PIA TRAINING CENTRE (PTC) Module 11A – AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS

Category – B1.1 Sub Module 11.6 - Electrical Power

MODULE 11A
Sub Module 11.6

ELECTRICAL POWER (ATA 24)

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 PIA TRAINING CENTRE (PTC) Module 11A – AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS
Category – B1.1 Sub Module 11.6 - Electrical Power

Contents
BATTERY INSTALLATIONS ----------------------------------------------1 PROTECTION CIRCUITS ----------------------------------------------- 53
BATTERY SYSTEMS -------------------------------------------------------4 OVERVOLTAGE PROTECTION --------------------------------------- 53
BATTERY CHARGING FROM EXTERNAL POWER --------------6 UNDERVOLTAGE PROTECTION ------------------------------------ 53
“ON-BOARD" BATTERY CHARGER UNITS -------------------------8 OVER-EXCITATION AND UNDER-EXCITATION
DC POWER GENERATION --------------------------------------------- 15 PROTECTION--------------------------------------------------------------- 55

DC GENERATOR CONTROL & PROTECTION ------------------ 18 UNDERFREQUENCY AND OVERFREQUENCY

PROTECTION--------------------------------------------------------------- 55
VOLTAGE REGULATION ----------------------------------------------- 18
DIFFERENTIAL CURRENT PROTECTION ------------------------ 55
VIBRATOR TYPE VOLTAGE REGULATOR ----------------------- 19
MERZ-PRICE PROTECTION SYSTEM ----------------------------- 57
CARBON PILE VOLTAGE REGULATOR --------------------------- 20
SOLID-STATE GCU ------------------------------------------------------- 58
PARALLEL OPERATION OF DC GENERATORS ---------------- 21
VOLTAGE-REGULATOR SECTION ---------------------------------- 58
EQUALIZING CIRCUITS ------------------------------------------------- 21
CONTROL SECTION ----------------------------------------------------- 60
REVERSE CURRENT PROTECTION ------------------------------- 23
PROTECTIVE FUNCTIONS -------------------------------------------- 60
CURRENT LIMITER ------------------------------------------------------- 30
FAULT ANNUNCIATION ------------------------------------------------- 60
TRANSISTOR VOLTAGE REGULATORS -------------------------- 31
CIRCUIT PROTECTION ------------------------------------------------- 61
AC POWER GENERATION --------------------------------------------- 34
EXTERNAL AND GROUND POWER -------------------------------- 67
POWER SOURCES ------------------------------------------------------- 41
D.C. SYSTEMS ------------------------------------------------------------- 67
AC GENERATOR CONTROL & PROTECTION------------------- 41
A.C. SYSTEMS ------------------------------------------------------------- 73
VOLTAGE REGULATION ----------------------------------------------- 41
AUXILIARY POWER UNITS -------------------------------------------- 76
LOAD SHARING OR PARALLELING OF CONSTANT
FREQUENCY SYSTEMS ------------------------------------------------ 44 EMERGENCY POWER GENERATION ----------------------------- 79

REAL LOAD-SHARING -------------------------------------------------- 44 APU GENERATORS ------------------------------------------------------ 79

REACTIVE LOAD-SHARING ------------------------------------------- 48 INVERTERS ----------------------------------------------------------------- 79

SYNCHRONIZING LIGHTS --------------------------------------------- 50 ROTARY INVERTERS---------------------------------------------------- 80

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Category – B1.1 Sub Module 11.6 - Electrical Power

STATIC INVERTERS ----------------------------------------------------- 80

TRANSFORMER RECTIFIER UNITS -------------------------------- 83
POWER DISTRIBUTION SYSTEMS --------------------------------- 85
A SIMPLE ELECTRICAL SYSTEM ----------------------------------- 86
MAIN POWER DISTRIBUTION SYSTEMS ------------------------- 88
SINGLE-ENGINE AIRCRAFT ------------------------------------------ 88
TWIN-ENGINE AIRCRAFT ---------------------------------------------- 88
POWER DISTRIBUTION ON COMPOSITE AIRCRAFT -------- 95
LARGE-AIRCRAFT ELECTRICAL SYSTEMS --------------------- 97
POWER DISTRIBUTION SYSTEMS --------------------------------- 97
THE SPLIT-BUS SYSTEM ---------------------------------------------- 98
PARALLEL ELECTRICAL SYSTEMS ------------------------------ 103
SPLIT PARALLEL SYSTEM ------------------------------------------ 106
POWER DISTRIBUTION HIERARCHY ---------------------------- 109
CONTROL OF THE POWER DISTRIBUTION SYSTEMS --- 109

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Category – B1.1 Sub Module 11.6 - Electrical Power

BATTERY INSTALLATIONS
Some systems use an acid trap. These traps are bottles
Depending on the requirements for the operation of the inserted in the line between the battery and fuselage which
essential services under emergency conditions a single battery have a neutralizing agent in them to prevent acid being sprayed
or several batteries (usually connected in parallel) are used. on to the aircraft fuselage.
The batteries are installed in areas where adequate heat
dissipation will occur and ventilation of gases can take place.
They are normally mounted and clamped to a tray, which is
secured to the aircraft structure.
The gasses generated by the battery are usually vented to
atmosphere via holes in the side of the fuselage. One such
method uses non-corrodible piping connected from the battery
to the outside vent. The airflow outside causes a venture effect
which draws the air from battery.

Fig.2BATTERY VENTING

Most modern aircraft use the effect of pressurization to cause

air flow across the top of the battery to atmosphere.

Batteries are usually connected to the aircraft DC system using

a single screw attached two pole plug. A typical battery
connector is shown in figure 3. Clockwise rotation of hand wheel
disengages the sockets and pins. Note also the electrical
connection to the internal thermostat, which monitors the
temperature of the battery.

The battery will have structural clamps to secure it in position.

Fig.1 BATTERY VENTING

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Fig.4 BAe 146 INSTALLATIONS

Fig.3 BATTERY CONNECTORS The batteries consist of twenty individual cells linked in series
and assembled in a steel case with a detachable lid. Each cell
Figure 4 shows the battery installation of BAe 146. There are vent incorporates a safety valve set to avoid internal pressure
two 24V 23Ah nickel cadmium batteries which supply their build-up beyond the limits of cell. Inside the battery case, the
respective battery bus bars (hot bus) at all times and to provide cells are held tightly packed by insulator limiting. Vertical
power to emergency DC bus bar (essential bus) should there be movement of the cells is further restrained by a molded silicon
a failure of all generated power. rubber linear attached to the inside of the lid assembly. The lid
assembly also includes a non-return valve and an integral hold
down bar and strap which accepts hold down securing
attachments.
The non –return valve, in conjunction with a ventilation pipe
fitted at the side of the battery case, provides passage for
cooling air and the extraction of battery gasses.

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Category – B1.1 Sub Module 11.6 - Electrical Power

Battery temperature sensing is provided by two thermostats, The main battery/battery charger provides a dedicated source of
one set at 57ºC to give a flight deck indication of HI TEMP and DC power for the operation of standby and auto-land systems.
the other set at 71ºC will inhibit the battery supply to the A separate APU battery/battery charger provides power for the
emergency DC bus bar. APU start.
Electrical connection is by an ELCON quick release connectors, The main and APU batteries are identical 20 cell Ni-Cad
and a six-pin connector on the battery front face connects the batteries with individual cell venting pressure at2psi to 10psi. A
temperature sensing elements to their respective circuits. Thermistor thermal sensor provides the battery charger with
Figure 5 shows the battery/battery charger layout for a Boeing battery temperature information. If the battery reaches a set
757 aircraft. temperature the battery charger is de-energized.
The chargers are identical and have an input of 115V, 400Hz, 3
phase. They have forced air and convection cooling and can be
used as an alternative 28V DC supply.
Figure 6, shows the battery shunts and current monitors. The
main battery shunt is connected on the ground-side of the main
battery and the APU battery shunt is connected on the ground
side of the APU battery charger.

Fig.6 BATTERY LOCATIONS

Fig.5 B757 INSTALLATION

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Category – B1.1 Sub Module 11.6 - Electrical Power

BATTERY SYSTEMS

Figure 1,shows the circuit arrangement for a battery system, Under emergency conditions, e.g. a failure of the generator
which is employed in a current type of turboprop airliner; the supply or main busbar occurs, the batteries must be isolated
circuit serves as a general guide to the methods adopted. Four from the main busbar since their total capacity is not sufficient
batteries, in parallel are directly connected to a battery busbar tokeep all services in operation. The power selector switch must
which, in the event of an emergency, supplies power for a therefore be put to the "off" position, thus de-energizing the
battery relay. The batteries then supply the essential services
limited period to essential consumer services, i.e. radio, fire
for the time period pre-calculated on the basis of battery
warning and extinguishing systems, a compass system, etc. capacity and current consumption of the essential services.
Direct connections are made to ensure that battery power is
available at the busbar at all times. The reverse current circuit breaker in the system shown is of
the electromagnetic type and its purpose is to protect the
The batteries also require to be connected to ensure that they
are maintained in a charged condition. In the example illustrated batteries against heavy current flow from the main busbar.
this is accomplished by connecting the batteries to the main d.c. Should this happen the current reverses the magnetic field
Busbar via a battery relay, power selector switch and a reverse causing the normally closed contacts to open and thereby
current circuit breaker. interrupt the circuit between the batteries and main busbar, and
the battery relay coil circuit.
Under normal operating conditions of the d.c. supply system,
the power selector switch is set to the "battery" position (in
some aircraft thus may be termed the "flight" position) and, as
will be noted, current flows from the batteries through the coil of
the battery relay, the switch, and then to ground via the reverse
current circuit breaker contacts. The current flow through the
relay coil energizes it, causing the contacts to close thereby
connecting the batteries to the main busbar via the coil and
second set of contacts of the reverse current circuit breaker.
The d.c. services connected to the main busbar are supplied by
the generators and so the batteries will also be supplied with
charging current from this source.

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 7: Circuit arrangement for a battery system

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Category – B1.1 Sub Module 11.6 - Electrical Power

The battery system in some types of turboprop-powered aircraft The power selector switches are left in the "battery" position so
is so designed that the batteries may be switched from a- that when the engine-driven generator is switched onto the
parallel configuration to a series configuration for the purpose of busbar, charging current can flow to the batteries.
starting an engine from the batteries. The circuit arrangement
of one such system using two 24-volt nickel-cadmium batteries BATTERY CHARGING FROM EXTERNAL POWER
is shown in simplified form in Fig7.
In some single-engine aircraft systems, the battery may be
Under normal parallel operating conditions, battery 1 is charged when an external power unit is plugged into the aircraft.
connected to the battery busbar via its own battery relay, and This is achieved by a battery relay closing circuit connected
also contacts 1 a-1 b of a battery-switching relay. Battery 2 is across the main contacts of the relay as shown in Fig. 8.
directly connected to the busbar via its relay.
With the external power connected and switched on, power is
When it is necessary to use the batteries for starting an engine, available to the battery relay output terminal via the closed
i.e. to make an "internal" start, both batteries are first connected contacts of the external power relay. At the same time, power is
to the battery busbar in the normal way, and the 24-volt supply applied to the battery relay closing circuit via its diode and
is fed to the starter circuit switch from the busbar. Closing of resistor which reduces the voltage to the input side of the
the starter switch energizes the corresponding starter relay, battery relay's main contacts and coil.
and at the same time the 24-volt supply is fed via the starting
circuit, to the coil of the battery switching relay thereby When the battery master switch is selected to "on", sufficient
energizing it. Contacts 1a-1b of the relay is now opened to current flows through the coil of the battery relay to energize it.
interrupt the direct connection between battery 1 and the The closed contacts of the relay then allow full voltage from the
busbar. Contacts 3a-3b are also opened to interrupt the external power unit to flow to the battery for the purpose.
grounded side of battery 2. However, since contacts 2a-2b of
the switching relay are simultaneously moved to the closed
position, they connect both batteries in series so that 48 volts is
supplied to the busbar and to the starter motor.

After the engine has started and reached self-sustaining speed,

the starter relay automatically de-energizes and the battery
switching relay coil circuit is interrupted to return the batteries to
their normal parallel circuit configuration.

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 8: Battery Charging From External Power

Figure 1: Connecting Parallel Battery To Series

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Category – B1.1 Sub Module 11.6 - Electrical Power

“ON-BOARD" BATTERY CHARGER UNITS

In most types of turbojet transport aircraft currently in service,

the battery system incorporates a separate unit for maintaining The charger/battery relay is of the dual type, one relay being
the batteries in a stage of charge. Temperature-sensing a.c. operated and the other d.c. operated. The a.c. relay-coil is
elements are also normally provided in order to automatically supplied with power from one phase of the main three-phase
isolate the charging circuit whenever there is a tendency for supply to the battery charger, and as will be noted from the
battery overheating to occur. The circuits of "on-board" charger diagram, the relay is energized by current passing to ground via
units as they are generally termed, vary between aircraft types, the contacts B1-B2 of the sensing relays, the battery switch and
and space limits description of all of them. We may however, the emergency switch. Energizing of the relay closes the upper
consider two examples, which highlight some of the variations set of contacts (A1-A2) to connect the d.c. positive output from
to be found. the battery charger to the batteries, thereby supplying them with
charging current.
The much-simplified circuit shown in Figure 9 is based on the
system adopted for the McDonnell Douglas DC-10. In this
particular application, the required output of 28 volts is achieved
by connecting two 14-volt batteries in series. Furthermore, and
unlike the system shown above, the batteries are only
connected to the battery busbar whenever the normal d.c.
Supply (in this case from transformer/ rectifier units) is not
available. Connection to the busbar and to the charger unit is
done automatically by means of a "charger/battery" relay and
by sensing relays.

When power is available from the main generating system,

d.c.is supplied to the battery busbar from a transformer/rectifier
unit and, at the same time, to the coils of the sensing relays.
With the relays energized, the circuit through contacts A2-A3 is
interrupted while the circuits through contacts B1-B2 are made.
The battery switch, which controls the operation of the
charger/battery relay, is closed to the "batt" position when the
main electrical power is available, and the emergency power
switch is closed in the "off” position.
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Category – B1.1 Sub Module 11.6 - Electrical Power

In the event of main power failure, the battery charger will

become inoperative, the a.c. charger relay will de-energize to
the center off position, and the two sensing relays will also de-
energize, thereby opening the contacts B1-B2 and closing the
contacts A2-A3. The closing of contacts A2-A3 now permits a
positive supply to flow direct from the battery to the coil of the
d.c. battery relay, which on being energized also actuates the
a.c. relay, thereby closing contacts B1-B2 which connect the
batteries direct to the battery busbar. The function of the
battery relay contacts is to connect a supply from the battery
busbar to the relays of an emergency warning light circuit. The
charging unit converts the main three-phase supply of 115/ 200
volts a.c. into a controlled d.c. output at constant current and
voltage, via a transformer and a full-wave rectifying bridge
circuit made up of silicon rectifiers and silicon controlled
rectifiers. The charging current is limited to approximately 65 A,
and in order to monitor this and the output voltage as a function
of battery temperature and voltage, temperature-sensing
elements within the batteries are connected to the S.C.R.
"gates" via a temperature and reference voltage control circuit,
and a logic circuit. Thus, any tendency for overcharging and
overheating to occur is checked by such a value of gate circuit
current as will cause the S.C.R to switch off the charging current
supply.

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 9: On-board Battery Charger System

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Category – B1.1 Sub Module 11.6 - Electrical Power
pulsed charge and the process is repeated. The pulsing
continues until the control circuits within the charger change.

