Generators and Related Control Circuits

Generators and Related Control Circuits
 Ever since the first aircraft to use any kind of electric equip-
ment was launched, the electrical loads on airplanes and
other flying devices have increased. Today modern jet airlin-
ers are equipped with scores of different electrical systems,
~ each requiring a substantial amount of electric energy.
Generators were the first means of supplying electric
power for aircraft. Currently, generators or generator deriva-
tives called alternators are found in a wide variety of sizes
and output capacities. A typical alternator used on a large
commercial aircraft can produce over 6000 W of electric
power. On multiengine aircraft, one generator is driven by
each engine to allow for redundancy in the event of a genera-
tor failure.
An electric generator can be defined as a machine that
changes mechanical energy into electric energy. On aircraft
the mechanical energy is usually provided by the aircraft’s
engines. Light aircraft use 14- or 28-V dc generators. Large
aircraft typically employ generators that produce an alternat-
ing current of 208 or 117 V at 400 Hz. Compared with a 28-V
dc system, a higher-voltage ac system will develop several
times as much power for the same weight; hence it is a great
advantage to use ac systems where heavy electrical loads are
Electricity is produced in a generator by electromagnetic
induction. As explained in Chapter 1, it is a fundamental
principle that when there is a relative movement between a
magnetic field and a conductor held perpendicular to the line
of flux, an emf is produced in the conductor. If the ends of the
conductor are connected together, the emf will cause a cur-
rent to flow, as shown in Figure 10-1. The direction of cur-
rent flow is determined by the direction of the magnetic flux
and the direction in which the conductor is moved through
the flux.
À simple way to determine the direction of current flow is
to use the left-hand rule for generators. Extend the thumb,
index finger, and middle finger so they are at right angles to
one another, as illustrated in Figure 10-2. Turn the hand so
the thumb points in the direction of movement of the conduc-
tor and the index finger points in the direction of the magnetic
flux. Then the middle finger will be pointing in the direction of
the current flow. Remember, current flow is from negative to
FIGURE 10-2 Left-hand rule for generators.
positive. Flux direction is considered to be from north to
Simple AC Generator
A simple ac generator can be constructed by placing a single
loop of wire between the poles of a permanent magnet and ar-
ranging it so that it can be rotated as shown in Figure 10-3.
The current is taken from the wire loop by means of brushes,
which make continuous contact with the collector rings (slip
rings). One collector ring is connected to each end of the wire
loop. In Figure 10-3, the sides of the loop are designated AB
and CD. As the loop rotates in the direction indicated by the
arrow, side AB will be moving up through the magnetic field.
If we apply the left-hand rule for generators, we find that a
voltage 1s induced that will cause current to flow from A to B
in one side of the loop and from C to D in the other side of the
loop. This is because AB is moving up through the field and
CD is moving down through the field.
The voltage induced in the two sides of the loop add to-
gether and cause the current to flow in the direction ABCD,
through the external circuit, and then back to the loop. As the
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90° 180° 270° 360°
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FIGURE 10-3 A simple ac generator and associated voltage
loop continues to rotate toward a vertical position, the sides
will be cutting fewer lines of flux, and when it reaches the
vertical position, the sides of the loop will not be cutting any
lines of flux but will be moving parallel to them. At this posi-
tion, no voltage is induced in the loop because a conductor
must cut across flux lines in order to induce a voltage. By ro-
tating the loop through the vertical position and back to the
horizontal, a voltage will be induced again, but it will be in
the opposite direction in the loop because side AB will now be
moving down through the field and side CD will be moving
up through the magnetic field. Soon the loop 1s once again in
the vertical position, and no flux lines are being cut. When the
loop is exactly perpendicular to the magnetic flux lines, no
voltage is being produced. The current flow then repeats its
cycle as long as the loop is rotated inside the magnetic field.
The voltage waveform produced by this type of generator is
called the sine wave.
By examining the sine wave of Figure 10-3, it can be seen
that the voltage is at zero when the loop is in a vertical posi-
tion, and then it climbs to a maximum value when the loop 1s
in the horizontal position. This is indicated on the sine curve
from O to 90”. As the loop continues to turn, we find that the
voltage is maximum at 90°, zero at 180°, maximum at 270°,
and zero again at 360°.
Essential Parts of a Sample AC Generator
The essential parts of a simple ac generator are shown in
Figure 10—4. These are a magnetic field, which may be pro-
duced by a permanent magnet or by electromagnet field coils;
a rotating loop or coil called the armature or rotor; slip
rings; and brushes by which the current is taken from the ar-
mature. The poles of the magnet are called field poles. In
most generators, these poles are wound with coils of wire
called field coils. The path of the magnetic flux is called the
magnetic circuit and includes the yoke connecting the field
poles as well as the armature.
FIGURE 104 Essential parts of an ac generator.
‘ Value of Induced Voltage
The voltage induced in a conductor moving across a mag-
netic field depends on two principal factors: the strength of
the field (the number of lines of force per unit area) and the
speed with which the conductor moves across the lines of
force. In other words, the voltage depends on the number of
lines of force cut per second. For example, if a conductor cuts
lines of force at the rate of 100,000,000 lines per second, an
emf of 1 V will be established between the ends of the con-
Simple DC Generator
DC generators are needed for many aircraft electrical sys-
tems, for battery charging, and for various other applications.
For this reason an ac generator will not meet all power re-
quirements unless a means of rectifying the alternating cur-
rent is provided. Figure 10-5a shows an elementary type of
dc generator that is quite similar to the simple ac generator
explained previously.
A pulsating direct current can be obtained from the illus-
trated generator by using a commutator in place of the slip
rings found on the ac generator. A commutator is a switch-
ing device that reverses the external connections to the arma-
ture at the same time that the current reverses in the armature.
The commutator in Figure 10—5a is a split ring that turns with
the armature. One end of the rotating loop is connected to one
half of the ring, and the other end of the loop is connected to
the opposite half of the ring. The two sections of the commu-
tator are insulated from each other. Two brushes are placed in
a position relative to the commutator so that as the commuta-
tor turns, the brushes pass from one segment of the commuta-
tor to the other at the same time that the current is reversing;
there is then practically no emf between the two segments.
