
Figure 111.-—Starting circuits assembled from complete wiring diagram.
179
TM 1-406
107
ARMY AIR FORCES
used to indicate this connection. By use of this method, the features of the circuit may be simplified to the extent shown in (2).
(2) An approximate lay-out of the basic items of a single-engine aircraft electrical system is shown in figure 113. A diagram of this
14	20	86
JT1.
A	14	18____ 84
zX+\	—	___ :	63
(	-----8----) >--7----1	; O
( A+ F +	„	,	>----- I O--------;------4 9-------
9	* >--10---/	i & i
i	।	(TO—j----	I-----,
-^3-	‘	cr-o—;--- •_________!
’	.-.	'	0-0—----- ; -==- I
---------86------ 4R	.-----.	,	;
—--------20------>---------------- 1 -T _ .
rp	y
-----10------)
----7-------/	221	~ o'
17	8	60
I---06+ B+O----102 -o-----104 --> 0 OPEN IN RESISTOR------@ SHORT BETWEEN SWITCH TERMINALS-----------------(5) GROUNDED WIRE
OPENy
jyrjrr--------------------------T^I---------------	------------J®u---------------
_	tfjy	'5*	'£*	l£v o]v.	I2JV o[v
-=- (v)		-=_ (y) (v)
jL _t_ _t_ „t_ -i-	-A. jl _ - X- -A-.- -X-	- =■
@ LAMP ON-NORMAL READINGS	(S) LAMP OUT - OPEN IN FUSE
/ °*eM	^-OPCH
_g—pryr-------------------------T3!--------------- _r—TTT7---------------------------------------Fi 7—
"*	I2»v.	I2»y.	I2jv.	lajv.	o[v.	*"*	I2(y.	12 Jy	I2^v	I2jy	l2Jy	12] v.	OjV.
•	:'v'	V;	:V)	(v:	(v'J	(V*.	."♦	(V;	tv';	(vj	t'v:	(v;	(v;	•v)
=	T11	f	f	T	t	IT	I	i	i	I
X -A- -i- .-i.-	JL	_ _	X -i- -A-.- -.-L-	-k- -i-- •=■
(•) LAMP OUT-OPEN IN LAMP	(1) LAMP OUT - OPEN IN GROUND CONNECTION
Figubh 114.—Continuity testing.
TM 1-406
AIRCRAFT ELECTRICAL SYSTEMS	108
108. Circuit testing.—If an electrical device malfunctions, there are two general possibilities—the device itself may be at fault (burned out, damaged mechanically, etc.) or that part of the circuit leading to or from the device may be at fault. Continuity testing refers to checking for the existence of a complete electrical circuit between two points.
a.	The circuits illustrated in figure 114®, ®, and ® include a portable type of continuity tester wherein dry cells are used as a source of voltage. A lamp within the portable tester serves as the indicator.
(1)	If the alligator clips of the tester unit were touched together, a complete circuit would be established and the lamp would be lighted. If, as shown in ®, the clips are brought into contact with the terminals of the resistor (or coil, etc.) under test, and the lamp does not light, an open circuit in the resistor is indicated. This test is conclusive only if the resistance of the unit under test is sufficiently low to permit the lamp to light if the unit is not defective; if the resistance is too high (more than approximately 10 ohms), a voltmeter (mounted on the panel of the tester) can be connected in the circuit in place of the lamp; if no deflection of the voltmeter pointer occurs, an open circuit is indicated.
(2)	In ® the tester is shown indicating the presence of a short circuit across the terminals of a switch in the open position.
(3)	To determine whether a length of wire is grounded at some point between its terminals, the wire is disconnected at each end; one test clip is affixed to wire, and the other clip grounded as shown in ®. If the wire is grounded the lamp will light.
(4)	When this method of continuity testing is applied to installations on aircraft, the disconnection of a number of wires is necessitated.
b.	To test for open circuits in wiring and equipment installed in aircraft, the aircraft storage battery may be used as the source of voltage, and a voltmeter with long flexible leads used to test the circuit. The procedure in this method of circuit testing may be illustrated by reference to a simple aircraft circuit containing a 12-volt battery, fuse, switch, and landing lamp. The procedure may be applied to any branch circuit which is fed by the battery.
(1)	An essential preliminary is to draw a simple wiring diagram of the circuit involved, as shown in figure 114®. For this purpose, the wiring blueprint or the service instruction diagrams should be consulted.
183
TM 1-406
108
ARMY AIR FORCES
(2)	If the lamp functions when the switch is closed, the indication to be expected in each position of the voltmeter is shown by the voltmeter symbols in @. If the lamp does not function, check the battery for normal voltage. The negative test clip of the voltmeter is attached to any convenient ground connection; the positive test clip is connected to the battery side of the fuse. If the voltmeter reads zero, it must then be determined whether the battery is at fault, or whether the battery leads to ground or fuse are at fault. If the reading of the voltmeter is approximately 12 volts, no defect is indicated up to this point, and the testing may be continued along the circuit.