The d.c. supply for battery charging is obtained from a The operation to the low mode, approximately two minutes after
transformer rectifier unit (TRU) within the charger, and it pulse charging commences.
maintains cell voltage levels in two modes of operation: High
and Low. Under normal operating conditions of the aircraft's The second example shown in Figs. 11 and 12, are based on
power generation system, the charging level is in the high mode that used in the Boeing 737. The charger operates on 115 volt
since as will be noted from the figure 10, the mode control relay 3-phase a.c. power supplied from a "ground service" busbar,
within the charger is energized by a rectified output through the which in turn, is normally powered from the number 1 generator
battery thermal switch, and the relaxed contacts of both the busbar, and/or from an external power source. Thus, the
battery bus relay and the external power select relay. Above 16 aircraft's battery is maintained in a state of charge both in flight
amps the charger acts as an unregulated transformer-rectifier and on the ground.
unit, and when the battery has sufficient charge that the current In flight the a.c. supply is routed to the charger through the
tends to go below 16 amps, the charging current is abruptly relaxed contacts of a battery charger transfer relay and an APU
reduced to zero. start interlock relay.

In addition to the control relay within the battery charger, there In the event that the number one generator supply fails there
are three other ways in which the charging mode can be will be a loss of a.c. power to the ground service busbar, and
controlled, each of them fulfilling a protective role by interrupting therefore, to the battery charger. However, with number two
the ground circuit to the mode control relay and so establishing generator still on line, a transfer signal from its control unit is
a low mode of charge. They are : (i) opening of the battery automatically supplied to the coil of the battery charger transfer
thermal switch in the event of the battery temperature relay, and as may be seen from Fig. 12, its contacts change
exceeding 46 °C (115 °F); (ii) loss of d.c. power from the over to connect the charger to the a.c. supply from number two
designated transformer-rectifier unit causing the battery transfer generator, and so charger operation is not interrupted.
relay to relax and the battery bus relay to energize; and (iii)
The APU start interlock relay is connected in parallel with a
energizing of the fueling panel power select relay when external
relay in the starting circuit of the APU, and is only energized
a.c. power is connected to the aircraft. The latter is of
during the initial stage of starting the APU engine. This prevents
importance since if the charger was left to operate in the high
the starter motor from drawing part of its heavy starting current
mode, then any fault in the regulation of the external power
through the battery charger. The interlock relay releases
supply could result in damage to the aircraft battery. The current
automatically when the APU engine reaches 35% rev/min.
remains at zero until the battery voltage drops below the charge
voltage, at which time the charger provides the battery with a

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 10: Showing Low &High Mode

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 12: Battery Charger Transfer Relay In The Event of No.1 Generator Failure

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Category – B1.1 Sub Module 11.6 - Electrical Power

DC POWER GENERATION

DC generators transform mechanical energy into electrical

energy. As the name implies, DC generators produce direct
current and are typically found on light aircraft. In many cases,
DC generators have been replaced with DC alternators. Both
devices produce electrical energy to power the aircraft’s
electrical loads and charge the aircraft’s battery. Even though
they share the same purpose, the DC alternator and DC
generator are very different. DC generators require a control
circuit in order to ensure the generator maintains the correct
voltage and current for the current electrical conditions of the
aircraft. Typically, aircraft generators maintain a nominal output
voltage of approximately 14 volts or 28 volts.
Electromagnetic induction is the process of producing a voltage
Fig ’1’
(EMF) by moving a magnetic field in relationship to a conductor.
When a conductor (wire) is moved through a magnetic field (Fig
It is important to remember that the voltage being produced by
‘1’), an EMF is produced in the conductor. If a complete circuit is
this basic generator is AC, and AC voltage is supplied to the slip
connected to the conductor, the voltage also produces a current
rings. Since the goal is to supply DC loads, some means must
flow. These principles show that voltage is induced in the
be provided to change the AC voltage to a DC voltage.
armature of a generator throughout the entire 360° rotation of
Generators use a modified slip ring arrangement, known as a
the conductor. The armature is the rotating portion of a DC
commutator, to change the AC produced in the generator loop
generator. As shown, the voltage being induced is AC. Since
into a DC voltage. The action of the commutator allows the
the conductor loop is constantly rotating, some means must be
generator to produce a DC output.
provided to connect this loop of wire to the electrical loads. Slip
By replacing the slip rings of the basic AC generator with two
rings and brushes can be used to transfer the electrical energy
half cylinders (the commutator), a basic DC generator is
from the rotating loop to the stationary aircraft loads. The slip
obtained. The red side of the coil is connected to the red
rings are connected to the loop and rotate; the brushes are
segment and the amber side of the coil to the amber segment.
stationary and allow a current path to the electrical loads. The
The segments are insulated from each other. The two stationary
slip rings are typically a copper material and the brushes are a
brushes are placed on opposite sides of the commutator and
soft carbon substance.

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The voltage generated by the basic DC generator in varies from
are so mounted that each brush contacts each segment of the zero to its maximum value twice for each revolution of the loop.
commutator as the commutator revolves simultaneously with This variation of DC voltage is called ripple and may be reduced
the loop. The rotating parts of a DC generator (coil and by using more loops, or coils.
commutator) are called an armature. As the number of loops is increased, the variation between
As the loop rotates the brushes make contact with different maximum and minimum values of voltage is reduced (Fig ‘3’),
segments of the commutator (Fig ‘2’). In positions A, C, and E, and the output voltage of the generator approaches a steady
the brushes touch the insulation between the brushes; when the DC value. For each additional loop in the rotor, another two
loop is in these positions, no voltage is being produced. In commutator segments is required.
position B, the positive brush touches the red side of the
conductor loop. In position D, the positive brush touches the
amber side of the armature conductor. This type of connection
reversal changes the AC produced in the conductor coil into DC
to power the aircraft. An actual DC generator is more complex,
having several loops of wire and commutator segments.

Fig ‘2’

Because of this switching of commutator elements, the red

brush is always in contact with the coil side moving downward,
and the amber brush is always in contact with the coil side
moving upward. Though the current actually reverses its
direction in the loop in exactly the same way as in the AC
generator, commutator action causes the current to flow always
in the same direction through the external circuit or meter. Fig ‘3’
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Category – B1.1 Sub Module 11.6 - Electrical Power

TYPES OF DC GENERATORS PARALLEL (SHUNT) WOUND DC GENERATORS

There are three types of DC generator: series wound, parallel A generator having a field winding connected in parallel with the
(shunt) wound, and series-parallel (or compound wound). The external circuit is called a shunt generator. It should be noted
appropriate generator is determined by the connections to the that, in electrical terms, shunt means parallel. Therefore, this
armature and field circuits with respect to the external circuit. type of generator could be called either a shunt generator or a
The external circuit is the electrical load powered by the parallel generator Figure2.
generator. In general, the external circuit is used for charging
the aircraft battery and supplying power to all electrical
equipment being used by the aircraft. As their names imply,
windings in series have characteristics different from windings in
parallel.

SERIES WOUND DC GENERATORS

The series generator contains a field winding connected in
series with the external circuit. Series generators have very
poor voltage regulation under changing load, since the greater
the current is through the field coils to the external circuit, the
greater the induced EMF’s and the greater the output voltage is.
When the aircraft electrical load is increased, the voltage
increases; when the load is decreased, the voltage decreases.
Since the series wound generator has such poor voltage and
current regulation, it is never employed as an airplane
Figure2:Parallel Wound
generator. Generators in airplanes have field windings, that are
In a shunt generator, any increase in load causes a decrease in
connected either in shunt or in compound formats. Figure 1. the output voltage, and any decrease in load causes an
increase output voltage. This occurs since the field winding is
connected in parallel to the load and armature, and all the
current flowing in the external circuit passes only through the
armature winding (not the field).
The output voltage of a shunt generator can be controlled by
means of a rheostat inserted in series with the field windings. As
the resistance of the field circuit is increased, the field current is
reduced; consequently, the generated voltage is also reduced.
Figure1:Series Wound
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As the field resistance is decreased, the field current increases

and the generator output increases. In the actual aircraft, the DC GENERATOR CONTROL & PROTECTION
field rheostat would be replaced with an automatic control
device, such as a voltage regulator. VOLTAGE REGULATION

COMPOUND WOUND DC GENERATORS During the discussion of generator theory, it was explained
that the voltage produced by electromagnetic induction depends
A compound wound generator employs two field windings one on the number of lines of force being cut per second by a
is series field winding and the other is parallel field winding. conductor. In a generator, the voltage produced depends on
This arrangement takes advantage of both the series and three factors:
parallel characteristics described earlier. The output of a
1. The speed at which the armature rotates,
compound wound generator is relatively constant, even with
changes in the load. The torque produced by a DC generator is 2. The number of conductors in series in the armature,
directly proportional to the product of the current flowing in the and
rotating armature winding. Figure 3.
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 requirements.

It is obvious that the speed of a generator cannot be varied

according to load requirements if the engine directly drives the
generator. 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
Figure 3:Compound Wound the field coils, and this current can be controlled by a variable
resistor in the field circuit outside the generator.

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Figure 1: A simple voltage regulator (variable resistor type)

The simplest type of voltage regulation is accomplished as VIBRATOR TYPE VOLTAGE REGULATOR
shown in the Figure 1. In this arrangement a rheostat (variable
resistor) is placed in series with the shunt field circuit. A generator system using a vibrator-type voltage regulator is
shown in Figure1. A resistance that is intermittently cut in and
If the voltage rises above the desired value, the operator can out of the field circuit by means of vibrating contact points is
reduce the field current with the rheostat, thus weakening the placed in series with the field circuit. The contact points are
field and lowering the generator voltage. An increase in voltage controlled by a voltage coil connected in parallel with the
is obtained by reducing the field circuit resistance with the generator output. When the generator voltage rises to the
rheostat. All methods of voltage regulation in aircraft electrical desired value, the voltage coil produces a magnetic field strong
systems employ the principle of a variable or intermittent field enough to open the contact points. When the points are open,
resistance. Modern voltage regulators have been developed to the field current must pass through the resistance. This causes
such a high degree of efficiency that the emf of a generator will a substantial reduction in field current, with the result that the
vary only a small fraction of a volt throughout extreme ranges of magnetic field in the generator is weakened. The generator
load and speed. voltage then drops immediately, causing the voltage coil
electromagnet to lose strength so that a spring can close to the
Voltage regulators or controls for modem aircraft are usually of contact points. This allows the generator voltage to rise, and the
the solid-state type; that is, they employ transistors and diodes cycle is then repeated. The contact points open and close many
as controlling elements. Because there are still many older times a second, but the actual time that they are open depends
airplanes in use that employ vibrator-type and variable-
on the load being carried by the generator and the generator
resistance voltage regulators, we shall examine these in the
following sections. (engine) rpm. As the generator 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 points.

Often temperature greatly affects the generator output;

therefore, this adjustment should be made only under specific
conditions set up by the manufacturer.

Because the contact points do not bum or pit appreciably,

vibrator-type voltage regulators are satisfactory for generators

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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- CARBON PILE VOLTAGE REGULATOR
duty generator systems.
The carbon-pile voltage regulator derives its name from the fact
If the regulating resistance becomes disconnected or burned
that the regulating element (variable resistance) consists of a
out, the generator voltage will fluctuate, and excessive arcing
stack, or pile, of carbon disks (see Figure). Usually, the carbon
will occur at the contact points. When inspecting a vibrator-type
voltage regulator, make sure that the connections to the pile has alternate hard carbon and soft carbon (graphite) disks
resistance are secure and that the resistance is in good contained in a ceramic tube with a carbon or metal contact plug
condition. 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 electromagnet in a position where it
will release the spring pressure on the disks as the voltage rises
above a predetermined value, a stable and efficient voltage
regulator is obtained.

The carbon-pile voltage regulator is connected in a generator

system the same way any other regulator is connected, that is,
with a resistance in the field circuit and an electromagnet to
control the resistance. The carbon pile is in series with the
generator field, and the voltage coil is shunted across the
generator output. A small manually operated rheostat is
connected in series with the voltage coil to provide for a limited
amount of adjustment, which is necessary when two or more
generators are connected in parallel to the same electrical
system.
Figure 1: Vibrator type voltage regulator circuit

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PARALLEL OPERATION OF DC GENERATORS

EQUALIZING CIRCUITS

When two or more generators are connected in parallel to a

power system, the generators should share the load equally. If
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 1). The equalizing 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 potential between the negative terminals
of the generators that is proportional to the difference in load
Figure1: Carbon pile voltage regulator 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 Figure1 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 1and 0.25 V across the shunt of

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generator 2. This will make a net potential difference of 0.25 V
between the negative terminals of the generators. Since the
equalizing circuit is connected between these points, a current
will flow through the circuit.

The current 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 current in
the field windings, and the generator will take more of the load.
To summarize, the effect of an equalizing circuit is to lower the
voltage of a generator that is taking too much of the load and to
increase the voltage of the generator that is not taking its share
of the load.

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 maintain a
satisfactory load balance. A periodic inspection of the ammeters
should be made during flight to see that the generator loads are Figure 1 : Equalizing circuit
remaining properly balanced.

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REVERSE CURRENT PROTECTION When the generator voltage reaches a value slightly above that
of the battery in the system, the voltage coil in the relay
REVERSE CURRENT CUTOUT RELAY magnetizes the soft-iron core sufficiently to overcome the spring
tension. The magnetic field closes the contact points and
In every system in which the generator is used to charge thusconnects the generator to the electrical system of the
batteries as well as to supply operating power, an automatic airplane. As long as the generator voltage remains higher than
means must be provided for disconnecting the generator from the battery voltage, the current flow through the current coil will
the battery when the generator voltage is lower than the battery be in a direction that aids the voltage coil in keeping the points
closed. This means that the field of the current coil will be in the
voltage. If this is not done, the battery will discharge through the
same direction as the magnetic field of the voltage coil and that
generator and may burn out the armature. Numerous devices
the two will strengthen each other.
have been manufactured for the purpose of automatically
disconnecting the generator, the simplest being the reverse- When an airplane engine is slowed down or stopped, the
current cutout relay. The figure1 is a schematic diagram generator voltage will decrease and fall below that of the
illustrating the operation of such a relay. battery. In this case the battery voltage will cause current to
start flowing toward the generator through the relay current coil.
A voltage coil and a current coil are wound on the same soft- When this happens, the current flow will be in a direction that
iron core. The voltage coil has many turns of fine wire and is creates a field opposing the field of the voltage winding. This
connected in parallel with the generator output; that is, one end results in a weakening of the total field of the relay, and the
of the voltage winding is connected to the positive side of the contact points are opened by the spring, thus disconnecting the
generator output, and the other end of the winding is connected generator from the battery. The contact points may not open in
to ground, which is the negative side of the generator output. normal operation until the reverse current has reached a value
This is clearly shown in the diagram. The current coil consists of of 5 to 10A.
a few turns of large wire connected in series with the generator
output; hence it must carry the entire load current of the Generally speaking, the tension of the spring controlling the
generator. A pair of heavy contact points is placed where it will contact points should be adjusted so that the points will close at
be controlled by the magnetic field of the soft-iron core. When approximately 13.5 V in a 12- V system and at 26.6 to 27 V in a
the generator is not operating, these contact points are held in 24- V system.
an open position by a spring.

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SWITCHED REVERSE CURRENT RELAY

This relay is adopted in the d.c. generator systems of some

types of small aircraft, its purpose being to permit switching of a
generator on to the main busbar, and at the same time retain
the disconnect function in the event of reverse current. The
circuit arrangement is shown in Figure1-2.