This system of changing the alternating current of the arma-
ture to direct current in the external circuit 1s called commu-
tation. |
Referring to Figure 10—5a, observe that the side of the
loop moving up through the field will always be connected to
Generator Theory 191
FIGURE 10-5 A simple dc generator and associated voltage wave.
the positive brush and that the side of the loop moving down
through the field will always be connected to the negative
As illustrated in Figure 10-5b, the current from the gener-
ator will then be traveling in one direction in the external cir-
cuit, but it will pulsate; that is, it will vary in intensity from
zero to maximum and back to zero through each half turn of
the armature. A current of this type is called a pulsating direct
current and is not suitable for many uses.
Elimination of DC Ripple
Since the pulsating direct current of the simple dc generator is
not satisfactory for all purposes, it is necessary to construct a
generator that will produce an almost constant voltage. This
is accomplished by increasing the number of coils in the ar-
FIGURE 10-6 Comparing voltages from a single-coil armature
and a multiple-coil armature.
Chapter 10 Generators and Related Control Circuits
mature and/or the number of field coils. Figure 10—6a illus-
trates the nature of the voltage from a single-coil generator,
and Figure 10—6b shows the curve for a generator with four
armature coils. Notice the great difference in the nature of the
Armature coils are wound on a laminated soft-iron core.
The iron core concentrates the field flux and greatly increases
the voltage generated. The laminations reduce the effects of
eddy currents induced in the core.
The current from any dc generator will have a slight pul-
sation known as commutator ripple, but this ripple does not
interfere in ordinary electric circuits for purposes such as
lighting and operating electric motors. For radio circuits, the
commutator ripple must be eliminated because it causes a
hum in the radio output. A capacitor of correct capacitance
shunted across the dc power leads of a radio receiver will
greatly reduce the amount of ripple. For a more effective fil-
ter, an inductance or choke coil is connected in series with the
dc line along with the capacitor in parallel with the dc line.
Remember that an inductance coil opposes any change in the
current flow. A capacitor, a choke coil, or a combination of
the two connected in a circuit to reduce ripple is called a rip-
ple filter.
Residual Magnetism
An electromagnet will produce a much greater field strength
per given size and weight than a permanent magnet. For this
reason field coils are used to provide the magnetism required
for the generation of current. In dc generators the field coils
are usually energized by current from the generator. Fortu-
nately, any substance that has been magnetized will retain a
certain amount of magnetism. Materials such as soft iron give
up most of their magnetism very quickly when removed from
the magnetizing influence. However, they do retain a small
amount, which is known as residual magnetism.
It is this residual magnetism that makes it possible to start
a generator without exciting the field from an outside source
of magnetism. The residual magnetism in the field poles
causes a weak voltage to be generated when a generator be-
gins to rotate. This small voltage is then used to power the
generators electromagnetic field, thus increasing the field
The increase in field strength causes a corresponding in-
crease in generator output voltage, and a mutual increase in
field strength and voltage continues until the voltage reaches
the proper value for the generator. If the residual magnetism
should be lost because of excessive heat or shock, it can be re-
stored to the field by passing a direct current through the field
windings in the correct direction. This procedure is called
flashing the field and is discussed in a later section of this
Characteristics of DC Generators
DC generators are classified as shunt-wound, series-
wound, or compound-wound, according to the manner of
connecting the field coils with respect to the armature.
The internal connections for a shunt-wound generator
are shown in Figure 10-7, where it can be seen that the field
coils are connected in parallel with the armature. In this type
of generator, itis necessary to have a resistance or some other
means of regulation in the field circuit to prevent the devel-
opment of excessive voltage. If such a generator were to run
without a load, the entire output would go through the field
coils, thus producing a very strong field. This field would, of
course, increase the voltage of the generator, and the field
strength would also increase. The result would be a continued
increase of both the field strength and the voltage until the
generator burned out.
Shunt generators without field-current regulation are sat-
isfactory only for operation at a constant speed and with a
constant load. In practice, it is doubtful that such a generator
could be used except experimentally.
A series-wound generator contains a field winding that
is in series with respect to the armature winding. A diagram
of a series-wound generator is shown in Figure 10-8. In this
FIGURE 10-7 Diagram of a simple shunt-wound generator.
FIGURE 10-8 Diagram of a simple series-wound generator.
FIGURE 10-9 Diagram of a simple compound-wound genera-
type of generator the resistance of the load controls the field
current. If the load resistance decreases because more electri-
cal load is applied, the field current increases and the genera-
tor’s output voltage increases. If the electrical load decreases
(increasing resistance), the current through the load and the
generator’s field decreases and the generators output voltage
decreases. From these relationships it can be seen that an un-
regulated series-wound generator will not maintain a con-
stant voltage output. Series-wound generators that are not
regulated can be used in situations where a constant rpm and
a constant load are applied to the generator, but they are not
suitable for aircraft applications.
A compound-wound generator combines the features of
the series- and shunt-wound generators. As illustrated in
Figure 10-9, this generator contains a field winding in series
and one in parallel with respect to the armature. In this type of
generator, when the load increases (decreasing resistance),
the series field current increases and the parallel field current
decreases. The output voltage remains constant. If the load
decreases (increasing resistance), the series field current de-
creases and the parallel field current increases. Once again,
output remains constant.
In theory, the series-wound, shunt-wound, and com-
pound-wound generators each have certain advantages and
disadvantages. On modern aircraft, however, all generators
contain some means of controlling the output voltage and
current. The shunt-wound generator used in conjunction with
a voltage regulator is the most common type of dc generator
system for aircraft. The voltage regulator adjusts the current
to the shunt field in order to maintain the necessary output
under a variety of rpm and load conditions. Voltage regulator
circuits will be discussed later in this chapter.
Generator Theory 193
FIGURE 10-10 Armature circuit with battery analogy.
Analysis of an Armature Circuit
In a previous paragraph it was explained that a practical gen-
erator ha% many coils of wire in the armature. These coils are
connected to the commutator segments in such a manner that
they are in series with one another. Figure 10—10a shows the
connections for a typical commutator in a two-pole genera-
tor. Assume that the armature has eight coils of two turns
each wound around the armature through oppositely posi-
tioned slots. If the magnetic flux 1s horizontal, no voltage will
be induced in the vertical coils because the coil sides will be
moving parallel to the lines of force and will not be cutting
any of them. The coils in positions B and B’ will be cutting
across a maximum number of flux lines and will therefore
have a maximum emf induced in them.
