ATA 24 Hawker 750/800/800XP/850XP/900XP

ELECTRICAL P OWER TRAINING MANUAL (B1/B2)

ATA 24
ELECTRICAL POWER

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OBJECTIVE
• The Objective of this course is to understand :
• How electrical power generated in the aircraft.
• The working principal of system involved in generation of electrical power.
• And the components used in that systems.

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Zone Identification

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Electrical Zone Area

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Electrical Zone Area

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• The Aircraft Electrical Power Generation Methods

• AC Generation
• DC Generation

• AC Generation
• Constant Frequency system
• Constant Frequency Main Inverter
• Standby Inverter
• Random Frequency Windshield Power System
• AC Generator

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• DC GENERATION
• No 1 And 2 Generator Control Unit
• Starter/ Generator
• Battery
• Battery Thermostat
• Bus Bar Protection
• Over Voltage Sensor
• Main Lead Acid Battery
• Auxiliary Lead Acid Battery
• Auxiliary Nickel-cadmium Battery (Marathon Power)
• APU GENERATION (Turbomach)
• APU GCU

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AC GENERATION

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CONSTANT FREQUENCY SYSTEM

• 115 volts 400 HZ single phase AC supplies are provided to 2 main inverters.
• Third (STAND BY ) static 250 VA inverter provides 115 volts 400 HZ single phase AC supplies.
• The standby inverter will work only if both main fails.

The Main inverters and Standby inverters are installed in rear equipment bay.
• Three switches
• INV 1,
• INV 2 and
• STBY INV l
• Located on roof panel CG, control the inverters.
• Each main inverter is connected to its busbar (distribution terminal block) through its related relay on panel XZ, and XE.
• Each main inverter incorporates integral protection against abnormal output voltage and frequency conditions, which will de-
energize the inverter contactor should a fault occur.
• The functioning of No.1 and No.2 inverter circuits are basically similar.
• INV1 FAIL, XS1 FAIL and XE FAIL annunciators light, the ELECT repeater annunciator flashes and the master warning lamps flash.

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During normal conditions, No.1 and No.2 inverters are operating together, No.1 inverter supplying XS1 and XE, and No.2 inverter
supplying XS2; the standby inverter control switch is set to the ARM position.
In the event of both main inverters failing, both INV FAIL and XS FAIL annunciators will light and the ELECT repeater on the MWS will
flash.

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STANDBY INVERTER
• The standby inverter, Type PC-250, provides 115 volts 400Hz single phase a.c. to supply the aircraft essential services should the
main inverters fail. A 26 volts 400Hz single phase a.c. supply is also provided but not used on the aircraft.
• The generation of the alternating signal and the regulation are accomplished by a single integrated regulating pulse width
modulator circuit.
• Auto shut off if the output Voltage rise to 125 to 138volts a.c. or the frequency drift below 370Hz or above 430Hz

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RANDOM FREQUENCY WINDSHIELD POWER SYSTEM

• Electrical power for the windshield anti-ice system is obtained from two 3-phase a.c. generators.
• Mounted on each engine accessory gearbox.
• It is controlled by a generator control unit, mounted in the rear equipment bay.
• The left and right systems normally operate independently of one another.
• The left generator supplies the left windshield panel 'A' and right windshield panel 'B'.
• The right generator supplies the right windshield panel 'A' and left windshield panel 'B‘.
• In the event of a single generator failure, both panel 'B' supplies and the stall vane heater normal supplies are load shed.
• The remaining generator will supply both 'A' panels.

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RANDOM FREQUENCY WINDSHIELD POWER SYSTEM

• AC generator
• Each generator is a 3-phase.
• Output rating of 7.4 KVA at 208 volts rms over the speed range 6,600 to 12,882 rpm.
• Operates at frequency of 400 Hz when driven at a speed of 12,000 rpm.
• The generator comprises three stages
• A permanent magnet pilot exciter, main exciter and main alternator.
• The main rotor, exciter rotor and pilot exciter rotor are mounted on a common shaft.
• To protect the engine accessory gearbox against a seizure of the generator.
• Cooling of the generator is provided by an axial fan fitted to the non-drive end of the shaft
• Generator control unit
• Each generator control unit provides associated generator voltage regulation, channel protection and control of the
generator line contactor. The regulator circuit controls the generator output voltage within the limits 113.5 to 116.5 volts
rms over the speed range of 6 600 to 12882 rpm, by varying the excitation current in the main exciter field winding.
• An excitation control relay (ECR) completes the main excitation circuit, when energized, and short circuits the recirculating
diode and generator main exciter field winding, when de-energized.
• A generator control relay (GCR) controls the generator line contactor of the generating channel.