In addition to a current coil the relay has a voltage coil, and a

pair of contacts actuated via a contactor coil. When the voltage
output is at a regulated value, the current through the voltage
coil is sufficient to actuate its contacts, which then connect the
generator switch and contactor coil to ground. The contactor coil
is thus energized from the A+ output of the generator and so the
auxiliary and main contacts close to connect the generator
output to the battery and main busbar. The magnetic effect of
the current passing through the current coil assists that of the
voltage coil in keeping the pilot contacts closed.

During engine shut-down, the generator output voltage

decreases thereby initiating a reverse current condition, and
because the magnetic effect of the current through the current
coil now opposes that of the voltage coil, the pilot contacts
Figure 1: Reverse Current Cutout Relay Circuit
open to de-energize the contactor coil; thus, the main and
auxiliary contacts are opened to disconnect the generator from
the battery and main busbar.

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Category – B1.1 Sub Module 11.6 - Electrical Power

REVERSE CURRENT CIRCUIT BREAKERS

These circuit breakers are designed to protect power supply

systems and associated circuits against fault currents of a
magnitude greater than those at which cut-outs normally
operate. Furthermore, they are designed to remain in a "locked-
out" condition to ensure complete isolation of a circuit until a
fault has been cleared.

An example of a circuit breaker designed for use in a d.c.

generating system is shown in Figure 2. It consists of a
magnetic unit, the field strength and direction of which are
controlled by a single-turn coil connected between the generator
positive output and the busbar via a main contact assembly. An
auxiliary contact assembly is also provided for connection in
series with the shunt-field winding of the generator. The opening
of both contact assemblies is controlled by a latching
mechanism actuated by the magnet unit under heavy reverse
current conditions. In common with other circuit breakers,
resetting after a tripping operation has to be done manually, and
is accomplished by a lever which is also actuated by the
latching mechanism. Visual indication of a tripped condition is
provided by a colored indicator flag which appears behind a
window in the circuit breaker cover. Manual tripping of the unit is
effected by a push-button adjacent to the resetting lever.

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Figure 1-2:Switched Reverse Current Relay

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Figure 2:Reverse Current Circuit Breaker

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Figure 3 is based on the circuit arrangement of a d.c. generating

system used in a particular type of aircraft, and is an example of The reverse current passing through the circuit breaker coil
the application of a reverse current circuit breaker in conjunction would continue to increase in trying to overcome mechanical
with a cut-out relay. Unlike the circuit shown in above, the relay loads due to the engine and generator coupling, and so the
controls the operation of a line contactor connected in series increasing reverse field reduces the strength of the magnet unit.
with the coil of the reverse current circuit breaker. Under normal When the reverse current reaches the pre-set trip value of the
current flow conditions closing of the relay energizes the line circuit breaker, the field of the magnet unit is neutralized and
contactor, the heavy-duty contacts of which connect the repelled, causing the latch mechanism to release the main and
generator output to the busbar via the coil and main contacts of auxiliary contacts to completely isolate the generator from the
the normally closed reverse current circuit breaker. The busbar. The breaker must be reset after the circuit fault has
magnetic field set up by the current flow assists that of the been cleared.
magnet unit, thus maintaining the breaker contacts in the closed
position. The generator shunt field circuit is supplied via the
auxiliary contacts.

When the generator is being shut down, or a failure of its output

occurs, the reverse current resulting from the drop in output to
a value below that of the battery flows through the circuit as
indicated, and the cut-out relay is operated to de-energize the
line contactor which takes the generator "off line". Under these
conditions the reverse current circuit breaker will remain closed;
since the current magnitude is much lower than that at which a
specific type of breaker is normally rated (some typical ranges
are 200-250 A and 850-950 A).

Let us consider now, what would happen in the event of either

the cut-out relay or the line contactor failing to open under the
above low magnitude reverse current conditions, e.g. contacts
have welded due to wear and excessive arcing. The reverse
current would feed back to the generator, and in addition to its
motoring effect on the generator, it would also reverse the
generator field polarity.

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Category – B1.1 Sub Module 11.6 - Electrical Power

Figure 3:Reverse Current Circuit Breaker In Conjunction With A Cut-Out Relay

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The current limiter described above should not be confused with
the fuse-type current limiter.
CURRENT LIMITER

In some generator systems a device is installed that will reduce The fuse-type limiter is merely a high-capacity fuse that permits
the generator voltage whenever the maximum safe load is a short period of overload in a circuit before the fuse link melts
exceeded. This device is called a Current Limiter and is
designed to protect the generator from loads that will cause it to
overheat and eventually bum 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 the figure 4).

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 corresponding
decrease in generator current. Since the magnetism produced
by the current-limiter coil is proportional to the current flowing and breaks the circuit
through it, the decrease in generator load current also weakens
the magnetic field of the current coil and thus permits 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 generator, the

contacts of the current limiter will continue to vibrate, thus Figure 4: Current limiter circuit
holding the current output at or below the minimum safe limit.
The contact points are usually set to open when the current flow
is 10 percent above the rated capacity of the generator.

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Category – B1.1 Sub Module 11.6 - Electrical Power

TRANSISTOR VOLTAGE REGULATORS This current flow excites the field of the alternator, and the
output of the alternator quickly rises to the desired level. In the
A typical transistor voltage regulator is shown in the photograph circuit of Fig. 2, it can be seen that there is a circuit from ground
of Fig. 1. This regulator has no moving parts and consists through R6, R5, the zener diode, R3, and R, to A. There is also
primarily of a control diode, a control resistor, a power a circuit from the zener diode through the base-emitter circuit of
transistor, field diodes, capacitors, and resistors. The principles the control transistor TR2 and through R1 to terminal A of the
of operation for diodes and transistors have been described regulator. The zener diode blocks the flow of current from R5
previously and should be reviewed to obtain a good until the voltage between ground and A reaches approximately
understanding of the operation of transistor voltage regulator. 14.5 V. At this point the zener diode begins to conduct and
applies a forward bias through the base-emitter circuit of TR2,
A circuit diagram to illustrate the operation of a transistor the control resistor. TR2 then becomes conductive, and current
voltage regulator is shown in Fig. 2. In this description of the flows through the collector-emitter section from ground. This
operation, each item will be explained in terms of actual current current flow is from ground through R2, TR2, R1, and out A. The
flow from negative to positive. For example, when the battery is effect of this is to short-circuit the base-emitter circuit of TR1 and
furnishing current to the circuit, current flows from the negative this causes TR1 to stop conducting field current for the
terminal to ground and current flows from the circuit into the alternator. The alternator voltage immediately drops, and the
positive terminal of the battery. When the battery is being zenerdiode stops conducting, thus removing the forward bias
charged, current is flowing into the negative terminal of the from TR2, which also stops conducting. This returns the forward
battery and out the positive terminal. bias to TR1, which starts conducting field current again, and
the cycle repeats. This cycle repeats about 2000 times per
In the circuit of Fig. 2, when the alternator master switch is second thus producing a reasonably steady voltage of
closed, the battery and alternator are connected to the relay and approximately 14.5 V from the alternator.
through the relay to the positive terminal (A) of the regulator.
There is then a complete circuit from around through the The two key points to understand with respect to the operation
resistor R2, the base of the power transistor TR1 the diode D1 of the transistor voltage regulator are the zenerdiode operation
the resistor R1 and back to the battery and the positive terminal and the control of the power transistor by the control transistor.
of the alternator. If the output of the alternator is below the The zener diode may be compared to a relief valve, which
voltage for which the regulator is set, transistor TR 1 will have opens at a given pressure in a hydraulic system. When the
forward bias and current will flow from the F 1 terminal of the zener diode conducts current, it causes the control transistor to
alternator to the F terminal of the regulator and through the shut off the power transistor.
collector-emitter circuit of TR1. The circuit is completed through
D1, R1, and the relay to the alternator.

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Figure 1

Figure 2

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The reason that the control transistor can stop the flow of
current through the emitter-base circuit of the power transistor is
that there is a difference in the voltage drops across the emitter-
base circuits of the two transistors when the control-transistor
emitter-base circuit is conducting. The diode D1 causes
approximately 1V drop in potential across the emitter-base
circuit of the power transistor when the circuit is conducting.
When the emitter-collector circuit of the control transistor begins
to conduct, there is no appreciable voltage drop across the
control transistor; hence, a 1V reverse bias becomes effective
across the emitter-base circuit of the power transistor. This, of
course, stops the emitter-base current in the power transistor.

Adjustment of alternator voltage output is accomplished through

the variable resistor R5. A change in the resistance of this
resistor will change the voltage level across the zener diode,
thus raising or lowering the level of alternator output voltage
required to cause the zener diode to conduct.

The resistor R1 and capacitor C1 act to reduce the time required

for the field voltage to change between maximum and minimum
values. This prevents overheating of the transistors. The
capacitor C2 reduces the voltage variations, which appear
across the resistors R4 and R5, thus making the regulator more
accurate. Resistor R3 prevents leakage current from the emitter
to the collector in the control transistor. Resistor R 4 is a special
temperature sensitive type, which acts to increase the alternator
voltage slightly at lower temperatures. This aids in maintaining
adequate charge current for low-temperature operation. Diode
D2 aids in controlling field-current flow as the power transistor
rapidly turns the field current on and off.

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AC POWER GENERATION

Alternating current (ac) generators supply the electrical energy The ac generator and many of the system’s control and
for operating aircraft avionics equipment. A generator is a protection components are lighter. Twelve kilowatts is the
machine that converts mechanical energy into electrical energy practical limit to the size of an aircraft dc generator. Aircraft now
by electromagnetic induction. Ac power systems result in better have ac generators with ratings up to 120 kilovolt-ampere
design and use of equipment than older electronic equipment (kVA).
powered by direct current (dc), which have inverters for ac
power and dynamotors for supplying higher voltage dc power.
These components are very heavy compared to their relative
power outputs. They are not reliable and increase maintenance
costs and time. In today’s aircraft, the same ac-powered
equipment obtains various ac voltages and dc power by using
simple transformers and transformer-rectifiers. These
components are lightweight, simple, and reliable. Modern
aircraft use the three-phase, 115 volt, 400-hertz (Hz) ac power
system. The number of magnetic poles and rotor revolutions per
minute (rpm) determine the voltage frequency of the generator.
With a fixed number of poles, constant frequency requires
constant rotor rpm. An ac generator-rotating field has 12 poles
with adjacent poles of opposite polarity. Each pair of poles
produces one cycle per revolution; therefore, each revolution
produces six cycles. The output frequency of the generator
varies in direct proportion to the engine drive speed. A
generator operating at 6,000 rpm is operating at 100 revolutions
per second or at 600 Hz. The 115volt, 400-Hz, three-phase ac
power system has many advantages over the 28-volt dc
system. It requires less current than the 28-volt dc system
because of higher voltage and a ground neutral system. The
current required is a fraction of that required for the same power
in a 28-volt dc system. This permits the use of smaller aircraft
wiring, saving weight.

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Category – B1.1 Sub Module 11.6 - Electrical Power

TYPES OF AC GENERATORS

Aircraft ac generators range in size from the tachometer

instrument generator up to the 120,000 volt-ampere generators.
Generators are categorized as either brush-type or brushless.
Regardless of weight, shape, or rating, practically all of these
generators have the following common characteristics:

The stator (stationary armature winding) provides the ac output.

The ac generator field (rotor) is a rotating magnetic field with
fixed polarity. Regulating the rpm of the rotating magnetic field
controls the voltage frequency. Controlling the strength of the Figure 2
magnetic field regulates the voltage.
Each output winding produces 115 volts as measured from n to
Present specifications require that the basic aircraft ac power a, b, or c (phase voltage). When measuring two separate phase
system produce voltage with a value of 115volts. A three-phase voltages together (line voltage), such as a to b, a to c, or b to c,
generator is actually three separate power sources enclosed in the voltage is 1.73 times the single-phase voltage, 115volts.
one 1-1 housing (figure.1). To produce the required 115vollt The line voltage found in a three-phase, wye-connected system
output, external connections form a wye (figure.2). is the vector sum of the voltages generated by two separate
phase windings. Because a 120-degree phase difference exists
between the voltages, they reach their peak amplitudes at
different times. They add vectorially and not directly. In the four-
wire, grounded-neutral, wye-connected system, the neutral wire
attaches to the frame of the aircraft (ground). The three-phase
wires run to buses, which supply power to various loads. A bus
is a primary power distribution point that is connected to the
main power source. The connections for loads requiring 115
volts are between one of the buses and the aircraft frame.

Figure 1

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BRUSH-TYPE

It consists of an ac generator and a smaller dc exciter generator

as one unit. The output of the generator supplies ac to the load.
The only purpose for the dc exciter generator is to supply the
direct current required to maintain the ac generator field.

Figure 2

Figure 2 is a simplified schematic of the generator. Look at

figures as you read this section. The exciter is a dc, shunt-
wound, self-excited generator. The exciter field (2) creates an
area of intense magnetic flux between its poles. When the
exciter armature (3) rotates in the exciter-field flux, voltage is
induced in the exciter armature windings. The output from the
exciter commutator (4) flows through brushes and slip rings (5)
to the generator field.
Being dc, already converted by the exciter commutator, the
current always flows in one direction through the generator field
(6). Thus, a fixed-polarity magnetic field is maintained in the
generator field windings. When the field winding rotates, its
magnetic flux passes through and across the generator
armature windings (7).
Figure 1

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The ac in the ac generator armature windings flows through Figure 3 is an expanded view of the main assembly of a
fixed terminals to the ac load. The stationary member of the brushless generator. The brushless generator shown in figure is
generator consists of the ac armature and the dc exciter field. a salient 8-pole, 6,000-rpm, ac generator. It has a 12-pole ac
Both ac and exciter terminal boards are easily accessible. All exciter and a three-phase, half-wave diode rectifier rotating with
brush rigging is on the generator and has a brush cover. The the exciter armature and main generator field assembly. The
slotted-hole mounting provides for ease in attaching to the exciter rotor is a hollow frame assembly with the main ac field
engine pad. The capacitors connected between the exciter mounted on the inside and connected to a common drive shaft.
armature terminals and ground suppress radio noise. A single-phase permanent magnet generator (PMG) furnishes
control voltage and power for the voltage regulator. Three half-
BRUSHLESS-TYPE wave rectifiers are on the exciter rotor and connected to the
Most aircraft are using brushless generators for voltage exciter armature windings. A generator shaft shear section
generation. The advantage of a brushless generator over a prevents possible damage to the engine or drive unit if the
brush-type is its increased reliability and greater operating time generator seizes. A fan at the drive end of the generator
between overhaul. provides cooling airflow for the rotor and stator windings and the
drive bearings. The end bell not only holds the generator
together, but also houses the PMG stator and the permanent
magnetic rotor core. The generator to air duct adapter allows a
vent tube to be attached to the generator so that built-up air and
fumes created by the spinning generator can be vented to the
outside of the aircraft. Some aircraft have oil-cooled generators.
The aircraft engine or gearbox oil enters the generator through
an inlet port and leaves through an exit port in the mounting
flange of the generator. As the oil passes through the generator,
it absorbs the heat from the rotor and stator. At the same time, it
cools the rotating seals, lubricates and cools the bearings, and
is used for the constant speed drive operation. As the generator
shaft rotates, the PMG supplies single-phase, ac voltage to the
voltage regulator and other protective circuits as shown in
figure4.