For the purpose of illustration, we shall assume that this
emf is 6 V. The coils at positions A, A’, C, and C’" will then
have an induced emf of approximately 4 V each. The result is
that three voltage-producing coils are connected in series in
each half of the armature.
Figure 10-105 shows a battery analogy of the armature
circuit. In each of the two series circuits in the armature, there
are two 4-V coils and one 6-V coil. The total emf from each
series is 14 V, and since the two circuits are connected in par-
allel, the amperage will be twice that of one series circuit.
The armature-winding arrangement illustrated in Figure
FIGURE 10-11 Armature reaction.
Chapter 10 Generators and Related Control Circuits
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14 V
10-10 1s known as progressive lap winding. There are sev-
eral different types of windings used for generators and mo-
tors, but the one shown here is adequate for the purpose of
this discussion.
Armature Reaction
Since an armature is wound with coils of wire, a magnetic
field is set up in the armature whenever a current flows in the
coils, as in Figure 10—11a. This field is at right angles to the
generator field shown in Figure 10-115 and is called cross
magnetization of the armature. The effect of the armature
field 1s to distort the generator field and shift the neutral
plane, as illustrated in Figure 10-11c. Remember, the neutral
plane 1s the position where the armature windings are moving
parallel to the magnetic flux lines. This effect is known as ar-
mature reaction and is proportional to the current flowing in
the armature coils.
The brushes of a generator must be set in the neutral plane;
that is, they must contact segments of the commutator that are
connected to armature coils having no induced emf. If the
brushes were contacting commutator segments outside the
neutral plane, they would short-circuit “live” coils and cause
arcing and loss of power. Armature reaction causes the neu-
tral plane to shift in the direction of rotation, and if the
brushes are in the neutral plane at no load, that is, when no ar-
fo =>
mature current is flowing, they will not be in the neutral plane
when armature current is flowing. For this reason it is desir-
able to incorporate a corrective system into the generator
There are two principal methods by which the effect of ar-
mature reaction is overcome. The first method is to shift the
position of the brushes so that they are in the neutral plane
when the generator is producing its normal load current. In
the other method, special field poles, called interpoles, are
installed in the generator to counteract the effect of armature
The brush-setting method is satisfactory in installations in
which the generator operates under a fairly constant load. If
the load varies to a marked degree, the neutral plane will shift
proportionately, and the brushes will not be in the correct po-
sition at all times. The brush-setting method is the most com-
mon means of correcting for armature reaction in small
generators (those producing approximately 1000 W or less).
Larger generators require the use of interpoles.
Interpoles. The use of interpoles is the most satisfac-
tory method for maintaining a constant neutral plane in a gen-
erator. The windings of the interpoles are in series with the
load; hence the interpole effect is proportional to the load.
The polarity of the interpoles is such that their effect is oppo-
site to that of the armature field; that is, each interpole is of the
same polarity as the next field pole in the direction of rota-
tion. With this polarity, the interpole may be said to pull the
generator field into the correct position. A typical interpole
system is shown in Figure 10-12.
In many generators a compensating winding is used to
help overcome armature reaction. This winding consists of
conductors embedded in the field-pole faces with one coil
surrounding sections of two field poles of opposite polarity
(see Figure 10-13). The compensating winding is in series
with the interpole windings; hence it works with the inter-
poles and increases their effectiveness. The sparkless com-
mutation obtained by the use of interpoles and a
compensating winding increases the life of the brushes and
commutator, reduces radio interference, and greatly im-
proves the efficiency of the generator.
FIGURE 10-13 Generator with interpoles and compensating
Aircraft dc generators have for the most part been replaced by
dc alternators on modern aircraft. However, there are still
several dc generators currently in operation on older aircraft.
Two of these generators are illustrated in Figure 10-14. One
of these is a high-output generator used on large aircraft with
heavy electrical loads; the other is a dc generator typical of
those found on smaller light aircraft.
Armature Assembly
The armature assembly (Figure 10-15) consists of a lami-
nated soft-iron core mounted on a steel shaft, the commutator
at one end of the assembly, and armature coils wound through
the slots of the armature core. The core is made of many soft-
iron laminations coated with an insulating varnish and then
stacked together. The purpose of the laminations is to elimi-
nate or reduce the eddy currents that would be induced in a
solid core. The effect of these currents was explained in
Chapter 9. The laminations for the armature core are stacked
together in such a manner that the slots are lined up so that the
armature coils can be placed in them. Before the coil wind-
ings are installed, insulating paper or fabric is placed in the
slots to protect the windings from wear and abrasion.
Insulated copper wire of a size large enough to carry the
maximum armature currents is wound in coils through the
slots of the armature.
Each end of the copper wire is then connected to a seg-
ment of the commutator. If the generator contains two
brushes, the armature wire ends connect 180° apart; if four
brushes are used, the winding ends connect to the commuta-
tor segments 90° apart.
After an armature is wound, the coils are held in place by
means of nonmetallic wedges placed in the slots. On some
models, bands of steel are placed around the armature to pre-
vent the windings from being thrown out by centrifugal force
when the armature is driven at high speeds.
DC Generator Construction 195
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COMMUTATOR oO FIGURE 10-16 Cross section of a commutator.
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Field-Frame Assembly
The heavy iron or steel housing that supports the field poles is
called the field frame, field ring, or field housing. It not only
supports the field poles but also forms a part of the magnetic
circuit of the field. The pole shoes are held in place by large
countersunk screws that pass through the housing and into
A | | e the shoes.
WOODRUFF 4 | | POLE SHOES / re RAND — Small generators usually have two to four poles mounted
ARMATURE in the field-frame assembly, and large generators can have as
DRIVE END FIELD COIL A many as eight main poles and eight interpoles. The pole
pieces are rectangular and in most instances are laminated.