(3)	The negative clip is kept connected to ground. The positive clip is moved progressively along the circuit, using the diagram as a guide, testing each unit and length of wire (by attachment of positive clip to a terminal) for indication of normal battery voltage until the first zero reading is obtained. Between the last point at which voltage was indicated and the point of first zero reading, an open circuit is indicated. This is illustrated at @ by an open fuse, at ® by an open lamp filament, and at © by an open lamp-to-ground connection.
(4)	It is to be noted that in ® the voltmeter reads 12 volts when the positive clip is Connected to either terminal of the lamp. This means that there is no potential difference across the lamp. This is reasonable, because no current is flowing through the lamp. (Actually, the voltmeter does allow some current flow through it (par. 37) but this is so small that its effects are negligible.)
c.	An ammeter should not be used in the methods of continuity testing described in this section because the ammeter has a very low resistance and would short-circuit the battery.
184
TM
1-406
185

INDEX
Paragraph Page
Aircraft circuit__________________________________________  14	15
Alternating current________________________________________ 10	10
Alternating e.m.f_______ ------------___________________ 48	56
Ammeter____________________________________________________ 36	43
Ampere_____________________________________________________ 10	10
Ampere hour________________________________________________ 60	80
Ampere turns________________________________________________27	31
Armature------------------------------------------ 33, 66, 67 38, 91, 94
Atom________________________________________________________ 3	3
Back e.m.f------------------------------------------------- 52	65
Background-------------------------------------------------- 1	1
Battery ignition------------------------------------------- 83	132
Bonding_____________________________________________________ 7	6
Booster coil______________________________________________  54	66
Booster magneto-------------------------------------------- 82	130
Break-down, insulation______________________________________ 5	5
Brushes___________________________________ .	___ __	33	38
Build-up-------------------------------------------------   51	60
Capacity___________________________________________________ 44	52
Charge, separation__________________________________________ 6	5
Charging, battery------------------------------------------ 63	85
Chemical reaction------------------------------------------- 4	4
Circuit---------------------------------------------------  13	13
Circuit testing___________________________________________ 108	183
Cold operating_ „__________________________________________ 86	137
Commutator_________________________________________________ 33	38
Commutation________________________________________________ 33	38
Compass____________________________________________________ 21	25
Compound generator----------------------------------------- 51	60
Condenser—
Break-down_____________________________________________ 45	52
Capacity_______________________________________________ 44	52
Construction___________________________________________ 42	50
Operation______________________________________________ 43	50
Primary________________________________________________ 54	66
Purpose________________________________________________ 41	50
Conductor_______________________________________________ 5, 11	5, 11
Conservation of energy______________________________________ 2	2
Conventional current_______________________________________ 10	10
Coulomb____________________________________________________  6	5
Counter e.m.f_______________________________-___________ 52	65
TM 1-406
INDEX
Paragraph Page
Critical point__________________________________________________ 51	60
Current_________________________________________________________ 10	10
Dielectric______________________________________________________ 42	50
Direct current__________________________________________________ 10	10
Direct current circuits:
Aircraft____________________________________________________ 14	15
Ohm’s law___________________________________________________ 15	16
Power_______________________________________________________ 19	20
Solution of simple problems_________________________________ 20	21
Units in—
Parallel_______________________________________________ 16	18
Series______________________2______________________ 17	19
Series-parallel________________________________________ 18	20
Direct current motor_________________,______________________ 33	38
Drop in potential________________________________________________ 8	7
Eddy currents___________________________________________________ 56	73
Electrical—
Current_____________________________________________________ 10	10
Force________________________________________________________ 3	3
Ignition____________________________________________________ 54	66
Resistance__________________________________________________ 11	11
Electricity, uses________________________________________________ 1	1
Electrolyte_________________________________________________L_ 58	- 76
Electromagnet___________________________________________________ 29	33
Electromagnetic field-----------------------------------------   23	26
Electromagetic induction:
D-c motors, characteristics_________________________________ 52	65
Effects_____________________________________________________ 46	53
Electrical ignition_________________________________________ 54	66
Generation of e.m.f_________________________________________ 50	59
Loqds_______________________________________________________ 49	58
Magneto, theory_____________________________________________ 55	69
Mutual induction____________________________________________ 53	66
Practical d-c generator_____________________________________ 51	60
Self induction______________________________________________ 47	54
Transformer_____________________________________________•_	56	73
Variations in flux linkage__________________________________ 48	56
Electromotive force (e.m.f.)___________________i____________ 9	8
Electron________________________________________________________  3	3
Electronic unit________________________________________________   3	3
Elements_________________________________________________________ 3	3