Figure 3

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PMG power is rectified and supplied to the exciter field. The

electromagnetic field, built by the excitation current flowing in
the exciter, induces current flow in the rotating three-phase
exciter rotor. This current is half-wave rectified by rotating
rectifiers. The resultant dc goes to the rotating field winding of
the ac generator. The rotating electromagnetic field induces an
ac voltage in the three-phase, wye-connected, output winding of
the generator stator. Varying the strength of the exciter
stationary field regulates voltage. Using an integral ac exciter
eliminates the need for brushes within the generator, which
minimizes radio noise in other avionics equipment. Two three-
phase differential transformers provide protection against shorts
in the feeder lines between the generator and the bus (called
feeder fault). One transformer is on the generator. Its coils
sense the current flow through each of the legs that connect the
ground side of the generator stator to ground. The other
transformer is at the main bus and senses current flow through
the three feeder lines. A short in the feeder line would cause the
transformers sensing a difference in current to trip the generator
off line. A generator mechanical failure warning device is
incorporated in the generator. It consists of a soft copper strip
embedded in and insulated from the generator stator assembly.
A bearing beginning to fail allows the rotor to rub against the
copper strip, completing a warning light circuit to ground.

Figure 4

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mounting technique allows for quick line repair of an aircraft that
CONSTANT SPEED DRIVE (CSD) has a defective generator CSD.
A normal operating temperature for the cooling oil is
In an ac power system it is usually necessary to maintain a fairly approximately 2000F. in order to maintain the correct cooling
constant speed in the ac generator. This is because the capacity, the oil level should be monitored periodically. A sight
frequency of the ac generator is determined by the speed with glass, as shown in figure 1, is employed on most constant
which it is driven. It is especially important to maintain constant speed drives to allow the technician a quick check of the oil
generator speed in installations in which the generators operate level. In the case of an in-flight oil loss or an over-temperature
in parallel. In this case it is absolutely essential that generator condition, a warning indicator will light up on the flight deck. In
speed be kept constant within extremely close limits. In order to this situation the CSD should be disengaged immediately and
provide constant speed generator operation in modern ac inspected upon landing. Most constant speed drive units are
electrical systems, it is common practice to use a constant equipped with an electrically activated generator drive
speed drive (CSD), such as that manufactured by Sundstrand disconnect mechanism. This disconnect couples the CSD input
Aviation Electric Power Division of the Sundstrand Corporation. shaft to the CSD input spline. The CSD disconnect is operated
CSD units are manufactured in many designs to fit a variety of manually from the aircraft’s flight deck or automatically by a
applications. The principle of operation for all CSDs is generator control unit. The disconnect is activated in the event
essentially the same. of certain generator system failures.
The complete CSD system consists of an axial gear differential
(AGD), whose output speed relative to input speed is controlled
by a flyweight-type governor that controls a variable delivery
hydraulic pump. The pump supplies hydraulic pressure to
hydraulic motor, which varies the ratio of input rpm to output
rpm for the AGD in order to maintain a constant output rpm to
drive the generator and maintain an ac frequency of 400 Hz.
A typical ac generator and constant speed drive assembly is
shown in figure 1. In this view the CSD is on the left end of the
assembly, and the generator is at the right end. The generator
is cooled by an oil spray delivered by the CSD section. Most
CSDs are equipped with a quick attach/detach (QAD) adapter.
This unit allows the technician to remove and replace a
generator and CSD assembly in a matter of minutes. The QAD
ring is mounted to the CSD by means of several bolts through
the mounting flange. To remove the generator from the aircraft,
one must only release the QAD using one fastener. This

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STARTER MOTOR

The starter provides initial power for rotating the components of

the gas turbine to self-sustaining speeds. It rotates the
compressor to a speed high enough for correct airflow for
combustion. Also, the starter assists acceleration after light-off,
preventing excessive turbine temperature at low speeds. The
starter has a duty cycle of 1 minute on and 4 minutes off. The
starter motor armature shaft is splined and pinned to the clutch
assembly. The starter clutch assembly performs two functions:
As a friction clutch, it prevents excessive torque between the
starter and accessory drive gears to protect both. As an
overrunning clutch, it provides the means of automatically
engaging the starter with the gear train for starting. The clutch
automatically releases it when the unit has reached a condition
allowing it to accelerate and run without assistance. The friction
clutch section provides over-torque protection. Due to the inertia
the engine offers when the starter motor pawls first engage with
the gearbox ratchet, the overrunning clutch flange and splined
clutch plates remain stationary. When in this state, the motor,
Figure 1: A typical constant speed drive and generator assembly clutch housing, and keyed clutch plates rotate. Slippage occurs
until engine and starter speeds have increased enough to
develop less than the specified torque value. The starter
normally is de-energized by the centrifugal switch at 35%. If the
switch does not cut out at this speed, the starter may fail from
overheating or it may fail mechanically from over-speed. If the
overrunning clutch does not release properly, mechanical over-
speed failure of the starter will result.

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POWER SOURCES
AC GENERATOR CONTROL & PROTECTION
Electrical power is provided to the buses from following sources:
VOLTAGE REGULATION
 Left main generator
 Right main generator The control of the output voltages of a.c. generators is also an,
 Left backup generator essential requirement, and from the foregoing description of
 Right backup generator excitation methods, it will be recognized that the voltage
regulation principles adopted for d.c. generators can also be
Two engine-driven generator transmissions supply ac. Each applied, i.e. automatic adjustment of excitation current to meet
one is coupled to a 115volt ac, 400-Hz, three-phase brushless changing conditions of load and/or speed. Voltage regulators
ac generator. These generators supply electrical power to the normally form part of generator system control and protection
main, essential, and monitor ac buses. Either of the two units.
generators can supply the entire electrical demand of the
aircraft in the event one generator fails. Two transformer- The regulation of the output of a constant-frequency system is
rectifiers (TR) supply dc power. Each TR unit receives power also based on the principle of controlling field excitation, and
from its respective main ac bus. The TR units convert 115 Vac some of the techniques thus far described are in many
to 28 Vdc for distribution to the secondary bus system. Either of instances applied. In installations requiring a mufti-arrangement
the TRs is capable of supplying the entire dc requirements of of constant-frequency generators, additional circuitry is required
the aircraft. to control output under load-sharing or parallel operating
A hydraulically driven generator provides ac and dc emergency conditions and as this control also involves field excitation, the
power for essential equipment only. The emergency generator overall regulation circuit arrangement is of an integrated, and
automatically actuates upon multiple generator or multiple TR sometimes complex, form. At this stage, however, we are only
unit failure. Emergency generator operation terminates upon concerned with the fundamental method of regulation and for
reactivation of either main generator. this purpose we may consider the relevant sections or stages
of the circuit shown schematically in Figure 1.

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Figure 1: AC Voltage Regulation Circuit

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Voltage regulation circuit is comprised of three main sections: a, If the a.c. Line voltage should go above or below the fixed
voltage error detector, pre-amplifier and a power amplifier as value, tire voltage drops across R1 and R2 will differ causing an
shown in figure 1 (above). The function of the voltage error unbalance of the bridge circuit and a flow of current to the
detector is to monitor the generator output voltage, compare it "error" control winding of the pre-amplifier. The direction and
with a fixed reference voltage and to transmit any error to the magnitude of current flow will depend on whether the variation,
pre-amplifier. It is made up of a three-phase bridge rectifier or error in line voltage, is above (positive error signal) or below
connected to the generator output, and a bridge circuit of which (negative error signal) the balanced nominal value, and on the
magnitude of the variations.
two arms contain gas-filled regulator tubes and two contain
resistances. The inherent characteristics of the tubes are such
When current flows through the "error" control winding the
that they maintain an essentially constant voltage drop across
magnetic flux set up alters the total flux in the cores of the
their connections for a wide range of current through them and
amplifier, thereby establishing a proportional change in the
for this reason they establish the reference voltage against
amplifier output, which is applied to the signal winding of the
which output voltage is continuously compared. The output side
power amplifier. If the error signal is negative it will cause an
of the bridge is connected to an "error" control winding of the
increase in core flux, thereby increasing the power amplifier
preamplifier and then from this amplifier to a "signal" control
output current to the generator exciter field winding. For a
winding of a second stage or power amplifier. Both stages are
three-phase magnetic amplifiers. The final amplified signal is positive error signal the core flux and excitation current will be
reduced. Thus, the generator output is controlled to the preset
then supplied to the shunt windings of the generator a.c. exciter
value, which on being attained restores the error detector bridge
stator.
circuit to lire balanced condition.
The output of the bridge rectifier in the error detector is a d.c.
Regulators normally incorporate torque-limiting circuitry, which
voltage slightly lower than the average of the three a.c. line
limits the torque at mechanical linkages to a safe value by
voltages; it may be adjusted by means of a variable resistor
(RV1) to bring the regulator system to a balanced condition for limiting the exciter field current.
any nominal value of line voltage. A balanced condition of the
bridge circuit concerned is obtained when tire voltage applied
across the bridge (points "A" and "B") is exactly twice that of tire
voltage drop across the two tubes. Since under this condition,
the voltage drop across resistors R1 and R2 will equal the drop
across each tube, then no current will flow in the output circuit to
the error control winding of the pre-amplifier.

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LOAD SHARING OR PARALLELING OF CONSTANT REAL LOAD-SHARING

FREQUENCY SYSTEMS
The sharing of real load between paralleled generators is
These systems are designed for operation under load sharing or determined by the real relative rotational speeds of the
paralleling conditions and in this connection regulation of the generators, which in turn in influence the voltage phase
two parameters, real load and reactive load is required. Real relationships. As we learned earlier speed of a generator is
load is the actual working load output in kilowatts (kW) available determined by the initial setting of the governor on its
for supplying the various electrical services, and the reactive associated constant speed drive. It is not possible, however, to
load is the so-called "wattless load" which is in fact the vector obtain exactly identical governor settings on all constant speed
sum of the inductive and capacitive currents and voltage in the drives employed in any one installation, and so automatic
system expressed in kilovolt-amperes reactive (VAR). control of the governors becomes necessary.

Since the real load is directly related to the input power from the A.C. generators are synchronous machines. Therefore when
prime mover, i.e. the aircraft engine, real load-sharing control two or more operate in parallel they lock together with respect
must be on the engine. There are, however, certain practical to frequency and the system frequency established is that of the
difficulties involved, but as it is possible to reference back any generator whose output is at the highest level: Since this is
real load unbalance to the constant-speed drive unit between controlled by speed-governing settings then it means that the
engine and generator, real load-sharing control is effected at generator associated with a higher setting will carry more than
this unit by adjusting torque at the output drive shaft. its share of the load and will supply energy which tends to motor
the other machines in parallel with it. Thus, sharing of the total
Reactive load unbalances are corrected by controlling the real load is unbalanced, and equal amounts of energy in the
exciter field current delivered by the voltage regulators to their form of torque on the generator rotors must be supplied.
respective generators, in accordance with signals from a
reactive load-sharing circuit. Fundamentally, a control system is comprised of two principal
sections: one in which the unbalance is determined by means
of current transformers, and the other (load controlling section)
in which torques are established and applied. A circuit diagram
of the system as applied to a four-generator installation is
shown schematically in Figure 1.

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Figure 1: The Sharing Of Real Load Between Paralleled Generators

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The current transformers sense the real load distribution at The three amps difference divides equally between the other
phase "C" of the supplyfrom each generator, and are connected generators and so the output of each corresponding current
in series and together they form a load-sharing loop. Each load transformer is reduced by one amp, a difference that flows
controller is made up of a two-stage magnetic amplifier through the error sensing elements of the load controllers. The
controlled by an error-sensing element in parallel with each error signals are then applied as d.c. control signals to the two-
current transformer. The output side of each load controller is, in stage magnetic amplifiers and are fed to electromagnetic coils
turn, connected to a solenoid in the speed governor of each which are mounted adjacent to permanent magnet flyweights
constant speed unit. and form part of the governor in each constant speed drive unit.
The current and magnetic field simulate the effects of centrifugal
When current flows through phase "C" of each generator, a forces on the flyweights and are of such direction and
voltage proportional to the current is induced in each of the magnitude as to cause the flyweights to be attracted or repelled.
current transformers and as they are connected in series, then
current will flow in the load sharing loop. This current is equal to Thus, in the unbalanced condition we have assumed, i.e. No.1
the average of the current produced by all four transformers. generator running at a higher governor setting, the current and
field resulting from the error signal applied to the corresponding
Let us assume that at one period of system operation, balanced load controller flows in the opposite sense and repels the
load sharing conditions are obtained under which the current flyweights, thereby simulating a decrease of centrifugal force.
output from each transformer is equal to five amps, then the The movement of the flyweights causes oil to flow to under-
average flowing in the load sharing loop will be five amps, and drive and the output speed of the constant speed unit drive
no current circulates through the error sensing elements. If now decreases, thereby correcting the governor setting to decrease
a generator, say No.1, runs at a higher speed governor setting the load being taken by No.1 generator. The direction of the
than the other three generators, it will carry more load and will current and field in the load controller sensing elements of the
increase the output of its associated current transformer. remaining generators is such that the governor flyweights in
their constant speed drive units are attracted, allowing oil to flow
The share of the load being carried by the other generators falls to overdrive, thereby increasing the load being taken by each
proportionately, thereby reducing the output of their current generator.
transformers, and the average current flowing to the load
sharing loop remains the same, i.e. five amps. If, for example, it
is assumed that the output of No.1 generator current
transformer is increased to eight amps a difference of three
amps will flow through the error-sensing element of its relevant
load controller.

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Figure 1: The Sharing Of Real Load Between Paralleled Generators

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REACTIVE LOAD-SHARING (ii) An air gap in the iron care to produce a phase
displacement of approximately 90 degrees between
The sharing of reactive load between paralleled generators the primary current and secondary voltage. They
depends on the relative magnitudes of their output voltages, serve the purpose of delivering signals to the voltage
which vary, and as with all generator systems are dependent on regulator, which is proportional to the generator's
the settings of relevant voltage regulators and field excitation reactive load only.
current. If, for example, the voltage regulator of one generator is
set slightly above the mean value of the whole parallel system, When a reactive load unbalance occurs, current transformers
the regulator will sense an under-voltage condition and it will detect this in a similar manner to those associated with the real
accordingly increase its excitation current in an attempt to raise load-sharing circuit and they cause differential currents to flow
the whole system voltage to its setting. However, this results in in the primary windings of their associated mutual reactors.
a reactive component of current flowing from the "over-excited" Voltages proportional to the magnitude of differential currents
generator, which flows in opposition to the reactive loads of the are induced in the secondary windings and will either lead or lag
other generators. Thus, its load is increased while the loads of generator current by 90 degrees. When the voltage induced in a
the other generators are reduced and unbalance in reactive particular reactor secondary winding leads associated generator
load sharing exists. It is therefore necessary to provide a circuit current it indicates that a reactive load exists on the generator;
to correct this condition. in other words, that it is taking more than its share of the total
load. In this condition, the voltage will add to the voltage sensed
In principle, the method of operation of the reactive load-sharing by the secondary winding at phase "C". If, on the other hand,
circuit is similar to that adopted in the real load-sharing circuit the voltage lags the generator current then the generator is
described earlier. A difference in the nature of the circuitry absorbing a reactive load, i.e. it is taking less share of the total
should however be noted in figure 1. Whereas in the real load- load and the voltage will subtract from that sensed at phase "C".
sharing circuit the current transformers arc connected directly to
the error detecting elements in load controlling units, in a The secondary winding of each mutual reactor is connected in
reactive load-sharing circuit they are connected to the primary series with an error detector in each voltage regulator, the
windings of devices called mutual reactors. detector functioning in the same manner as those used for
voltage regulation and real load sharing.
These are, in fact, transformers, which have

(i) A power source connected to their secondary

windings in addition to their primaries; in this
instance, phase "C" of the generator output, and

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Figure 1: The Sharing Of Reactive Load Between Paralleled Generators

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Category – B1.1 Sub Module 11.6 - Electrical Power

Let us assume that No.1 generator takes the greater share of

the load, i.e. it has become overexcited. The voltage induced in
the secondary winding of the corresponding mutual reactor will
be additive and so the error detector will sense this as an SYNCHRONIZING LIGHTS
overvoltage. The resulting d.c. error signal is applied to the pre-
amplifier and then to the power amplifier the output of which is In some power-generating systems a method of indicating
adjusted to reduce the amount of exciter current being delivered synchronization between generator outputs forms part of the
to the No.1 generator. In the case of the other three generators paralleling system, and consists of lights and frequency
they will have been carrying less than their share of the reactive adjustment controls. A schematic diagram of the method based
load and, therefore, the voltages induced in their mutual on that adopted for the triple generator system of the Boeing
reactors will have lagged behind the currents from the 727 is given in Figure 1.
generators, resulting in opposition to the voltages sensed by the
secondary windings. Thus, the output of each power amplifier The lights are connected into phases "A" and "C" of each
will be adjusted to increase the amount of exciter current being generator between the generator breakers and a synchronizing
delivered to their associated generators until equal reactive busbar, via a selector switch. The switch is also used for
load-sharing is restored between generators within the connecting a voltmeter and a frequency meter to each
prescribed limits. generator output phase "B". The frequency adjustment controls
are connected into the circuit of the load controllers.