FIGURE 10-14 Two typical dc generators. (a) High-output; The main shunt field windings consist of many turns of com-
(b) low-output. paratively small insulated copper wire. Series windings, such
as those on the interpoles, consist of a few turns of insulated
copper wire large enough to carry the entire load current
without overheating. A typical field-frame assembly 1s
shown in Figure 10-17.
FIGURE 10-15 A typical armature assembly.
The commutator consists of a number of copper segments
insulated from the armature structure, and from each other,
with mica. The segments are constructed to be held in place
by wedges located between the shaft and the segments. A
cross section of a typical commutator is shown in Figure
10-16. Each commutator segment has a riser to which is sol-
dered a lead from an armature coil. The surface of the com-
mutator is cut and ground to a very smooth cylindrical
surface. The mica insulation between the segments is under-
cut approximately 0.020 in. [0.051 cm] to make certain that it
does not interfere with the contact of the brushes with the
commutator. FIGURE 10-17 Field-frame assembly.
196 Chapter 10 Generators and Related Control Circuits
FIGURE 10-18 Brush holder assembly.
Brush Rigging
The brush rigging assembly (Figure 10-18) is located at the
commutator end of the generator. The brushes are small
blocks of a carbon and graphite compound soft enough to
give minimum commutator wear but sufficiently hard to pro-
vide long service. Special brushes have been designed for
generators operated at extremely high altitudes. These are
needed because arcing increases at high altitudes and will
cause the rapid deterioration of ordinary brushes.
As brushes wear, they slide in their metal holders and are
held firmly against the commutator by means of springs. The
tension of these springs should be sufficient to provide a
brush pressure of approximately 6 psi [41 kPa] of contact sur-
face. A flexible lead is connected from the brush to the brush
frame to ensure a good electrical connection. Brushes of sim-
ilar polarity are connected together electrically with a metal
strip or wire.
End Frames
The generator end frames support the armature bearings and
are mounted at each end of the field frame. The frame at the
commutator end of the generator also supports the brush
rigging assembly. The frame at the drive end is flanged to
provide a mounting structure. On some generators the end
frames are attached to the field-frame assembly by means of
long bolts extending entirely through the field frame. On oth-
ers the end frames are attached by machine screws into the
ends of the field frame.
Generator bearings are usually of the ball type, prelubri-
cated and sealed by the manufacturer. Prelubricated bearings
do not require any service except at overhaul or in case of
damage. The bearings fit snugly into the recesses in the end
frames and are held in place by retainers attached to the end
frames with screws.
Cooling Features
Since a generator operating at full capacity develops a large
amount of heat, it is necessary to provide cooling. This is
accomplished by means of passages leading through the gen-
erator housing between the field coils. In high-output genera-
tors there are also cooling air passages through the armature.
Cooling air is forced through the passages either by a fan
mounted on the generator shaft, or by pressure from a ram air
duct leading into an air scoop mounted on the end of the gen-
erator, or by bleed air from the compressor of a turbine en-
gine. Openings are provided in the end frame opposite the fan
or air fittings to allow the heated air to pass out of the genera-
tor housing.
A starter-generator is a combination of a generator and a
starter in one housing. A typical starter-generator is shown in
Figure 10-19. Starter-generators are typically employed on
small turboprop and turbine-powered aircraft such as the
Beechcraft King Air. Most starter-generators contain at least
two sets of field windings and only one armature winding.
While in the start mode, the starter-generator employs a low-
resistance series field. At this time a high current flows
through both the field and armature windings, producing the
high torque required for engine starting.
While in the generator mode, the starter-generator is capa-
ble of supplying current to the aircraft’s electrical system. A
typical starter-generator can supply a direct current of up to
300 A at 28.5 V while in the generator mode. To generate
electric power, the shunt winding of the starter-generator is
energized, and the series field is de-energized. The shunt
winding is a relatively high-resistance coil that produces the
magnetic field to induce voltage into the armature. The volt-
age produced in the armature sends current to the aircraft bus,
where it is distributed to the various loads of the aircraft.
FIGURE 10-19 A typical starter-generator. (Lear Siegler, Inc.,
Power Equipment Division)
Starter-Generators 197
It should be noted that several types of starter-generators
are currently in use. Some employ two separate field wind-
ings as stated above; others use only one (shunt) field wind-
ing. If only one field winding is used, special circuitry is
needed in the generator control unit (GCU) to increase start-
ing torque to an appropriate level. The technician should
become familiar with any specific starter-generator before
beginning maintenance procedures.
One advantage of the starter-generator 1s that only one
drive gear mechanism is used for both the start and the gener-
ator modes. Therefore, the starter motor drive gear need not
be engaged to or disengaged from the engine drive gear. Also,
the starter-generator reduces both size and weight as com-
pared to a conventional system that employs two units: a
starter and a generator. The main disadvantage of starter-
generators is that they are unable to maintain fuil output at a
low rpm. Most starter-generators therefore must be used on
turbine-powered aircraft that consistently maintain a rela-
tively high engine rpm.
Starter-Generator Components
Starter-generators are designed to provide torque for engine
starting and generate dc electric power for the aircraft’s elec-
trical systems. The starter-generator shown in Figure 10-19
contains a self-excited four-pole generator. Four interpoles
and a compensating winding are used to help overcome ar-
mature reaction. An integral fan is used to draw air through
the unit during rotation. The cooling air is required to main-
tain temperature limits during high power generation. A
clutch damper may be used on some units to connect the ar-
mature to the starter-generator’s drive shaft. This clutch pro-
vides friction damping of any torsional loads that may be
applied to the armature during operation. Changes in tor-
sional loads occur whenever the aircraft’s electric equipment
is turned on or off. If the armature is connected directly to the
engine, without a clutch, the torsional loads may overstress
the drive shaft and cause generator failure. Some starter.
generators employ a drive shaft shear section, which is used
to protect the engine’s gearbox in the event the generator me.
chanically fails and cannot rotate. In this situation the shear
section breaks (shears) and disconnects the generator from
the drive gear.
Ways to Monitor Generator Output
There are two ways to monitor the output of a generator. The
voltage produced by the generator can be indicated by a volt-
meter, or the current flow from the generator can be displayed
by an ammeter. One or both of these instruments are usually
mounted on the aircraft’s flight deck. If only one instrument
is used, the ammeter is preferred. A voltmeter should never
be used without an ammeter.