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RANDOM FREQUENCY WINDSHIELD POWER SYSTEM

System control and indication
• Each generator is controlled by an ON-OFF switch on roof panel CG.
• When a switch is selected OFF, the related generator is de-excite and the associated generator line contactor is de-energized.
• The contactor open circuits the generator output lines to the windshield heater elements, connects a fault signal from busbar PE
on panel ZK to light an associated ALTA FAIL annunciators on roof panel CG via the MWS, which flashes the ICE PROT repeater
annunciators on the centre instrument panel CC.
• The A SCRN and B SCAN circuit breakers on panel 'DA-D' for LH Windscreen Panel A and RH Windscreen Panel B heat control
have white indicator labels added to satisfy smoke drill procedures for windscreen heat equipment supplied from the PE busbar.

RANDOM FREQUENCY WINDSHIELD POWER SYSTEM

• BITE
• A built-in-test facility is provided on each GCU to check that the generator will trip if an over-voltage, under voltage or over-load
occurs on either phase A, B or C.
• The BITE consists of a 3-position selector switch and three associated test points idented OV, UV and OL. These test points are
actually miniature switches which, when closed using a sharp object, simulate over voltage, under-voltage and overload.
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RANDOM FREQUENCY WINDSHIELD POWER SYSTEM

Operation
• If you select control switch to ON connects the permanent magnet generator supply to provide the control unit with internal
circuit supplies and generator field excitation.
• It energize the related generator line contactor (GLC).
• The energized GLC connects the generator output to the windshield heater circuit, extinguishes the relevant ALTR FAIL
annunciator, and the ICE PORT repeater annunciator.
• To prevent overload of the remaining generator, both panel B heater thermal controllers are switched off by the relaxing of the
'both generators on line' relay. If the generator is brought back on line each generator will assume its related windshield load
and the ALTA FAIL and ICE PROT annunciators will go out.
• If a generator develops under or overvoltage symptoms, the control unit sensing circuits develop a signal to de-energize the
excitation control relay and generator control relay. The relays are latched out and the generator de-excites.
• The generator line contactor is de-energized to disconnect the generator output to the windshield heaters and connect a supply
from busbar PE to light the ALTA FAIL annunciator together with the MWS ICE PROT flashing annunciator.
• Continuity of supply is maintained to both heated windshields as previously described. The control unit logic circuit 'latch-in'
function will maintain this condition and thus prevent fault cycling. After fault clearance, the 'latch-in' logic circuit can be reset
by setting the generator control switch to OFF, and then to ON.

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DC GENERATION
General
• Two identical, engine-driven, dual purpose, starter generators are employed with their associated control components to
function initially in an engine starting mode and, when this mode is satisfactorily completed, to provide d.c. electrical power at
28 volts 18 kW (9 kW per generator) to operate the d.c. electrical services.
• Distribution of this power is effected through a busbar arrangement which can be split, or integrated, through a switched bus-tie
contactor.
• The main busbars, designated PS1 and PS2, can be split or tied to enable individual, or paralleled generator system functioning,
as necessity dictates.
• Busbars PS1 and PS2 supply the major (non-essential) loads and also feed, through blocking power diodes, an essential services
busbar PE and PE2.
• PS1 and PS2 also provide charge current for two main airplane batteries. The blocking diodes prevent feed-back from PE to PS1
and PS2.
• Batteries are isolated from the non-essential services when the generators are off line.
• To assure supplies to the essential services in the latter instance, batteries No.1 and 2 are connected to busbar PE via individual
contactors (Emergency condition).