The generators are connected to their respective load busbars

and the synchronizing busbar, via generator breakers and bus-
tie breakers respectively; each breaker being closed or tripped
by manual operation of switches on a panel at the Flight
Engineer's station. The breakers also trip automatically in the
event of faults detected by the generator control and protection
system. The field relays are similarly operated. Indicator lights
are located adjacent to all switches to indicate either of the
closed or tripped conditions.

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Prior to engine starting, the bus-tie breakers and field relays are
closed (indicator lights out) and the generator breakers are As the third engine is started, the meter selector switch is
tripped (indicator lights on). As the first engine is started, the positioned at GEN 3, and by following the same procedure just
meter selector switch is positioned at GEN 1 to connect phases outlined, number 3 generator is connected to its load bulbar.
"A" and "C" of this generator to the synchronizing bulbar via the With all three generators thus connected their subsequent
synchronizing lights. Phase "B" is connected to both voltmeter operation is taken care of automatically by the load-sharing
and frequency meter readings of which are then checked. Since sensing circuits of the associated control and protection unit.
at this moment, only the number 1 generator is in operation,
then with respect to the other two, it will of course, produce It is important to note that a generator must never be connected
maximum voltage and phase difference and both synchronizing to its load busbar when the synchronizing lights are on. Such
lights will flash at a high frequency as a result of the current action would impose heavy loads on the generator or CSD and
flow through them. The frequency control knob for the generator possibly cause damage to them. If, at any time the
is then adjusted until its load controller has trimmed synchronizing lights flash alternately, a phase reversal is
CSD/generator speed to produce a "master" frequency of about indicated and the appropriate generator should not be used.
403Hz and simultaneous flashing of both synchronizing
lights.

When the second engine is started, the meter selector switch is

positioned at GEN 2 to connect the synchronizing lights and
meters to the appropriate phases of number 2 generator, and its
frequency is also adjusted in the manner just described. The
number 1 generator is then connected to its load bulbar by
closing its generator breaker. This action also connects the
generator to the synchronizing busbar, and since the
synchronizing lights are now sensing the output of the second
oncoming generator, their flashing frequency will be very much
less as a result of less voltage and phase difference between
the two generator outputs. The frequency of the second
generator is then adjusted to obtain the greatest time interval
between flashes of the synchronizing lights, and while the lights
are out (indicating both sources of power are in phase) the
number 2 generator is connected to its load busbar by closing
its breaker.

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Figure 1: Synchronizing Lights

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PROTECTION CIRCUITS UNDERVOLTAGE PROTECTION

OVERVOLTAGE PROTECTION In a constant frequency a.c. system, the circuit arrangement for
under voltage protection is similar in many respects to that
An overvoltage protection system adopted in one example of a shown in Fig., since it must also trip the generator control relay,
constant frequency (non-paralleled) a.c. Generating system is the generator breaker, and must also annunciate the condition.
shown in basic form in Figure 1. The voltage level at which the circuit operates is less than 100 ±
3 volts. A time delay is also included and is set at 7 ± 2
The detector utilizes solid-state circuit elements, which sense all seconds; its purpose being to prevent tripping due to transient
three phases of the generator output, and is set to operate at a voltages, and also to allow the CSD to slow down to an under-
level greater than 130±3 volts. An overvoltage condition is an frequency condition on engine shutdown and so inhibit tripping
excitation-type fault probably resulting from loss of sensing to, of the generator control relay.
or control of, the voltage regulator such that excessive field
excitation of a generator is provided. When generators are operating in parallel, under-voltage
protection circuits are allied to reactive loadsharing circuits.
The signal resulting from an overvoltage is supplied through an
inverse time delay to two solid-state switches. When switch S1
is made it completes a circuit through the coil of the generator
control relay, one contact of which opens to interrupt the
generator excitation field circuit. The other contact closes and
completes a circuit to the generator breaker trip relay, this in
turn, de-energizing the generator breaker to disconnect the
generator from the busbar. The making of solid-state switch S2
energizes the light relay causing it to illuminate the annunciator
light, which is a white one in the actual system on which Fig. is
based. The purpose of the inverse time delay is to prevent
nuisance tripping under transient conditions.

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Figure 1: Over Load Protection

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OVER-EXCITATION AND UNDER-EXCITATION DIFFERENTIAL CURRENT PROTECTION

PROTECTION
The purpose of a differential current protection system is to
Over-excitation and under-excitation are conditions, which are detect a short-circuited feeder line or generator busbar which
closely associated with those of over voltage and under voltage, would result in a very high current demand on a generator, and
and when generators are operating in parallel, the conditions possibly result in an electrical fire. Under these conditions, the
are also associated with reactive current. Protection is therefore difference between the current leaving the generator and the
afforded by a mixing circuit. If the reactive current is the same in current arriving at the busbar is called a differential fault or a
the generators paralleled, there will be no output from the feeder fault. In an a.c. system, current comparisons are made
circuit. When an unbalance occurs, e.g. a generator is phase for phase, by two three-phase current transformers, one
overexcited, voltages will be produced in both over-excitation on the ground or neutral side of the generator (ground DPCT)
and under-excitation sections of the circuit, and these voltages and the other (the load DPCT) on the downstream side of the
will be fed to the overvoltage and under voltage circuits. As a busbar. Figure 1 illustrates the arrangement and principle of a
result, the overvoltage circuit will be biased down so that it will system as applied to a single-phase line.
trip the generator breaker at a lower level. The under voltage
circuit will also be biased down so that it will trip the breaker at a If the current from the generator is I, and the fault current
lower voltage. Since the overall effect of over-excitation is to between the generator and bulbar equals If, then the net current
raise the busbar voltage then the overvoltage circuit provides at the busbar will be equal to I- If. The fault current will flow
the protective function. through the aircraft structure and back to the generator through
the ground DPCT. The remainder of the current I - If, will flow
With an under-excited generator, the voltages fed to the through the load DPCT, the loads, the aircraft structure, and
overvoltage and under voltage circuits cause the biasing to then back to the generator via the ground DPCT. Thus, the
have the opposite effect to over-excitation. Since under- ground DPCT will detect the generator's total current (I - If) + (If)
excitation lowers the busbar voltage, then the under voltage which is equal to I, and the load DPCT will detect I -If.
circuit provides the protective function.
If the difference in current (i.e. the fault current; between the two
UNDERFREQUENCY AND OVERFREQUENCY current transformers on the phase line is sensed to be greater
PROTECTION than the specified limit (20 or 30 amperes are typical values) a
protector circuit within a generator control unit will trip the
Protection against these faults applies only to a.c. generating generator control relay.
systems and is effected by the real loadsharingcircuit of a
generating system.

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Figure 1: Differential Current Protection

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MERZ-PRICE PROTECTION SYSTEM

This system is applied to some a.c. generating systems to

provide protection against faults between phases or between
one of the phases and ground. The connections for one phase
are shown in Figure 1.; those for other phases (or other feeders
in a single-phase system) being exactly the same. Two similar
current transformers are connected to the line, one at each end,
and their secondary windings are connected together via two
relay coils. Since the windings are in opposition, and as long as
the currents at each end of the line are equal, the induced
emfare in balance and no current flows through the relay coils.
When a fault occurs, the fault current creates an unbalanced
condition causing current to flow through the coils of the relays
thereby energizing them so as to open the line at each end.

Figure 1: Merz-Price Protection

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SOLID-STATE GCU VOLTAGE-REGULATOR SECTION

A solid-state GCU for large aircraft is shown in the photograph Voltage regulation in the GCU shown in Fig. 1 is accomplished
of Fig. 1. This unit combines the functions of voltage regulation, primarily by means of a zener diode connected in a circuit with a
power control, contactor and relay control, and a variety of voltage divider system and other controlling elements. In
protective functions. The GCU shown is completely solid-state addition to maintaining a constant voltage of 115/200 V, three-
(static) except for the relays, which are hermetically sealed. The phase average, the voltage regulator also provides for equal
circuitry is mounted in a standard case described as a 3/8 short division of the reactive portion of the load when the generators
ATR (Air Transport Rack) case. It is easily fitted to or removed are operating in parallel and limits the output current to 635 A
from its installation in a standard ARINC (Aeronautical Radio, maximum. It further limits the demand of the generator to 240
Inc.) rack by use of the handle on the front of the case. A latch hp (179 kW).
device on the handle holds the unit securely in place and also
serves as a jack during the engaging or disengaging of the Reactive load is sensed by means of current transformers in the
external connector. A phase of each generator. If the reactive load is evenly divided,
no correction signal will be developed. When reactive load is not
The major circuit assemblies of the GCU are shown outside the evenly divided, a signal will be produced that causes the
case in Figure 2. It will be noted that the individual circuits are regulator to reduce the excitation of any generator carrying a
mounted on printed circuit boards that can be easily plugged in high portion of the load and increase excitation of generators
or removed from the case. This type of construction greatly carrying a low portion of the reactive load.
simplifies maintenance and troubleshooting. Circuit boards
having defects can be quickly removed and replaced with
functional circuits.

When the circuit boards (modules) are installed in the case,

they are interconnected by means of circuitry in the bottom of
the case. This circuitry is shown in next page.

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Figure 1: Solid State GCU Figure 2: Circuit Assemblies Of The GCU

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CONTROL SECTION  Undervoltage

 Differential fault
The control section of the GCU controls various contactors and
relays so they are closed only when satisfactory conditions  Over-excitation
exist, and it opens them automatically when designated
conditions exist. The following control functions are handled by  Under-excitation
the control section:
 Difference current
 Generator field control and indication  Over-frequency
 Generator circuit-breaker (GCB) control  Open phase
 Bus-tie circuit-breaker (BTB) control
The protective system automatically disconnects and
 Essential-power relay control deactivates any generator in the system when any of the listed
faults exist beyond a predetermined length of time.
 Automatic paralleling
 Anticycling FAULT ANNUNCIATION

 Bus-tie circuit-breaker automatic reclose The GCU system described here is designed to provide
command signals to an external maintenance annunciator to
PROTECTIVE FUNCTIONS identify certain system components that have developed faults.
Fault annunciation is provided for the following line-replaceable
Although the control functions mentioned in the foregoing system components:
paragraph may be considered protective in a number of
respects, the GCU exercises other functions for protection when  Generator
system faults occur. These protective functions prevent damage
 GCU
to generators and system components that would occur without
the actions of the protective system. The following protective  CSD
functions are provided:
 Load controller
 Under speed of any generator
 Generator feeders
 Overvoltage
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CIRCUIT PROTECTION
Fuse Carrier or Fuse Holder
INTRODUCTION
A carrier for holding a fuse-link, arranged to be easily inserted
Modern aircraft have numerous electrical circuits each of which between fixed contacts. A cartridge or other container of a fuse
has to be protected. element may form a fuse carrier or part of a fuse carrier.

Fuses and circuit breakers are devices which protect both the Current Rating
circuit and the power supply in the event of fault causing
excessive current flow. They are placed in series with the load A current, less than the minimum fusing current, stated by the
to take all the load current. manufacturer as the current that the fuse will carry continuously
without deterioration.
Let's look first at the terminology associated with fuses.
Minimum Fusing-current
FUSE TERMINOLOGY
The minimum current at which a fuse element in a fuse will melt.
The term 'fuse' is used generally either for the wire element In general, prolonged use near the minimum fusing current
itself or for the complete unit. Various expressions are used for causes 'ageing' and the fuse will eventually operate, whereas
ratings or capacities. occasionalmoderate overloads may be expected to have little
effect. Ageing may be minimized by using anti-oxidizing
Fuse elements such as silver plated copper or tin, and by designing
for low working temperatures.
A device for protecting electrical apparatus against the effect of
excess current; it consists of a piece of fusible metal which is Rupturing Capacity
connected in the circuit to be protected and which melts to
interrupt the circuit when an excess current flows. The term fuse This is the maximum current which a fuse may be expected to
also includes the necessary mounting and cover (if any). operate without explosion or shattering, and is usually specified
in conjunction with a maximum circuit voltage. Some aircraft
Fuse Element fuses have breaking capacities up to 30,000 ampsat voltages
substantially higher than the highest system voltages. There are
The essential part of a fusible cut-out. The part which is termed high rupturing capacity (HRC) fuses.
designed to melt and thus open the circuit.

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TEST
PROD
Voltage Rating HOLE
SPRING
PLATE
A voltage stated by the manufacturer’s as the highest declared
voltage that may be normally associated with the fuse. B.C. PINS
The voltage rating of each fuse in a circuit (which may consist of
only some of the conductors in a system) should not be less 7 AMP
FUSE
than the highest voltage (rmsa.c. or d.c.) between conductors in
that circuit. BODY

A fuse suitable for a.c. 50 cycle circuits may not be suitable for FIXING
a.c. circuits at other frequencies, i.e., 400 cycles, or for d.c. NUT
circuits. Similarly a fuse proved suitable for d.c. circuits may not
SCREW-IN
do for a.c. circuits. Figure PATTERN
B.C.
Generally speaking a low rating fuse will take two or three times PATTERN
Figure 1
as high a voltage on commercial 50 cycle a.c. as on d.c.; the
difference being less on high currents
TERMINA
L
HIGH RUPTURE CAPACITY (HRC) FUSES

HRC fuses are commonly used in aircraft electrical systems and

range from 1 amp to 500 amp ratings. They comprise ceramic
tubular bodies cemented to metal end caps which are
connected together by a fusible element, or· elements in
parallel, embedded in quartz dust. The quartz dust quenches
the arcs which occur when the fuse operates and gives this
device a high breaking capacity (in the order of 30,000 A).
Figure 2

Shown below are small type and larger type) HRC fuses.

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SLOW OPERATING AIRFUSE OR LIMITER Limiters are capable of handling short circuit currents of about
5000 amps at 28 volts without structural failure of the ceramic
housing. They are designed to isolate any portion of an
electrical distribution system which may be drawing fault
current, quickly enough to prevent voltage collapse or damage
to the generator system. They do however operate slowly
enough to be useful in circuits carrying occasional current
surges, such as motor starting currents.