The ammeter can be placed in the generator output lead, as
shown in Figure 10-20q, or in the battery positive lead, as
shown in Figure 10—20b. Ammeters located in the generator
output lead measure only the current leaving the generator;
therefore, they are calibrated in a positive scale only. A 30-A
generator would require an ammeter scaled from 0 to 30 A,
Any reading on this type of ammeter indicates that current is
leaving the generator and flowing to the bus.
Ammeters located in the battery positive lead must be cal-
ibrated from a negative value to a positive value. A typical
ammeter of this type reads from — 60 A to O to +60 A. This
is necessary because the current can flow through the amme-
ter either to or from the battery. If the battery 1s discharging,
the ammeter will indicate a negative value. If the battery is
charging, the ammeter will indicate a positive value. When
this type of ammeter is used, the indications must be positive
when the charging system is working properly.
ITS ger
0-60 S
& Ч ос
==) - 60 TO O TO 60
FIGURE 10-20 Ammeter placement in a generator system. (a) Located in the generator output lead;
(b) located in the battery lead.
198 Chapter 10 Generators and Related Control Circuits
A voltmeter is often necessary to correctly monitor a gen-
erator’s output on multiengine aircraft. The voltage of an
operating generator must be slightly higher than battery volt-
age. This is necessary to ensure that the battery receives a
charging current from the generator. Generators produce
nearly 14 V for systems using 12-V batteries and 28 V for
systems using 24-V batteries. With multigenerator systems it
often becomes necessary to monitor the output voltage of
each generator in order to determine which, if any, generator
has failed. Some multiengine aircraft contain only one volt-
meter, which can measure either bus voltage, right generator
voltage, or leit generator voltage by means of a control
switch. This saves weight and space on the instrument panel.
Before making any determination as to the condition of a
generator or generator system, be sure to run the aircraft and
monitor all related instruments. At that time consider the two
different types of ammeters, specifically what they measure.
This procedure will help to ensure a proper system diagnosis.
Principles of Voltage Regulation
In the section of this chapter describing generator theory, it
was explained that the voltage produced by electromagnetic
induction depends on the number of lines of force being cut
per second by a conductor. In a generator, the voltage pro-
duced depends on three factors: (1) the speed at which the
armature rotates, (2) the number of conductors in series
in the armature, and (3) the strength of the magnetic field.
In order to maintain a constant voltage from a generator
under all conditions of speed and load, one of the foregoing
conditions must be varied in accordance with operational re-
It is obvious that the speed of a generator cannot be varied
according to load requirements if the generator is directly
driven by the engine. Also, it is impossible to change the
number of turns of wire in the armature during operation.
Therefore, the only practical way to regulate the generator
voltage is to control the strength of the field. This is easily
accomplished, because the strength of the field is determined
by the current flowing through the field coils, and this current
can be controlled by a variable resistor in the field circuit out-
side the generator.
The simplest type of voltage regulation is accomplished as
shown in Figure 10-21. In this arrangement a rheostat (vari-
able resistor) is placed in series with the shunt field circuit. If
the voltage rises above the desired value, the operator can re-
duce the field current with the rheostat, thus weakening the
field and lowering the generator voltage. An increase in volt-
age 1s obtained by reducing the field circuit resistance with
the rheostat. All methods of voltage regulation in aircraft
electrical systems employ the principle of a variable or inter-
mittent field resistance. Modern voltage regulators have been
developed to such a high degree of efficiency that the emf of
a generator will vary only a small fraction of a volt through-
out extreme ranges of load and speed.
Voltage regulators or controls for modern aircraft are
usually of the solid-state type; that is, they employ transistors
and diodes as controlling elements. Because there are still
mu PP
А- LOAD —”
FIGURE 10-21 A simple voltage regulator (a variable resistor).
many older airplanes in use that employ vibrator-type and
variable-resistance voltage regulators, we shall examine
these in the following sections.
Vibrator-Type Voltage Regulator
A generator system using a vibrator-type voltage regulator is
shown in Figure 10—22. A resistance that is intermittently cut
in and out of the field circuit by means of vibrating contact
points 1s placed in series with the field circuit. The contact
points are controlled by a voltage coil connected in parallel
with the generator output. When the generator voltage rises
to the desired value, the voltage coil produces a magnetic
field strong enough to open the contact points. When the
points are open, the field current must pass through the resis-
tance. This causes a substantial reduction in field current,
with the result that the magnetic field in the generator is
weakened. The generator voltage then drops immediately,
causing the voltage coil electromagnet to lose strength so that
a spring can close to the contact points. This allows the gen-
erator voltage to rise, and the cycle is then repeated. The con-
tact points open and close many times a second, but the actual
time that they are open depends on the load being carried by
the generator and the generator (engine) rpm. As the genera-
tor load is increased, the time that the contact points remain
closed increases, and the time that they are open decreases.
Adjustment of the generator voltage is made by increasing or
decreasing the tension of the spring that controls the contact
Often temperature greatly affects the generator output;
therefore, this adjustment should be made only under specific
conditions set up by the manufacturer.
A+ LOAD + _
A 4 |
i ?
FIGURE 10-22 Vibrator-type regulator circuit.
Generator Control 199
Because the contact points do not burn or pit appreciably,
vibrator-type voltage regulators are satisfactory for genera-
tors that require a low field current. In a system in which the
generator field requires a current as high as 8 A, the vibrating
contact points would soon burn and probably fuse together.
For this reason a different type of regulator is required for
heavy-duty generator systems.
If the regulating resistance becomes disconnected or
burned out, the generator voltage will fluctuate, and exces-
sive arcing will occur at the contact points. When inspecting
a vibrator-type voltage regulator, make sure that the connec-
tions to the resistance are secure and that the resistance is in
good condition.
Carbon-Pile Voltage Regulator
The carbon-pile voltage regulator derives its name from the
fact that the regulating element (variable resistance) consists
of a stack, or pile, of carbon disks (see Figure 10-23).