Energy----------------------------------------------------------- 2	2
Farad___________________________________________________________ 44	52
Field coils___________________________________________________   33	38
Filter, condenser_______________________________________________ 51	60
Flux, magnetic__________________________________________________ 27	31
Footpound-------------------------------------------------------- 2	2
186
TM 1-406
INDEX
Paragraph Page
Force on current-carrying conductor__________________________ 30	35
Fuse_________________________________________________________ 12	13
Galvanometer_________________________________________________ 35	43
Gassing voltage______________________________________________ 63	85
Generation of e.m.f_____________________;_________________ 50	59
Generators:
Aircraft______________________________________________ 66, 67	91, 94
Armature testing_________________________________________ 75	115
Construction_____________________________________________ 66	91
Current control switch relay_____________________________ 74	114
Definition_______________________________________________ 50	59
Elementary________________________________________ _	50	59
Maintenance______________________________________________ 76	118
Plug-in models, voltage regulator________________________ 73	110
Practical d-c____________________________________________ 51	60
Regulation of voltage____________________________________ 68	103
Reverse current cut-out relay_______,_________________ 69	105
Twin-engine generator control circuit___________________  72	108
Two-element control panel________________________________ 70	106
Vibrator type current limiter____________________________ 71	106
Ground potential______________________________________________ 8	7
Ground .wire__________________________________________________ 7	6
Grounded_____________________________________________________  8	7
Horsepower_________________________________________ _ ____ 19	20
Hot operating------------------------------------------------ 86	137
Ignition:
Battery-------------------,___________________________ 83	132
Boosters_________________________________________________ 82	130
Coil_____________________________________________________ 54	66
Elements_________________________________________________ 77	119
Magneto:
Construction________________________________________ 79	121
Internal timing__________,_______________________ 81	130
Operation___________________________________________ 78	120
Types---------------------------------------------   80	128
Maintenance______________________________________________ 87	139
Spark plugs---------------------------------------------- 86	137
Switches_____•________________________________________ 84	133
Wiring------------------------■_______________________ 85	136
Inductance___________________________________________________ 46	53
Induction____________________________________________________ 46	53
Induction coil_______________________________________________ 54	66
Inductive unit_______________________________________________ 46	53
Inertia,----------------------------------------------------- 46	53
Insulation________________,_______________________________ 5	5
Insulator__________________________________________________    5	5
Interpole________________________________________________     66	91
187
TM 1-406
INDEX
Paragraph Page
Joule_______________________________________________________ 2	2
Kinetic energy______________________________________________ 2	2
Laminations________________________________________________ 56	73
Landing gear warning signal system________________________ 102	161
Lap wound__________________________________________________ 67	94
Lead-acid cells____________________________________________ 58	76
Lighting:
Cockpit----------------------------------------------- 104	165
External---------------------------------------------- 103	163
Instrument__________________________________________ 104	165
Maintenance___________________________________________ 105	168
Lightning--------------------------------------------------- 7	6
Line loss_________________________________________________  14	15
Lines of force_____________________________________________ 22	26
Load circuit_______________________________________________ 13	13
Magnetic—
Circuit________________________________________________ 27	31
Dipole_________________________________________________ 23	26
Field__________________________________________________ 22	26
Force__________________________________________________ 21	25
Induction______________________________________________ 25	30
Linkage------------------------------------------------ 48	56
Poles__________________________________________________ 21	25
Magnetism:
Cause__________________________________________________ 24	29
Coils__________________________________________________ 28	32
D-c motor______________________________________________ 33	38
Electromagnet_________________________________________  29	33
Force on wire__________________________________________ 30	35
Phenomena______________________________________________ 21	25
Retention______________________________________________ 26	30
Solenoid_______________________________________________ 29	33
Torque------------------------------------------------- 31	36
Weston meter movement__________________________________ 32	37
Magneto:
Construction___________________________________________ 79	121
Internal timing_____________________________________ 81	130
Operation___________________________________________•	78	120
Theory------------------------------------------------- 55	69
Types-------------------------------------------------- 80	128
Magnetomotive force	(m.m.f.)_______________________________ 27	31
Mass________________________________________________________ 3	3
Measurement, d-c:
Ammeter________________________________________________ 36	43
Galvanometer___________________________________________ 35	43
Thermocouple thermometer_______________________________ 40	48
188
TM 1-406
INDEX
Measurement, d-c—Continued.	Paragraph Page
Voltage divider (potentiometer)__________________________ 38	46
Voltmeter________________________________________________ 37	45
Wheatstone bridge________________________________________ 39	47
Metals_____________________________________________________     3	3
Microfarad__________________________________________________   44	52