The fusible element is a single strip of tinned copper drilled and

shaped at each end to form lug connections. The center is
waisted to the required width to form the fusing area and is
enclosed in a rectangular ceramic housing, one side of, which
has a window to show the state of the fuse. Fig 1

Shown in Fig 2. on the next page is a group of characteristics

for limiters between 35 and 500 amps.

Figure 1

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one of a higher rating is strictly forbidden.

Fuses which carry continuous current whose value may be near

to the fuse rating tending to deteriorate, and will eventually
rupture without the rated current being exceeded. It is therefore
necessary to inspect all fuses periodically; any which show
signs of sagging or discolored elements should be immediately
replaced. Spare fuses should also be inspected and any
showing signs of cracks, end-caps loose or other damage (due
mainly to vibration) must be replaced, particularly if they are
spares for emergency circuits.

INTRODUCTION

The expression circuit breaker or CB can be used to include any

type of switch or device that will open an electrical circuit. Within
this category are a number of items called relays, solenoids,
Figure 2 contactors, etc., most of which can be quoted as being within
the electromagnetic switch range. However, 'circuit breaker' in

the aircraft industry is synonymous with the popular thermally

SERVICING
operated circuit protective device used as an alternative to the
fuse.
The blowing of a fuse indicates that it has been carrying
excessive current, i.e., that a fault possibly exists in the circuit. THERMAL CIRCUIT BREAKERS
A replacement fuse should never be fitted until the reason for
the blowing of the previous fuse has been ascertained, and if Thermal circuit breakers are essentially circuit breakers in which
necessary, corrective action has been taken to prevent a further
the trip mechanism is actuated by heating a bi-metallic element,
occurrence of the same nature. either directly through the passage of current through the
element or indirectly by radiation from an adjacent element
Replacement fuses should always be of the correct rating for which carries the current.
the circuit. Do not be misled by the rating of the blown fuse
removed, it is sometimes safer to check the Maintenance
Manual. The practice of replacing a repeatedly blowing fuse by
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Push Type
A change in curvature of the bi-metal strip, caused by the
difference in the coefficients of expansion of the materials, The push types have push-buttons on the face. One of these is
operates the trip mechanism (some type of spring loaded dolly for closing the breaker, the amount by which the button projects
and toggle mechanism). from the face giving an indication of open or closed condition.

For general safety, the trip free circuit breaker is the only The second is a 'manual trip' button, sometimes recessed into
acceptable standard. It must be impossible for the contacts to the face to avoid accidental tripping, but requiring deliberate
be held closed under overload conditions. Earlier, the possibility attention (by ball point or match stick) to manually trip.
of holding an important circuit manually during a fault condition
had been envisaged. This was not an acceptable practice and it Push/Pull Type
is now a requirement of Airworthiness Authorities that trip free
breakers must always be used. The push/pull type are different in that they have only one
button on the face which is pushed to close with just sufficient of
Thermal circuit breakers have now been developed to a degree the button protruding to allow it to be gripped by the fingers and
of reliability to compete strongly with the fuse as a method of pulled out to open the breaker.
circuit protection. They are attractive in that they can be quickly Let's look more closely now at the push type of circuit breaker.
and easily reset by the flight crew, and they are capable of
replacing two components, i.e., the fuse and the switch. Circuit
breakers also compete with fuses in that development has
resulted in their size and weight being reduced.

A circuit breaker must be capable of carrying the normal current

indefinitely without deterioration and open in the event of
excessive current due to fault conditions.

On present day aircraft they can be found grouped on control

panels in positions where they are under flight crew supervision.

There are two main types of circuit breakers in current use:-

 Push type.
 Push/pull type.

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PUSH TYPE CIRCUIT BREAKERS PUSH/PULL TYPE CIRCUITBREAKERS

The push type circuit breaker (Fig. 1) is trip free and has two Take a look at Fig. 2 to familiarize yourself with the main parts
buttons, a close button and a trip button. of a push/pull circuit breaker.
The circuit is made when the close button is pushed in. The INSULATING BLOCK BI-METAL ASSEMBLY
circuit can be broken either by manual operation of the trip
button or when the bi-metal element trips the mechanism in an
overload condition.

CLOSE
EXTERNAL AND AUXILIARY POWER SUPPLIES

Fig. 2 Push/Pull Type Circuit Breaker

When the button is pushed in, the fixed and moving contacts
are held closed by the bi-metal assembly dropping into a latch.

TRIP
The CB can be tripped by the bi-metal element in an overload
condition or by pulling the actuating button.

When the white collar is in view it indicates that the CB is

tripped.
Fig. 1 push type circuit breaker
The figure on the button indicates the current rating of the CB.

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EXTERNAL AND GROUND POWER D.C. SYSTEMS

Electrical power is required for the starting of engines, operation A basic system for the supply of d.c.is shown in Fig. 2, and from
of certain services during "turn-round" servicing periods at this it will also be noted how, in addition to the external power
airports, e.g. Lighting, and for the testing of electrical systems supply, the battery may be connected to the main busbar by
during routine maintenance checks. The batteries of an aircraft selecting the "flight" position of the switch. As the name
are, of course, a means of supplying the necessary power, and suggests this is the position to which the switch is selected
although capable of effecting engine starts their capacity does when the aircraft is in flight since under this condition the
not permit wide scale use on the ground and as we have generator system supplies the main busbar and the battery is
already learned, they are restricted to the supply of power under constantly supplied with charging current.
emergency conditions. It is necessary, therefore, to incorporate
a separate circuit through which power from an external ground The external power connector symbol shown in the diagram
power unit (see Fig. 1) may be connected to the aircraft's represents a twin-socket type of unit, which although of an
distribution busbar system. In its simplest form, an external obsolete type is worth noting because it established certain
power supply system consists of a connector located in the aspects, which are basic in the design of present-day
aircraft at a conveniently accessible point (at the side of a connectors or receptacles as they are also called, namely the
fuselage for example) and a switch for completing the circuit dimensioning of pins and sockets, and the method of protecting
between the ground power unit and the busbar system. them. The pins were of different diameters to prevent a reverse
polarity condition, and the cover of the unit had to be rotated to
In addition to the external power supply system, some types of expose the sockets.
aircraft carry separate batteries which can supply the ground
services in the event that a ground power unit is not available in An example of a current type of unit is shown in Fig. 3. It
order to conserve the main batteries for engine starting. consists of two positive pins and one negative pin; one of the
positive pins is shorter and of smaller diameter than the
In the majority of large public transport aircraft, complete remaining pins. The pins are enclosed by a protective shroud,
independence of ground power units is obtained by special and the complete unit is normally fitted in a recessed housing
auxiliary power units installed within the aircraft. located at the appropriate part of the airframe structure. Access
to the plug from outside the aircraft, is via a hinged flap provided
with quick-release fasteners.

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Figure 1: External Ground Power Unit

Figure 2: DC Supply System

Figure 3: Receptacle

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The circuit of a three-pin receptacle system is illustrated in Fig. Indication that both busbars are also "tied" to the ground power
1,and from this it will be noted that the short positive pin is supply is provided by magnetic indicators "A" and "B" which are
connected in the coil circuit of the external power relay. The energized from the vital busbar via the auxiliary contacts of the
reason for this is that in the event of the external supply socket contactor.
being withdrawn with the circuit "live", the external power relay
will de-energize before the main pins are disengaged from the In some aircraft, and as an example we may consider the
socket. This ensures that breaking of the supply takes place at Boeing 737, a separate external power connector is installed for
the heavy-duty contacts of the relay thus preventing arcing at starting an auxiliary power unit in the event that the aircraft's
the main pins. battery is inoperative. The circuit arrangement is shown in Fig 3.

In some aircraft d.c. Power is distributed from a multiple busbar The receptacle is located adjacent to the battery together with
system and it is necessary for certain services connected to two circuit breakers indicated as "A" and "B" in the diagram. The
each of the bus bars to be operated when the aircraft is on the positive pin of the receptacle is coupled directly to the battery
ground. This requires a more sophisticated arrangement of the busbar via circuit breaker "A", and forms a parallel circuit with
external power supply system and the circuit of one such the battery. Before external power is applied, circuit breaker "B"
arrangement is shown in Fig. 2. In addition to the external must be tripped in order to prevent damage to battery charger.
supply relay or contactor, contactors for "tying" bus bars
together are provided, together with magnetic indicators to
indicate that all connections are made.

When the external ground power unit is connected to the aircraft

and the master switch is selected "on", it energizes the external
supply contactor, thus closing its auxiliary and main sets of
contacts. One set of auxiliary contacts complete a circuit to a
magnetic indicator, which then indicates that the external supply
is connected, and on ("C" in Fig. 2), a second set complete
circuits to the coils of No. 1 and No. 3 bustie contactors while a
third and main heavy-duty set connect the supply direct to the
"vital" and No. 2. bus bars. When both bus-tie contactors are
energized their main contacts connect the supply from the
external supply contactor to their respective bus bars.

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Figure 1: Three Pin Receptcle System

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Figure 2: External Power Supply Circuit System

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Figure 3: Starting an APU by External Power

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A.C. SYSTEMS This action connects a d.c. supply to the trip coil of the external
power breaker, thus releasing its main and auxiliary contacts
In aircraft, which from the point of view of electrical power are and isolating the external power from the a.c, main busbar.
principally of the "a.c. Type", then it is essential for the external
supply system of the installation to include a section through Figure 2 illustrates an external a,c. power receptacle and control
which an external source of a.c. Power may be supplied. The panel arrangement generally representative of that adopted in
circuit arrangements for the appropriate systems vary between large public transport aircraft. The receptacle is of the six-prong
aircraft types but in order to gain some understanding of the type three of the large prongs are for the corresponding a.c.
circuit requirements and operation generally we may consider power phases, and a fourth large prong for the ground
the circuit shown in Fig. 1. connection between the aircraft structure and external power
unit. The two small shorter prongs connect d.c. power for the
When external power is coupled to the receptacle three-phase operation of interlocking relays which connect the externala.c.
supply is fed to the main contacts of the external power breaker, power to the aircraft.
to an external power transformer/rectifier unit (T.R.U.) and to a
phase sequence protection unit. The T.R.U. provides a 28 volt The control panel contains three single-phase a.c. circuit
d.c. feedback supply to a hold-in circuit of the ground power breakers, and three more breakers, which protect relay control,
unit. If the phase sequence is correct the protection unit and indicating light circuits within the aircraft's external power
completes a circuit to the control relay coil, thus energizing it. A supply circuit. Indicator lights, interphone jack plug sockets,
single-phase supply is also fed to an amber light which comes and pilot's call button switch are also contained on the panel.
on to indicate that external power is coupled, and to a voltmeter
and frequency meter via a selector switch. The white indicator light is only illuminated whenever external
a.c. power is connected but is not supplying power to any a.c.
The circuit is controlled by an external power switch connected load busbar on the aircraft. The blue light is illuminated
to a busbar supplied with 28 volts d.c. from the aircraft battery whenever a.c, power is being supplied to the load bus bars.
system. When the switch is set to the "close" position current
flows across the main contacts of the energized control relay, to The pilot's call button switch and interphone jack plug sockets
the "close" coil of the external power breaker, thus energizing it provide for communication between ground crew and flight
to connect the external supply to the three-phase a.c. main crew.
busbar. The external power supply is disconnected by selecting
the "trip" position on the external power switch.

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Figure 1: AC System External Power

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Figure 2:external a.c. Power receptacle

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AUXILIARY POWER UNITS

Many of today's aircraft are designed so that if necessary, they

may be independent of ground support equipment. This is
achieved by the incorporation of an auxiliary power unit (A.P.U.)
in the tail section, which, after being started by the aircraft's
battery system, provides power for engine starting, ground air
conditioning and other electrical services. In some installations,
the A.P.U. is also used for supplying power in flight in the event
of an engine-driven generator failure and for supplementing the
delivery of air to the cabin during takeoff and climb.

In general, an A.P.U. consists of a small gas turbine engine, a

bleed-air control and supply system, and an accessory gearbox.
The gas turbine comprises a two stage centrifugal compressor
connected to a single stage turbine. The bleed-air control and
supply system automatically regulates the amount of air bleed
from the compressor for delivery to the cabin air conditioning
system. In addition to those accessories essential for engine
operation, e.g. fuel pump control unit and oil pumps, the
accessory gearbox drives a generator which is of the same type
as those driven by the main engines, and having the same type
of control and protection unit.
A motor for starting the A.P.U. is also secured to the gearbox
and is operated by the aircraft battery system or, when
available, from a ground power unit. In some types of A.P.U.
the functions of engine starting and power generation are
combined in a starter/generator unit. In order to record the
hours run, an hour meter is automatically driven by an A.P.U.
An external view of a typical unit and a typical installation, are
shown in Figs. 1 and 2 respectively.

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Figure 1: Auxiliary Power Unit (A.P.U.)

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Figure 2: Auxiliary Power Unit (A.P.U.)

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EMERGENCY POWER GENERATION

All aircraft are equipped with engine driven electrical power As a result the APU is capable of powering the entire electrical
generators. The number of generators is normally equal to the network in case of electrical emergency in flight.
number of engines installed on the respective aircraft. A secondary function of the APU generator is to power the
electrical system on ground whenever necessary
Since electrical power is an essential source of energy for the
operation of most of the components on the aircraft, it’s INVERTERS
necessary to ensure that even in case of an engine failure or
the failure of all the engine driven generators, there is an An inverter is a device for converting direct current into
alternate source of electrical power, to at least power the alternating current at the frequency and voltage required for
essential electrical components necessary for a safe landing. particular purposes. Certain systems and equipment in aircraft
electrical or electronic systems require 26- V 400-Hz ac power,
To full fill this requirement various aircraft manufacturers have and others require 115-V 400-Hz power. To provide this power,
come up with their own solutions. it is often necessary to employ an inverter.
Inverters are typically used on large aircraft for emergency
Here is a list of some of the available sources of alternate situations only. In this case the aircraft employs engine-driven
electrical power: generators (alternators) to supply needed ac power during
normal operating conditions. If all ac generators should fail, the
 APU Generators inverter would then be used to convert battery dc power into ac
 Inverters (static and rotary) power available for essential ac loads.
Many light aircraft employ static inverters during normal
APU GENERATORS operating conditions. These aircraft require a relatively small
amount of alternating current and therefore utilize engine driven
During normal operation it’s the engine driven generators that dc generators or alternators as their main electric power source.
power the entire electrical network. In case of loss of engine Aircraft using inverters to get the alternating current they need
driven generators, the APU generator can be made use of. In include the Beech craft King Air, most Cessna 421 and 310
most aircraft a generator, which is of the same type as those aircraft, and many small business jets, such as the Lear Jet 23.
driven by the main engines, is attached to the APU. Here These aircraft use alternating current to power a variety of
there’s no requirement of a CSD as the APU itself is runs at a components, including engine instruments, heated windshields,
constant speed. and lighting circuits. In some cases these components are
feasible only if operated by ac power; therefore, the inverter is
essential.

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There are two basic types of inverters, rotary and static. inverter should be sent to a repair shop that is equipped to
Modem aircraft employ static inverters because of their perform the electrical and electronic work and tests that may be
reliability, efficiency, and weight savings over rotary inverters. required.

ROTARY INVERTERS STATIC INVERTERS

For many years inverters were simply special types of motor A static, or solid-state, inverter serves the same functions as
generators; that is, a constant-speed motor was employed to other inverters. However, it has no moving parts and is
drive an alternator that was designed to produce the particular therefore less subject to maintenance problems than the rotary
type of power required. inverter.