Usually, the carbon pile has alternate hard carbon and soft
carbon (graphite) disks contained in a ceramic tube with a
carbon or metal contact plug at each end. At one end of the
pile, a number of radially arranged leaf springs exert pressure
against the contact plug, thus keeping the disks pressed
firmly together. For as long as the disks are compressed, the
resistance of the pile is very low. If the pressure on the carbon
pile is reduced, the resistance increases. By placing an elec-
tromagnet in a position where it will release the spring pres-
sure on the disks as the voltage rises above a predetermined
value, a stable and efficient voltage regulator 15 obtained.
The carbon-pile voltage regulator is connected in a gener-
ator system the same way any other regulator is connected,
that is, with a resistance in the field circuit and an electro-
magnet to control the resistance. The carbon pile is in series
with the generator field, and the voltage coil 1s shunted across
the generator output. A small manually operated rheostat is
connected in series with the voltage coil to provide for a lim-
ited amount of adjustment, which is necessary when two or
more generators are connected in parallel to the same electri-
cal system.
Equalizing Circuit
When two or more generators are connected in parallel to a
power system, the generators should share the load equally. If
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FIGURE 10-23 Carbon-pile voltage regulator circuit.
200 Chapter 10 Generators and Related Control Circuits
the voltage of one generator is slightly higher than that of the
other generators in parallel, that generator will assume the
greater part of the load. For this reason an equalizing circuit
must be provided that will cause the load to be distributed
evenly among the generators. An equalizing circuit includes
an equalizing coil wound with the voltage coil in each of the
voltage regulators, an equalizing bus to which all equalizing
circuits are connected, and a low-resistance shunt in the
ground lead of each generator (see Figure 10-24). The equal-
izing coil will either strengthen or weaken the effect of the
voltage coil, depending on the direction of current flow
through the equalizing circuit. The low-resistance shunt in
the ground lead of each generator causes a difference in po-
tential between the negative terminals of the generators that
is proportional to the difference in load current. The shunt is
of such a value that there will be a potential difference of
0.5 V across it at maximum generator load.
Assume that generator 1 in Figure 10-24 is delivering
200 A (full load) and that generator 2 is delivering 100 A
(half load). Under these conditions there will be a potential
difference of 0.5 V across the shunt of generator 1 and 0.25 V
across the shunt of generator 2. This will make a net potential
difference of 0.25 V between the negative terminals of the
generators. Since the equalizing circuit 1s connected between
these points, a current will flow through the circuit. The cur-
rent flowing through the equalizing coil of voltage regulator
1 will be in a direction to strengthen the effect of the voltage
coil. This will cause more resistance to be placed in the field
circuit of generator 1, thus weakening the field strength and
causing the voltage to be reduced. The drop in voltage will
result in the generator taking less load. The current flowing
through the equalizing coil of voltage regulator 2 will be in a
direction to oppose the effect of the voltage coil, thus causing
a decrease in the resistance in the field circuit of generator 2.
The generator voltage will increase because of increased cur-
rent in the field windings, and the generator will take more of
the load. To summarize, the effect of an equalizing circuit 1s
to lower the voltage of a generator that is taking too much of
the load and to increase the voltage of the generator that 1s not
taking its share of the load.
1 BUS —
FIGURE 10-24 Equalizing circuit.
Equalizing circuits can correct for only small differences
in generator voltage; hence the generators should be adjusted
to be as nearly equal in voltage as possible. If the generator
voltages are adjusted so that there is a difference of less than
0.5 V between any of them, the equalizing circuit will main-
tain a satisfactory load balance. A periodic inspection of the
ammeters should be made during flight to see that the gener-
ator loads are remaining properly balanced.
Reverse-Current Cutout Relay
In every system in which the generator is used to charge bat-
teries as well as to supply operating power, an automatic
means must be provided for disconnecting the generator from
the battery when the generator voltage is lower than the bat-
tery voltage. If this is not done, the battery will discharge
through the generator and may burn out the armature. Nu-
merous devices have been manufactured for the purpose of
automatically disconnecting the generator, the simplest being
the reverse-current cutout relay. Figure 10-25 1s a sche-
matic diagram illustrating the operation of such a relay.
A voltage coil and a current coil are wound on the same
soft-iron core. The voltage coil has many turns of fine wire
and is connected in parallel with the generator output; that is,
one end of the voltage winding is connected to the positive
side of the generator output, and the other end of the winding
is connected to ground, which is the negative side of the gen-
erator output. This is clearly shown in the diagram. The cur-
rent coil consists of a few turns of large wire connected in
series with the generator output; hence it must carry the entire
load current of the generator. A pair of heavy contact points is
placed where it will be controlled by the magnetic field of the
soft-iron core. When the generator is not operating, these
contact points are held in an open position by a spring.
When the generator voltage reaches a value slightly above
that of the battery in the system, the voltage coil in the relay
magnetizes the soft-iron core sufficiently to overcome the
spring tension. The magnetic field closes the contact points
and thus connects the generator to the electrical system of the
airplane. As long as the generator voltage remains higher
than the battery voltage, the current flow through the current
coil will be in a direction that aids the voltage coil in keeping
the points closed. This means that the field of the current coil
A+ r= 1
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a- VOLTAGE | | e —
FIGURE 10-25 Reverse-current cutout relay circuit.
Will be in the same direction as the magnetic field of the volt-
age coil and that the two will strengthen each other.
When an airplane engine 1s slowed down or stopped, the
generator voltage will decrease and fall below that of the bat-
tery. In this case the battery voltage will cause current to start
flowing toward the generator through the relay current coil.
When this happens, the current flow will be in a direction that
creates a field opposing the field of the voltage winding. This
results in a weakening of the total field of the relay, and the
contact points are opened by the spring, thus disconnecting
the generator from the battery. The contact points may not
open in normal operation until the reverse current has
reached a value of 5 to 10 A.
Generally speaking, the tension of the spring controlling
the contact points should be adjusted so that the points will
close at approximately 13.5 Vina 12-V system and at 26.6 to
27 V in a 24-V system.
Current Limiter
In some generator systems a device is installed that will re-
duce the generator voltage whenever the maximum safe load
is exceeded. This device is called a current limiter and is de-
signed to protect the generator from loads that will cause it to
overheat and eventually burn the insulation and windings.