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SYSTEM control
• At engine start initiation, the generator control unit (GCU) receives a signal to ensure that the auto-field latch relay (KFR) is reset,
and thereby completing the starter/generator field circuit.
• During engine start, the GCU field weakening regulation circuit (voltage regulator locked-out) will maintain starter current
control throughout the starting cycle to provide optimum acceleration for the engine.
• Starter cut-off is signalled by the GCU, derived from the starter/generator speed pick-up at 5500 rpm After cut off, the GCU field
weakening regulator is de-energized and the standard voltage regulator will gain control of its field power amplifier to build-up
the generator voltage.

• The GLC main contacts connect the generator output to PS1 (or PS2) and PE busbars and its auxiliary contacts change over to:
• Energize the corresponding battery contactor and connect the relevant battery to its charge and ammeter circuits.
• Complete the bustie contactor coil supply circuit to its BUSTIE switch, which, when set to CLOSE will link PS1 and PS2
busbars.
• Extinguish the GENerator FAIL annunciator.
• Manual control of each generator's output to the busbars is provided by a switch labelled GEN CLOSE-TRIP on the pilots' roof
panel CG
• Selecting the GEN switch to CLOSE position (momentary) will connect a supply to:

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• Excite the generator field tickle from busbar PE2, to promote correct polarity and sufficient residual voltage for build-up of
generator output voltage.
• Energize and reset the GCU field latch relay (KFR).
• Energize a generator control relay which inhibits the GLC (until switch is released) and prevents a generator output being
fed on to a possible busbar fault.
• When the generator is 'on-line', operating the GEN switch to its TRIP position (momentary) will
• Connect a supply from the generator output to energize the field latch relay trip coil, which then opens the GLC via the
open contacts of the field latch relay to disconnect the generator from the busbar.
• Energizes the trip coil of the generator trip relay. This isolates the generator output from the PS busbar by removing
power from the GLC coil and opening the contacts.

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• Field weakening starter control

• Field weakening regulator compares starter/generator interpole voltage with reference voltage. At engine start, the
regulator maintains a full field on the starter and locks out the generator voltage regulator. When engine self-sustaining
speed is reached the starter's armature current tends to fall (sensed at interpole winding) and the field weakening
regulator reduces field excitation to cause an increase in armature current to maintain starter current during the start
cycle.
• Automatic starter cut-off
• The starter/generator magnetic pick-up speed sensor connects a signal to the GCU, and at 5500 rpm the field weakening
circuit is de-energized and the generator voltage regulator gains control of the field power amplifier and begins to build up
generator voltage. The starter circuit is inoperative until the field circuit is closed. Application of power to pin W of the
GCU with 28 volts at pin S will close the field circuit relay (KFR reset).
• Field relay (KFR)
• The field and start lines are switched by a field latched relay (KFR) which forms an integral part of the GCU. When a start
signal is applied to pin W of the GCU the relay is pulsed, by solid state switching, to reset the contacts to a closed position
and, providing a supply is available at pin S, a supply is available at pin Z to initiate the start sequence. The relay also off-
loads the generator from the airplane busbars on receipt of signals from the ground fault and overvolt sense circuits.
• Voltage regulator
• Compares generated output voltage with a reference voltage, and the difference is used to control the field power
amplifier which adjusts the generator's shunt field excitation.

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• Paralleling
• When both generators are connected in parallel, an equalizing circuit is completed between generators and control units.
Each GCU compares its generator interpole winding voltage with that of the other generator, and the equalizing circuit
attempts to adjust the equalizing line difference to zero. Any difference in voltage is amplified and fed back to the voltage
regulator to adjust generator output voltages, so that the busbar loads are equalized between generators.
• Overvoltage protection
• The overvoltage sensing amplifier compares generator voltage with a reference voltage. When the generator voltage rises
to 32.5 volts the field latch relay (KFR) is tripped to:
• De-energize the GLC and isolate the faulty generator from the busbar.
• Activate the MWS system (Chapter 31, MASTER WARNING SYSTEM) to light the GENerator FAIL and ELECT repeater
annunciators.
• Open-circuit the generator field, and de-excite the generator output to residual voltage.
• Disconnect the equalizing circuit.