A typical rotary inverter is shown in Figure 1. The rotary The internal circuitry of a static inverter contains standard
section of this unit consists of a dc motor driving an ac electric and electronic components, such as crystal diodes,
generator. The rotors of the motor and the alternator are transistors, capacitors, and transformers. By means of an
dynamically balanced and are mounted on the same shaft. Fans oscillator circuit, the inverter develops the 400-Hz frequency for
are also mounted on the shaft to provide for air-cooling. which it is designed. This current is passed through a
transformer and filtered to produce the proper wave shape and
A four-pole, compound, compensating field winding and a wave- voltage. The unit shown in Figure 2 utilizes an input voltage of
wound armature are utilized in the motor. A damper winding in 18 to 30 V dc and produces an output of 115- V single-phase
the salient poles of the alternator aids in maintaining output alternating current with a frequency of 400 Hz. The unit weighs
wave shape under single-phase operating conditions. 18.5 Ib [8.4 kg].

This particular inverter utilizes an input voltage of 26 to 29 V dc. Static inverters are easily removed for testing. If they require
The output is 115 V, single-phase; 115 V, three phase; and repair, they should be sent to an approved facility that is
200V, three-phase. Frequency is 400Hz for all phases. equipped to perform the work required.

Maintenance of rotary inverters is similar to that of motors and

generators. Maintenance practices are set forth in the
manufacturer’s maintenance or service manual.

The control box on top of the inverter should not be

disassembled in the field. If it appears to be defective, the entire

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Figure 2: A Static Inverter

Figure 1: A Rotary Inverter

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Because of the miniaturization of electronic components, static

inverters have become relatively small and lightweight. This has
made it possible for light single-engine aircraft to employ an ac
electrical system.

An electro-luminescent (EL) panel is a high-efficiency lighting

system powered by alternating current. The panel is constructed
of phosphorous material laminated between two clear plastic
layers, as shown in Figure 1. The phosphorous material glows
when connected to an ac voltage. The front plastic layer is
painted black except where stencils of appropriate letters or
numbers are placed. The letters and numbers therefore remain
clear in order to transmit the light from the glowing phosphorous
material. Figure 2 shows a typical static inverter and electro-
luminescent panel.

Figure 2: an electro luminescent panel and associated static inverter

Figure 1: an electro luminescent panel

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TRANSFORMER RECTIFIER UNITS Most large aircraft AC generator systems have dedicated TRUs,
which operate on the same principle, although they are slightly
A transformer rectifier unit (TRU) is used to convert AC into more sophisticated. A typical unit is illustrated below.
relatively smooth DC An example of a simple TRU circuit is that
which is used in a car battery charger, as shown in figure

Figure1 – Transformer rectifier unit components

This device takes the mains 240 VAC and converts it to

approximately 14 VDC to charge the battery. This is achieved
by a transformer, which first steps down the AC voltage to a
Figure2
reasonable level and then converts it via a bridge rectifier
assembly into DC.

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The TRU that is fitted to an aircraft is typically supplied with 115

V 400 Hz three-phase AC, which is stepped-down through a
three-phase star-star wound transformer and changed to 28
VDC by asix-rectifier bridge assembly. The output from the TRU
is then fed to the aircraft’s DC bus bars.

Each TRU has the following basic protections:

Overheat

When operating, most TRUs are cooled by air from a

thermostatically controlled cooling fan. If the TRU overheats
(150°-200°) due to fan or other failure, a warning light
illuminates on the flight deck. The TRU should then be switched
off, either manually or automatically
.
Reverse Current.

When the TRUs are operating in parallel with some other power
source, the failure of a rectifier in a TRU can cause a reverse
current to flow into it and may even cause afire. Reverse current
protection in the failed TRU is designed to sense the fault
current when it reaches approximately 1 amp, and disconnect
the TRU automatically from the DC bus bars.

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Since only one wire (and the ground) is needed to operate
electrical equipment, this is known as a single-wire system.
POWER DISTRIBUTION SYSTEMS

Modern aircraft require a consistent and reliable supply of Single-wire systems are possible only where the airframe is
electric power. There are four common sources of electric constructed from a conductive material, such as aluminum. On
power used during normal aircraft operations. These sources composite aircraft, some type of ground (negative) conductor is
are: dc alternators, dc generators, ac alternators (generators), required. In some cases two wires (one positive, one negative)
are used; in other cases a ground plane is added to the
and the aircraft's storage battery. As discussed, the aircraft's
structure of the aircraft.
battery is typically used for emergency operations and any
intermittent system overloads. DC alternators are typically used
Larger, more complex aircraft typically contain several bus bars.
on piston engine aircraft. DC starter-generators are used on
Each bus has the specific task of distributing electric power to a
medium-sized turbine-powered aircraft. AC alternators are used
given group of electrical loads. Bus bars are often categorized
on transport-category aircraft and some military aircraft. Some
as ac and dc, left and right, and essential and nonessential
form of electrical distribution system must be employed on distribution buses. On multi engine aircraft each engine-driven
every aircraft containing an electrical system. A simple power
alternator typically employs its own distribution bus. These
distribution system consists of a basic copper conductor, called
generator buses are then connected to their respective loads
a busbar or bus. This type of system is found on most single-
via distribution buses and associated bus ties.
engine aircraft. The bus is a conductor designed to carry the
entire electrical load and distribute that load to the individual
As described earlier, alternators or generators are used on
power users. Each electric power user is connected to the bus
nearly every aircraft to produce electric power. Since both units
through a fuse or circuit breaker.
operate similarly, the terms alternator and generator are used
interchangeably throughout the aircraft industry. Although there
On almost all aircraft the bus bar is connected to the positive
are obvious differences between alternators and generators, in
output terminal of the generator and/or battery. The negative this chapter the reader should consider the terms synonymous.
voltage is distributed through the metal structure of the aircraft.
The following pages present the FAA recommendations
The metal airframe (negative side of the voltage) is often
concerning power distribution systems, examine the various
referred to as the ground; hence this type of distribution is often
types of systems, and present their related control circuits.
called a negative-ground system. In all negative-ground
aircraft, the positive voltage is distributed to any given piece of
electrical equipment through an insulated wire, and the negative
voltage is connected through the airframe.

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On multiengine systems, if only one alternator fails, the battery The main alternator power cables are also considerably larger
and the other generator(s) will supply the needed electric than the normal circuit wiring; however, they are usually smaller
power. If this condition overloads the operating generator(s), the than the cables required to carry full battery current. This is
pilot may then shut off some nonessential equipment and because the battery is used for starting the engine, and the
reduce the load to a suitable level. starting current is very large. During operation of the aircraft, the
battery is connected to the system but is not supplying power.
The principal concern of the aviation maintenance technician Instead, it is taking power from the alternator in order to
with respect to electrical load in an aircraft is a situation where it maintain a charge. All the normal load currents are supplied by
is desired to add electric equipment. If the addition of such the alternator during flight. The distribution bus receives power
equipment has been tested and approved by the FAA for a from the alternator and/or battery during different operating
particular installation, instructions will be available from the modes. The bus then distributes the electric current through the
manufacturer of the equipment or the aircraft setting forth all individual circuit breakers to their respective loads. As shown in
requirements for the installation. These instructions should be the schematic (see Figure1), the circuit breakers are connected
followed carefully. directly to the distribution bus. This is done to prevent any
accidental short to ground of an unprotected wire. It is always
“Do not Operate the air conditioner and windshield heat desirable to protect as much wiring as possible. Any wires that
simultaneously." This placard would be placed near the
are not protected by a fuse or circuit breaker must be as short
windshield heat and air conditioner control switches.
as practical and protected by insulated covers or "boots" at all
terminal connections.
A SIMPLE ELECTRICAL SYSTEM

A simple electrical system for a light aircraft consists of a battery

Although the schematic diagram in Figure shows an entire
circuit, an alternator circuit with associated controls, an engine aircraft electrical system, this is not typical. Most manufacturers
starter circuit, a bus bar with circuit breakers, control switches,
prefer to divide aircraft schematics into individual systems. This
an ammeter, lighting circuits, and radio circuits. A schematic
becomes a necessity when dealing with large, complex aircraft.
diagram of the basic power distribution system is shown in
If an entire electrical system were represented in one
Figure. The high-current -carrying cables in this system are
schematic, the diagram would be extremely cluttered and too
connected from the battery to the main battery relay, from the
difficult to read. The following paragraphs will discuss schematic
battery relay to the starter relay, and from the starter relay to
diagrams of several power distribution systems.
the starter. The ground leads for the starter and the battery are
also of heavy cable.

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Figure 1: Aircraft Electrical System

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MAIN POWER DISTRIBUTION SYSTEMS 2. TWIN-ENGINE AIRCRAFT

1. SINGLE-ENGINE AIRCRAFT The simplified power distribution schematic of the Cessna 421
twin-engine aircraft is shown in Figure 2. This system employs a
The Piper Tomahawk aircraft alternator and battery power diode in series with the wire connecting the main and
systems are shown in Figure 1. This schematic is typical of a emergency power distribution buses. This diode will allow
single-engine power distribution system. In the Piper current flow from the main bus to the emergency bus, but not in
Tomahawk, and in most other light aircraft, the master solenoid the reverse direction. This is done to isolate the main bus in the
coil is switched on the negative side of the circuit. The master event that it should short to ground. In that configuration the
switch contains two independent poles and throws. The battery emergency bus could still receive battery power without being
master, on the left half of the switch, connects the ground affected by the short circuit.
(negative voltage) to the master solenoid. The solenoid's
negative lead is switched to ensure proper system operation in This schematic also contains a diode in parallel with the battery
case of an electrical short to ground. That is, if wire number P2A relay coil. If a diode is placed in parallel with an electromagnetic
should short to ground, the master solenoid will remain closed. coil, it is used to "clip voltage spikes." As explained before,
If the solenoid is closed, battery power is connected to the when a current starts to flow in a coil, or when the current flow is
starter solenoid and the distribution bus, thus creating no stopped, the inductance of the coil creates a voltage opposing
immediate danger. The alternator master switch, on the right the applied voltage. Thus whenever the switch is opened or
side of the combination master switch, connects the voltage closed within the relay or solenoid coil circuit, a voltage spike
regulator to the bus, turning on the alternator. In many aircraft or transient voltage is produced. This reverse-polarity voltage
the alternator side of the master switch can be operated only if spike will damage sensitive electronic equipment if it is allowed
the battery master is also turned on. This is done to ensure that to enter the electrical system. The diode in parallel with the
the battery is connected to the bus prior to the alternator. relay's coil will short together any reverse-polarity voltage spike;
however, the applied voltage will be unaffected. A bi-directional
There are two notes listed on the bottom left side of this zener diode can also be used for this purpose. The zener diode
diagram. Always refer to any notes or effective serial numbers conducts and short-circuits the relatively high-value transient
prior to using a schematic for maintenance purposes. voltage. The lower system voltage is unaffected. Remember,
the zener diode is a voltage-sensitive device.

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Figure 1:Single-Engine Power Distribution System

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In general, diodes of all types are becoming more popular in

aircraft power distribution systems. If a diode is placed in
parallel with a coil, it is used to prevent damage from induced
voltage spikes. If a diode is placed in series, it is used to create
a one-way current path between units.

The power distribution system for a light twin-engine Piper

airplane is shown in Figure 3. Since the airplane is equipped
with all the avionic equipment necessary for electronic
navigation and optimum flight performance, it is necessary for
the alternators to have a comparatively high capacity.

The electrical system shown in Figure 3 includes a 24-V, 17-Ah

battery enclosed in a sealed stainless-steel battery box. Two
24-V, 70-A alternators driven by the engines supply all the
normal power requirements of the aircraft and its equipment.
The battery supplies power for starting the engines and for
emergency peak loads.

The alternators are paralleled by using one voltage regulator to

control the field current for both alternators. The circuit diagram
in Figure3 show this is accomplished.

An overvoltage relay in the system serves as a safety valve in

case either one or both of the alternators should produce
dangerously excessive voltage. This condition would exist in the
case of failure of the voltage regulator. In the event that the
main voltage regulator fails and the overvoltage relay
disconnects the alternator fields from the system, an auxiliary
voltage regulator is available. Failure of the alternators can be
Figure 2:Twin-Engine Power Distribution System (Cessna 421)

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detected by a discharge indication for the battery and a zero
output on both alternator test positions.

The output of each alternator is checked by pressing a PRESS-

TO-TEST switch and observing the ammeter in the overhead
switch panel.

The test switches are shown as left alternator switch and right
alternator switch in the circuit diagram in Figure3.

Electrical switches for the various systems, including the

MASTER SWITCH, are located on the aircraft instrument panel.
The circuit breakers, located below the switches, automatically
open their respective circuits in case of an overload. The circuit
breakers can be reset merely by pressing the RESET button. If
a circuit breaker continues to disconnect, the trouble should be
located and repaired before another attempt is made to operate
the circuit.

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Figure 3:Light Twin-Engine Piper Airplane

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The power distribution system for a gas-turbine-powered

airplane with two engines, the Beechcraft Super King Air 200, is
shown in Figure 4. This schematic diagram is presented to
show the complexity of a modem aircraft electrical system and
the many functions that require electric power.

The two-engine-driven starter-generators receive power from

the main battery bus for starting purposes. During the generator
mode, the starter-generator output current is directed to the
right and left generator buses, respectively. The two generator
buses are connected to the feed buses (numbers 1to4) through
diodes and circuit breakers. This arrangement allows for both
generators to power four feed buses during normal operations
or remain isolated during accidental short circuits to ground.

The two generator buses are connected to the isolation bus

through isolation limiters. The isolation limiters, which are
often referred to as current limiters, are simply high-amperage
fuses. The isolation limiters can carry 325 A before opening.
These isolation limiters open during overload conditions. For
example, if an overload exists (a direct connection to ground) on
the right generator bus, the right-side isolation limiter will open
and disconnect the battery (via the battery and isolation buses)
from the right generator bus. At the same time, the right
generator will be disconnected from the right generator bus by
the right generator control unit. The diodes placed between the
right generator bus and the four feed buses will be reverse-
biased in this event and therefore will isolate the feed buses and
prevent current flow from the feed buses to the right generator
bus.

The right generator bus is therefore completely isolated, and the

rest of the electrical system operates normally.

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Figure 4:Power Distribution System For A Gas-Turbine-Powered Airplane With Two Engines (Beechcraft Super King Air 200)

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3. POWER DISTRIBUTION ON COMPOSITE AIRCRAFT Two methods are used to connect electric equipment to ground
plane, direct electrical bonding and indirect electrical
Composite aircraft present an interesting challenge when it bonding.
comes to electric power distribution, control of static electricity,
and lightning strikes. Several experimental composite aircraft
are currently being flown throughout the world. Most of these The direct method is used where electric equipment is mounted
aircraft are light single-engine planes with limited electrical adjacent to the ground plane. To properly ground a component
when the direct method is used, one must first remove a thin
systems. On transport-category aircraft, some components are
layer of composite material, paint, or any resistive coating to
made from composite materials, but the fuselage and wing
expose the wire mesh. The wire mesh is then coated with an
structure are made of aluminum. The Beechcraft Starship is anticorrosive agent, and the electric component is mounted
currently the only production composite aircraft. directly to the ground plane. As seen in Figure 1, it is very
important to remove as little material as possible during this
This plane is a twin-engine, turboprop corporate aircraft. The process and still provide a sufficient area for a proper ground
power distribution system of this composite aircraft will be connection. The exposed area is refinished with a protective
addressed here. coating after component installation.