The current limiter operates on a principle similar to that
of the vibrator-type voltage regulator. Instead of having a
voltage coil to regulate the resistance in the field circuit of the
generator, the current limiter has a current coil connected in
series with the generator load circuit (see Figure 10-26).
When the load current becomes excessive, the current coil
magnetizes the iron core sufficiently to open the contact
points and add a resistance to the generator field circuit. This
causes the generator voltage to decrease, with a correspond-
ing decrease in generator current. Since the magnetism pro-
duced by the current-limiter coil is proportional to the current
flowing through it, the decrease in generator load current also
weakens the magnetic field of the current coil and thus per-
mits the contact points to close. This removes the resistance
from the generator field circuit and allows the voltage to rise
again. If an excessive load remains connected to the genera-
tor, the contacts of the current limiter will continue to vibrate,
thus holding the current output at or below the minimum safe
mo ver lan
CURRENT cutout
Lr | |
FIGURE 10-26 Current-limiter circuit.
Generator Control 201
limit. The contact points are usually set to open when the
current flow is 10 percent above the rated capacity of the
The current limiter described above should not be con-
fused with the fuse-type current limiter. The fuse-type limiter
is merely a high-capacity fuse that permits a short period of
overload in a circuit before the fuse link melts and breaks the
Two-Unit Control Panel
Generator systems for light aircraft often have the generator
control units mounted on a single panel. When the voltage
regulator and reverse-current cutout relay are mounted on a
single panel, the combination is called a two-unit control
panel or box (see Figure 10-27). In the voltage regulator on
this panel, an extra coil, called an accelerator winding, is
wound with the voltage coil. This coil is connected in series
with the field-regulating resistance and is wound in a direc-
tion opposite to that of the voltage winding. Its purpose is to
reduce the magnetism of the core when the contact points
open; this reduction causes the points to close more quickly
than they would without the neutralizing effect of a reverse
coil. The result of this arrangement is that the contact points
vibrate more rapidly and produce a steadier voltage from the
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FIGURE 10-27 Two-unit generator control.
Three-Unit Control Panel
A three-unit control panel consists of a voltage regulator, a
current limiter, and a reverse-current cutout relay mounted op
a single panel (see Figure 10-28). This combination will pro-
vide for both voltage regulation and protection from exces.
sive loads. A photograph of such a panel with the cover off is
shown in Figure 10-29.
The three-unit control panel has proved very successful
for the control of 12- and 24-V generator systems. Because of
its dependability and low cost, it was used almost exclusively
in light-aircraft generator systems before the development of
transistor regulators.
The wiring diagram in Figure 10-28 is only one of several
possible arrangements. In some systems the voltage regulator
1s placed in the ground side of the generator field circuit, but
the results are the same in either case.
Starter-Generator Control Systems
The control circuits of a starter-generator are relatively com-
plex; they must control current for both starting and generat-
ing operations. These circuits are contained in a device that is
often referred to as a generator control unit (GCU). The
GCU discussed here and some of the information presented
were produced by Lear Siegler, Power Equipment Division;
this GCU 1s used in conjunction with the starter-generator
discussed earlier. The GCU is pictured in Figure 10-30; it
contains a voltage regulator, the various control circuits for
the start and generator modes, and protection circuits used
during abnormal operating conditions. The GCU electronic
components are contained on three printed circuit boards.
Each board is mounted on a forged aluminum base, which
acts as a heat sink for those components which must dissipate
heat. The entire unit is enclosed by an aluminum cover that
allows for all external connections to be exposed.
GCU Functions
The start mode of a starter-generator is controlled through a
circuit independent of the GCU. During starting, battery or
auxiliary power unit (APU) power is sent to the starter-gener-
ator via a starter contactor. The starter contactor is energized
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FIGURE 10-28 Three-unit generator control.
Chapter 10 Generators and Related Control Circuits
FIGURE 10-30 A typical generator control unit for a starter-
generator. (Lear Siegler, Inc, Power Equipment Division)
by the engine start switch on the flight deck. During the gen-
erator mode, the GCU controls the generator output, genera- .
tor and system protection, and self-test functions. If a fault is
detected in the generator system, the GCU will illuminate the
appropriate annunciator fail light. The generator control unit
is capable of performing the following 10 functions.
1. Voltage Regulation The voltage regulator section of
the GCU maintains a constant generator voltage under vari-
ous loads, temperatures, and rotational speeds. The current of
the generator field circuit is controlled through the field tran-
sistor. This transistor varies the field current pulse time to
vary generator output.
2. Generator Line Contactor Control The generator
line contactor control provides a means of connecting the
generator output to the aircraft’s dc load bus. This circuit op-
erates with a time delay to ensure that generator voltage is
nearly equal to bus voltage immediately following initial en-
gine starting. Several inhibiting signals are also employed to
ensure proper contactor positioning (open or closed) when
failure conditions are sensed.
3. Overvoltage Protection The overvoltage protection
circuit prevents damage to aircraft equipment in the event an
excessive generator output occurs. If the generator output
voltage exceeds the preset limits, an integrator starts to func-
tion. This integrator is used as an inverse time delay, so that a
slight overvoltage condition is allowed for a much longer
time than a severe one before a trip occurs. In this way, un-
usually large but momentary voltage transients will not cause
a nuisance trip of the field relay. If a severe overvoltage is
sensed, the generator is de-energized and the line contactor is
opened. A completely separate circuit is used to open the
generator line contactor as soon as the voltage exceeds 40 V
dc. This feature not only provides redundant protection for
utilization equipment but also allows a faster response of the
line contactor after a failure. Unlike the overvoltage with in-
verse time delay, this function is not latched, so that manual
reset is not required after a temporary overvoltage condition.
4. Overload and Undervoltage Protection The over-
load and undervoltage protective functions cooperate to de-
energize the system in the event of an overload condition. An
overload condition is sensed by the GCU as either a genera-
tor overcurrent condition as indicated by an excessive gener-
ator interpole voltage or an undervoltage condition. When
the GCU senses either condition, an internal time delay 1s ini-
tiated. If the overload condition continues for a period of ap-
proximately 10 s, the GCU trips the field relay, de-energizing
the generator and opening the line contactor.