Molecule_______________________________________________________ 4	4
Mutual induction______________________________________________ 53	66
Neutral________________________________________________________ 6	5
Nonconductors__________________________________________________ 5	5
North pole____________________________________________________ 21	25
Nucleus________________________________________________________ 3	3
Ohm----------------------------------------------------------- 11	11
Ohm’s law_____________________________________________________ 15	16
Open circuit__________________________________________________ 13	13
Parallel circuit______________________________________________ 16	18
Pole piece____________________________________________________ 66	91
Potential_____________________________________________________  8	7
Potential energy_________________________________:________ 2	2
Potential level_______________________________________________  8	7
Potentiometer_________________________________________________ 38	46
Power_________________________________________________________ 19	20
Primary coil__________________________________________________ 54	66
Problems______________________________________________________ 20	21
Reluctance, magnetic------------------------------------------ 27	31
Residual magnetism____________________________________________ 26	30
Resistance____________________________________________________ 11	11
Resistive load_______________________________________________  13	13
Resistive unit________________________________________________ 46	53
Resistivity___________________________________________________ 11	11
Resistor______________________________________________________ 12	13
Reversible motor_____________________________________________ 100	159
Rheostat__________________________________________________ 12	13
Right-hand rule_______________________________________________ 23	26
Secondary coil________________________________________________ 54	66
Secondary condenser___________________________________________ 55	69
Self inductions_______________________________________________ 47	54
Separation of charge___________________________________________ 6	5
Series circuit______________________________________________   17	19
Series motor__________________________________________________ 33	38
Series-parallel_____x------------------------------------- 18	20
Shielding_____________________________________________________ 77	119
189
TM 1-406
INDEX
Paragraph Page
Short circuit__________________________________________________ 13	13
Shunt_________________________________________________________  36	43
Shunt motor.___________________________________________________ 33	38
Slip rings----------------------------------------------------- 50	59
Solenoid_______________________________________________________ 29	33
South pole_____________________________________________________ 21	25
Spark plugs---------------------------------------------------- 86	137
Starting systems:
Air injection______________________________________________ 98	157
Cartridge-type-------------------------------------------   97	155
Direct cranking hand and electric__________________________ 94	151
Hand-turning gear-type_____________________________________ 95	152
Inertia starter:
Combination hand and electric_________________________ 91	145
Electric motor________ _______________________________ 92	149
Hand_________________________________________________  90	143
Principle____________________________________________  89	142
Solenoid switch_______________________________________ 93	149
Maintenance________________________________________________ 99	157
Portable field_____________________________________________ 96	152
Requisites_____________________________________________ 88	142
Static electricity_________________________________________ 7	6
Storage batteries:
Charging_________________________________________________   63	85
Construction_______________________________________________ 59	78
Description____________________________________________ 57	75
Deterioration____________________________________________   61	82
Lead-acid cells____________________________________________ 58	76
Maintenance________________________________________________ 64	89
Ratings, __----------------------------------_--------- 60	80
Sulphation_________________________________________________ 61	82
Testing--------------------------------------------_____	62	82
Switches, ignition___________________________________________   84	133
Symbols---------------------------------------------------- _ _	13	13
Terminal voltage, effect of load_______________________________ 49	58
Thermocouple_____________________________________T_________	40	48
Thermojunction. _______________________________________________ 40	48
Torque-------------------------------------------_--------- 31	36
Transformer____________________________________________________ 56	73
Units in—
Parallel___________________________________________________ 16	18
Series____________________________________________________  17	19
Series-parallel____________________________________________ 18	20
Volt___________________________________________________________  8	7
Voltage divider________________________________________T___	38	46
Voltage drop_____________________________________\--------- 8	7
V oltmeter------------------------------------------------- 37	45
W0
TM 1-406
INDEX
Paragraph Page
Watt--------------------------------------------------- 19	20
Wave wound______________.______________________________ 67	94
Weston meter movement_____________________________________ 32	37
Wheatstone bridge_______________________________________   39	47
Wiring:
. Circuit testing-------------------------------------- 108	183
Diagrams_____________________________________________ 107	171
Features--------------------------------------------- 106	168
Ignition system--------------------------------------- 85	136
Work_______________________________________________________ 2	2
[A. G. 062.11 (2-7-42).]
By order of the Secretary of War :
G. C. MARSHALL,
Chief of Staff. Official :
J. A. ULIO,
Major General,
The Adjutant General.
Distribution :
Bn and Hl (6); Bn 9, 11 (2); IBn 1 (10); IC 9, 11 (3).
(For explanation of symbols see FM 21-6.)
191
rr> rr> rr> rn rn vn m ra
UNT LIBRARIES DENTON TX 76203
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