The entire fuselage and wing assemblies of the Beechcraft The indirect method is used in areas of the aircraft that are not
Starship are made from composite materials that have too high adjacent to the ground plane. The indirect method uses a
of a resistance to easily carry current. To counteract this high- flexible metal strap called a bonding jumper to connect the
resistance effect, a ground plane is integrated into the ground plane to the electric component. The bonding jumper is
composite airframe. The ground plane is made of an aluminum attached to the ground plane in a manner similar to the one
mesh material. This material is similar to an aluminum window described above. The jumper is then attached to the component
screen. The aluminum mesh is bonded into the composite requiring an electrical ground (see Figure 2).
material during the manufacturing process. The ground plane is
located toward the inside of the aircraft structure for ease of Lightning protection for a composite aircraft requires the
bonding to electric equipment. The mesh runs throughout the installation of aluminum wire, which is interwoven in the outer
airframe, including structural parts, bulkheads, floorboards, ply of the aircraft skin. If lightning strikes the aircraft, the current
instrument panels, and electric equipment shelves. Virtually any is distributed over a large area through the aluminum wire.
portion of the aircraft that has electric equipment has a ground Since lightning typically enters the airframe at one extremity
plane integrated into the composite material. and leaves at another, the aluminum wire covers the entire
structure of the aircraft. All sections of this lightning diversion
wire must be connected by a low-resistance attachment. The

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lightning protection wire must not be used for electrical
grounding.

Only the aircraft's ground plane is designed to carry the current

of electric equipment.

The distribution of the positive voltage for composite aircraft is

virtually identical with such distribution on metal aircraft.
Positive-voltage distribution on Starship employs an automated
control system powering five separate distribution buses. During
normal operation, the buses are fed by the right and left 28- V,
300-A generators. The battery is being charged. In the event of
a generator failure, the automated system isolates the generator
and performs load shedding as needed.

Figure 2: Bonding Jumper

Figure 1: Direct Electrical Bonding

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4. LARGE-AIRCRAFT ELECTRICAL SYSTEMS

Each inverter is capable of converting direct current, supplied by
the battery, into ac power, which is distributed by the essential
Large-aircraft electrical systems have many similarities to those
ac bus.
systems found on small aircraft. On large aircraft there is
typically one battery and two or more ac generators
The static inverters supply a relatively small amount ofa.c
(alternators), which supply power to several distribution buses.
power; however, their output is adequate to power all essential
The ac generators are connected to the ac buses. The dc
ac equipment.
battery is connected directly to the battery or emergency bus.
The ac power produced by the generators is converted to direct
There are two basic configurations that are used to distribute
current where needed for special applications. Essential
electric power, the split-bus system and the parallel system.
lighting, flight control systems, and communication and
The split-bus system is used on most twin-engine commercial
navigation radios are high-priority electrical systems. Galley
aircraft, such as the Boeing 737, 757, and 767; the McDonnell
power, nonessential lighting, and various other comfort systems
Douglas MD-80; and the Airbus Industries A-300 and A-310. In
are considered low-priority electrical systems. These
a split-bus system the engine-driven generators can never be
nonessential systems are usually turned off during a partial
connected to the same distribution bus simultaneously. Under
generator system failure. In the case of a complete generator
normal conditions each generator supplies power only to its
failure, the battery will supply power for all essential electric
associated loads.
equipment. A fully charged battery will normally supply
approximately 20 to 30 min of emergency power.
In a parallel electrical system, the entire electrical load is equally
shared by all the working generators. Parallel ac power
POWER DISTRIBUTION SYSTEMS
distribution systems are typically found on commercial aircraft
containing three or more engines, such as the Boeing 727 and
Modem large aircraft use both ac and dc electric power. The
early 747s, the Lockheed L-1011, and the McDonnell Douglas
output of a typical generator is three-phase 115V ac; this is
DC-10 and MD-11. A modified parallel system is used on some
converted by transformer-rectifier units (TRUs) where dc
modem four-engine aircraft, such as the Boeing 747-400. All
power is needed, a TRU incorporates a step-down transformer
four generators are not necessarily paralleled in this system.
and a full-wave rectifier. Its output is 28 V dc. Most large aircraft
The right-side generators and the left-side generators can be
contain two or more static inverters, which are used for
connected, or they can be isolated from each other by means of
emergency situations (generator failure).
a split system breaker.

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THE SPLIT-BUS SYSTEM

The split-bus electrical system contains two completely

isolated powergenerating systems. Each system, the left and
the right, contains its own ac generator, transformer-rectifiers,
and distribution buses. The right and the left generators power
their respective loads independently of other system operations.
In the event of a generator failure, the operating generator is
connected in such a manner as to feed all the essential
electrical loads, or the APU (auxiliary power unit) generator may
be employed to carry the electrical load of the inoperative
generator.

Figure 1 shows a simplified schematic of a typical split-bus

system. This schematic shows the external power contactor
(EPC) closed and a ground power supply connected to the
aircraft. The bus tie breakers (BTBs), 1 and 2, are closed,
connecting external power to both transfer buses and their
respective electrical loads. In this configuration the generator
breakers (GBs) are open, thus disconnecting the generators
from the electrical system.

It should be noted that the various contactors, such as the BTBs

and GBs, are controlling three-phase current. The contactors
are therefore actually made up of a set of three contacts, one
contact to open or close the "hot" wire for each phase. In some
cases the contactors may have four contacts, one for each
phase and one for the neutral wire. The buses also consist of
three distinct units, one for each phase. The power distribution
diagrams presented here are simplified diagrams and do not
show the actual wiring for each phase of the ac power
generated.

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Figure 1:The Split-Bus Electrical System

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In the case where the APU would be used to supply electric Thus the APU generator is connected to transfer bus 1, and
power to the entire aircraft, the EPC would open and the APU once again two independent generators are operating to
generator breaker would close. This would distribute the electric supplyall the aircraft electrical power.
power from the APU generator to both transfer buses.
The major advantage of split-bus system is that the generators
If both engine-driven generators are operating, the current flow operate independently; that is, generator output frequencies and
is from each generator toits respective transfer bus, as phase relationships need not be so closely regulated. Parallel
illustrated in Figure 1. At this time BTBs 1 and 2 are open, GBs systems require strict operating limits. Split-bus systems are, in
1 and 2 are closed, and the transfer relay is in its normal effect, more tolerant of frequency variance.
position. It can be seen from this distribution diagram that the
two generators operate completely independently of each other.

If one engine-driven generator fails, the opposite generator is

connected to both transfer buses in order to power the entire
electrical system. Under this configuration, nonessential loads
are automatically removed from the system in order to avoid a
generator overload. The schematic in Figure 2 shows a failure
in generator 1. The current from generator 2 is divided to
transfer buses 1 and 2 via the correct positioning of the transfer
relays. In this case transfer relay 1 is automatically activated to
its abnormal position, which connects generator bus 2 with
transfer bus 1. At this same time GB 1 opens to disconnect the
failed generator from the electrical system. This entire process
is controlled automatically within several microseconds, and the
flight continues uninterrupted. On certain aircraft, if a primary
generator fails, the flight crew may elect to employ the APU
generator. In this situation the APU must be started and its
generator connected by closing the APU generator breaker
(GB). This automatically closes BTB 1 and repositions transfer
relay 1 to its normal position.

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Figure 1:Split-Bus System

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Figure 2: Split-Bus System

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PARALLEL ELECTRICAL SYSTEMS

In a parallel electric power distribution system, all ac

generators are connected to one distribution bus. This type of If two or more generators fail, their respective generator circuit
system maintains equal load sharing for three or more ac breakers open, and they are isolated.
generators. Since the generators are connected in parallel to a
The remaining generator(s) supply the power to the entire
common bus, all generator voltages, frequencies, and their
system. In this situation nonessential electrical loads are
phase sequence must be within very strict limits to ensure automatically disconnected from the system. This prevents an
proper system operation. accidental overload of the operating generator(s).
The simplified block diagram in Figure 1 represents a typical
four-generator parallel system. This diagram shows a normal
operating configuration; all four generator circuit breakers
(GCBs) and bus tie breakers (BTBs) are closed. All four
generators are synchronized and connected in parallel by the tie
bus. The tie bus is often referred to as a synchronizing bus;
its purpose is to connect the output of all operating generators.
The load buses are used to distribute the generator current to
the various electrical loads.

If one generator fails, its receptive GCB opens. This isolates

that generator from its load bus; however, that load bus still
continues to receive power while connected to the tie bus. In
case of a load bus overload, the bus is isolated by the opening
of its GCB and BTB. This mode of failure is illustrated in Figure
2. In this diagram, load bus 3 has been isolated. The likely
suspicion is that load bus 3 has been shorted to ground and the
bus power control unit has automatically tripped the appropriate
contactors. This manner of isolation takes place whenever one
or more load buses are faulty.

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Figure 1: Parallel Electric Power Distribution System

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Figure 2: Parallel Electric Power Distribution System

Mode Of Failure

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SPLIT PARALLEL SYSTEM

A split parallel electric power distribution system is The ground-handling buses are used to power lighting and
illustrated in Figure 1. This system allows for flexibility in load miscellaneous equipment for cargo loading, aircraft fueling, and
distribution and yet maintains isolation between systems when cleaning. The GH buses are not powered during normal flight.
needed. When closed, the split system breaker connects all
generators together, thus paralleling the system. When open, The ground service (GS) buses are controlled from the flight
the split system breaker isolates the right- and left-hand attendants' station located at the number 2 left door of the
systems, thus creating a more flexible parallel system. aircraft. The control switch energizes the ground service relay
(GSR), which connects the GS buses to whichever is currently
A split parallel system is used on the Boeing 747-400 aircraft. on line, the APU or EXT power. The ground service buses are
As seen in Figure 2, this system employs four engine-driven used to light the interior of the aircraft and power the main
generators, and it can accept two separate external power battery charger and other miscellaneous systems required for
sources (EXT 1 and EXT 2). The B-747-400 uses an automated maintenance, cleaning, and initial start-up of the aircraft.
power distribution control system that features a no break power
transfer. The no-break power transfer will be discussed later in
this chapter. As seen in the schematic of the system, the four
integrated drive generators (IDGs) are connected to their
respective ac buses through generator control breakers (GCBs).
The ac buses are paralleled through the bus tie breakers
(BTBs) and the split system breaker (SSB). When the SSB is
open, the right system operates independently to any load bus,
and any combination of the IDGs can operate in parallel.

The external power or APU power can be connected to the ac

buses through their associated contactors. The ac ground
handling (GH) buses are powered by closing the ground-
handling relay (GHR) to either the APU or EXT power. The dc
GHbuses receive power from the transformer-rectifier (TR)
located directly above them on the diagram (Figure 2).

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Figure 1: Split Parallel Electric Power Distribution System

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Figure 2: Split Parallel Electric Power Distribution System

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POWER DISTRIBUTION HIERARCHY CONTROL OF THE POWER DISTRIBUTION SYSTEMS

All aircraft electrical systems are designed with a bus On modem aircraft employing a parallel or split-bus system, a
hierarchy. That is, each system is designed so that the most centralized means of controlling the power distribution between
critical components are the least likely to fail. On all aircraft the individual load buses is essential. For example, if a generator
most critical components must operate from battery power. fails or a bus shorts to ground, the appropriate bus ties and
Less critical components can operate from other power sources, generator circuit breakers must be set to the correct position.
such as an aircraft generator. For example, look at the Boeing Orin the event of a system overload, the control unit must
727 schematic in Figure 1. Here the least critical ac loads are reduce the electrical load to an acceptable level. This is called
powered by ac bus 1, 2, or 3. The least critical dc loads are Load Shedding. The aircraft's galley power is usually the first
powered by dc bus 1 or 2. The next most critical systems are nonessential load to be disconnected. Also, the control unit
powered by the essential (ESS) ac and dc buses, shown on the must automatically reconnect any essential loads to an operable
left side of the diagram. The essential ac bus can receive bus. This power manipulation must take place within a fractionof
power from any ac generator. The essential dc bus can be a second to ensure an uninterrupted flight. To achieve this goal,
powered either by dc bus 1 or 2 or by the essential TR unit. modem aircraft employ a solid-state bus power control unit
The most critical electrical loads on the aircraft are powered by (BPCU).
the standby buses, located at the top left of the diagram. These
busses will still receive power from the battery even if all three The BPCU receives data from the generator control units
generators fail. (GCUs), the ground power control unit (GPCU), and the various
bus ties and circuit breakers of the system. As discussed, GCUs
On the Boeing 747-400 the least critical loads are connected to are used in conjunction with each aircraft generator to monitor
ac and dc buses 1,2, 3, and 4. More critical loads may be and regulate generator activities. If a GCU detects a
connected to the captain's and first officer's transfer buses (AC malfunction, it will inform the BPCU. The BPCU will then ensure
CAPT XFR and AC FIO XFR). The most critical ac loads are the appropriate power distribution system configuration.
connected to the ac standby bus (AC STBY); dc loads are
connected to the main battery bus (MN BAT). This hierarchy
allows for safe operation of the aircraft even in the unlikely
event that all engine-driven generators fail.

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Figure 1:Power Distribution Hierarchy

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The BPCU also receives input information concerning system

loads from load controllers. Load controllers are electric Bus tie breakers are another type of unit used to connect or
circuits that sense real system current and provide control disconnect main electrical distribution points. A bus tie breaker
signals for the generator's constant-speed drive rpm governor. BTB)is similar to an electric solenoid in that it is used as
The constant-speed drive output rpm in turn affects generator aremote control switch. Each BTB is usually controlled by the
output frequency. Load controllers receive their input signals BPCU.
from current transformers, such as the one shown in Figure 1.
Current transformers consist of three inductive pickup coils On modern commercial passenger jets, the BPCU performs
that provide current-sensing signals. The main power leads control, test, protection, and fault identification functions. The
carrying the three-phase alternating current from each schematic in Figure 3 shows the GCUs, the BPCU and its
generator are routed through the corresponding holes in a related sensors, and the current transformer assemblies
current transformer. As the alternating current travels through (CTAs). Figure 4 shows the generator breakers (GBs) and the
the wire, the corresponding magnetic field induces a voltage bus tie breakers (BTBs), which are used to control system loads
into the current transformer (Figure 2). The electrical signals via signals from the BPCU.
from the current transformer, in conjunction with the GCU and
BPCU, are used to control protection circuitry and supply BPCUs are basically small computers designed for a specific
signals to load meters in the cockpit. function. Each aircraft typically contains two BPCUs for
redundancy in the event of a failure. Each BPCU constantly
The BPCU is the main control computer for all generator and monitors its input and output data using a digitally coded
electric power distribution. The BPCU receives input signals message. If a system fault occurs, the BPCU initiates the
from several current transformers in order to monitor the necessary corrective action and records the fault in a
electrical system. If the BPCU detects an abnormal condition, it nonvolatile memory. The nonvolatile memory is part of the built-
opens and/or closes the appropriate bus tie and/or circuit in test equipment (BITE) of the BPCU. Any fault data stored by
breaker. As illustrated earlier in this chapter by the schematics the BITE system can be recalled at a later time by a line
of parallel and split-bus systems, circuit breakers are located technician. This process greatly reduces maintenance time and
throughout the aircraft's electrical system and are used to enhances system reliability.
isolate or connect various generators and/or distribution buses.
Circuit breakers operate automatically according to GCU and
BPCU signals or manually via cockpit controls.

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Figure 1: Current Transformer

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Figure 2:Current Transformer

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Figure 3: GCUs,BPCU& Related Sensors

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Figure 4: Generator Breakers (GBs)&Bus Tie Breakers (BTBs)

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