5. Reverse-Current and Differential Voltage Protection
The reverse-current protection function senses generator in-
terpole voltage to determine whether the generator is acting
as a load on the system rather than a power source. If because
of a failure or during a normal engine shutdown current be-
gins to flow into the generator, this is sensed and the line con-
tactor is opened. An inverse time delay is used to quickly
open the contactor under severe conditions, while more time
is allowed during normal shutdowns. This prevents needless
cycling of the contactor during a transient condition. The cir-
cuit is not latched, and so no reset is required to reclose the
contactor after reverse current is sensed. The contactor is held
open owing to differential voltage sensing once reverse cur-
rent has been detected. The differential voltage function also
operates on generator buildup to keep the generator line con-
tactor from closing until the generator output voltage is
within 0.5 V dc of the bus voltage.
6. Reverse-Polarity Protection The reverse-polarity
protection function protects the utilization equipment from
reverse-polarity buildup of the generator. This protection
trips the field relay to de-energize the generator.
7. Anticycle Protection The anticycle protection fea-
ture prevents more than one reset attempt of the generator
field relay for each activation of the generator control switch.
Because the generator output voltage is used for GCU control
power, and this voltage disappears after a trip, the system
Generator Control 203
mem 0e 5 rm 7 RE Ly
“would repetitively build up in voltage and trip again if a fault
existed in the system.
8. Latching Field Relay Control A magnetic latching
field relay is used to de-energize the generator after a fault
condition has been sensed. The field relay is used to de-ener-
gize the generator by opening the generator shunt field exci-
tation path and open the line contactor by opening its power
input. The field relay is tripped by a protection function such
as overvoltage, overload, undervoltage, reverse polarity, or
open ground wire sensing; it may also be tripped by an exter-
nal switch applying a ground signal to the GCU.
9. Flash and Start Relay Control The field-flashing
relay and the associated circuitry ensure that the generator
output can be built up from the residual voltage without help
from any other power source. The residual voltage bootstraps
the generator upward to a point where the field relay is reset
and then to a higher voltage to energize the field-flashing
relay. Once the field-flashing relay is energized, the field flash
path is broken, but the normal voltage regulator circuitry is
able to operate at this voltage level, and so the generator con-
tinues to build up to the normal operating voltage.
10. Overvoltage and Overload Protection Self-Test
Provisions are made within the GCU to enable it to periodi-
cally exercise the overvoltage, overload, and undervoltage
protection circuits. A passive failure of the circuitry would
not otherwise be discovered until that function was required
to operate. If an external test switch applies generator output
voltage to the GCU, the protection will be biased to a point
where it will operate, even though normal voltage appears on
the generator output. If a trip of the channel results within a
few seconds after the voltage has been applied, the circuit 1s
working correctly.
Generator Load Balancing
When it is desired to balance the load among the generators in
any system, the technician should always follow the proce-
dure set forth by the manufacturer of the aircraft. This pro-
cedure will be found in the manufacturer’s service or
maintenance manual.
The balance procedure is usually begun by checking all
generators with a precision voltmeter. This is done after the
generators and engines are warmed up to normal operating
temperature. Under these conditions all generators are ad-
justed to exactly the same output voltage (28 to 29 V for a
24-V system). A substantial load is then turned on, and the
ammeters are examined. All generator loads should be within
+ 10 percent of one another. If the generator loads are not
within these limits, the generator with the greatest error
should be adjusted first. A small movement of the paralleling
potentiometer on the voltage regulator should produce an in-
stant change in the load current for the generator being ad-
justed. If the load on one generator is reduced, the other
204 Chapter 10 Generators and Related Control Circuits
generators will pick up load. All ammeters should be
watched while the adjustments are being made.
If the aircraft generators cannot be properly balanced, the
system should be repaired before the aircraft is returned to
service. The process of balancing generators is often referred
to as paralleling the generators. If both generators are pro-
ducing equal voltage, they will carry equal current loads if
connected in parallel.
Generator Troubleshooting
The first step in troubleshooting a generator circuit on an air-
plane is to determine what type of system is in use. If the sys-
tem is on a small airplane, it is likely that the control unit is a
three-element type, that is, that it contains a voltage regulator,
areverse-current cutout relay, and a current limiter. If the sys-
tem contains a 24-V generator or starter-generator, it is likely
that a more complex system of control is employed. Always
refer to the current manufacturer’s data and electrical wiring
diagrams prior to any troubleshooting. |
The two most likely indications of generator system fail
ure are (1) no or low voltage and (2) battery discharge. If the
aircraft's ammeter indicates a battery discharge, the genera-
tor may not be producing the proper voltage. The voltage at
the generator’s output terminal can then be measured, and if
approximately 2 to 6 V is present with the system operating,
the generator is operating from residual magnetism only. This
means that there is no current through the generator field coils
and some component of the field circuit is defective. Likely
suspects include wiring connections, the generator master
switch, the voltage regulator, and the generator’s field. If zero
volts is measured between the generator’s output terminal
and ground with the system operating, the generator has lost
its residual magnetism.
If it becomes necessary to determine if the voltage regula-
tor or the generator is defective, simply bypass the voltage
regulator circuit. This is done by connecting a voltage di-
rectly to the generator’s field circuit and monitoring genera-
tor output. If current is fed into the field of a rotating
generator, that generator should produce a normal or above-
normal output voltage. Since there are two different methods
of wiring a generator’s field, one must first determine which
method is being used. With one method, the field positive is
connected to the voltage regulator, as illustrated in Figure
10-3la; with the other method, the field negative is con-
nected to the voltage regulator, as shown in Figure 10-315.
To bypass the voltage regulator, the correct voltage signal
(positive or negative) must be connected to the generator F
terminal with the regulator disconnected and the generator
armature rotating. If the voltage measured at the generator
output terminal to ground is less than system voltage, the
generator is defective. If the generator output voltage 1s at or
above the normal value, the generator is good and the voltage
regulator most likely is defective. There may be some defec-
tive conditions that this test will not detect; however, for the
most part, bypassing the regulator is valid. Many factors must
be considered when one is troubleshooting a generator sys-
tem; always become familiar with the system before trou-
bleshooting begins. |
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