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• Ground fault protection

• Feeder fault protection between generator and GLC is provided during build-up of generator output when GLC is open
(Generator off-load).
• During build-up of generator output {GLC open) voltage detected across the generator interpole winding is low. If an earth
fault exists between generator and GLC the generator output would feed fault, the voltage across the interpole winding
would rise in proportion to current flow, and when equal to half of generator total load current rating, the GCU sensing
circuit will inhibit the GLC close circuit and trigger the overvoltage silicon-controlled rectifier (SCA). The SCA conducts to
energize the field latch relay (KFA) trip coil and de-excite the generator output to residual voltage.
• Over-excitation protection
• When generators are operating in parallel, if one generator voltage regulator is unable to properly control its generator
output, a condition may arise where one generator becomes over-excited and attempts to supply all the system loads. The
remaining good generator channel will act as load, and therefore, the busbar voltage should remain normal with a
tendency to slightly increase.
• The over-excitation circuit will detect this condition when the generator interpole voltage reaches 0.23 volts or more with
the busbar at or above 28.5 volts. The faulty channel will receive a de-excitation signal from the equalizing circuit because
it is carrying all the load, and consequently, an input signal is fed to the overvoltage circuit to trigger its SCA and trip the
generator field latch relay (KFA) to interrupt the GLC close circuit and de-excite the generator output to residual voltage.

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• Operation
• Normal conditions
• As the generator voltage rises, its GCU sensing circuit compares the generator and busbar voltages. When the generator busbar
voltage differential is 0.3 volt or less, a supply from the generator output is routed via the relaxed contacts of the generator relay
and ground power contactor to energize the GLC operating coil from its sensing circuit. The GLC main contacts close and connect
the generator output to power PS1, PS2 and PE busbars. The GLC auxiliary contacts change over to:
• Energize the related battery contactor and connect the relevant battery to its charge and ammeter circuits.
• Extinguish the GENerator FAIL annunciator.
• Complete the bustie contactor coil supply circuit to its BUSTIE switch, which, when set to CLOSE, will link PS1 and PS2
busbars.

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Operation
Normal conditions
• With both generators 'on-line', each generator output is connected to the split busbar system PS1 and PS2.
• The bustie contactor main contacts close to tie PS1 and PS2 busbar together and its auxiliary contacts will extinguish the BUSTIE
OPEN and ELECT i repeater annunciators, and through its auxiliary relay complete the equalizing circuit and link both battery
contactor coil circuits.
• If a generator output falls, reverse current will flow from busbar to generator. The voltage reference at the generator interpole
series winding will fall to switch off the GCU sensing circuit and break the GCU equalizing circuit.
• The GLC will be de-energized and disconnect the generator from the busbar; the GEN FAIL and ELECT i repeater annunciators will
operate.
• The total load current does not exceed 300A continuously.
• If load current exceeds 300A continuously, reduction is effected by switching off non-essential services.

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Operation
Abnormal conditions
• Generator over-excitation or over-voltage (32.5 volt) is detected by the control unit (GCU) sensing circuits. The GCU field latch
relay trip coil is energized automatically and de-excites the generator output to residual voltage. At high speeds, the over-
excitation can be sufficient to motor the healthy generator, thus opening its GLC on reverse current. Automatic reinstatement
occurs once the faulty generator is off line.
• Re-setting of a generator when its GCU field circuit relay has been tripped, is achieved by setting the GEN switch to the CLOSE
position (momentary).
• Releasing the switch will allow the GLC circuit to function and connect the generator to busbar.
• If protection circuits fail to isolate a faulty generator channel from the busbar automatically, disconnection is effected by
manually selecting the GEN switch to its TRIP position (momentary). In the TRIP position the generator output at the GLC is
connected to trip the GCU field latch relay (KFR) and de-excite the generator output to residual voltage.

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Operation
Abnormal conditions
• If the busbar voltage rises to 29.5 ±0.5 volts or falls to 24.5 ±0.5 volts the protection unit output silicon-controlled rectifier (SCA)
is switched 'on' and connects a PE supply through the BUSTIE switch in its central position to energize the bustie contactor trip
coil.
• With external ground supplies connected and switched on, the d.c. busbar system is energized. Selecting the EXT BATT CHG
switch to ON, will energize the battery contactors.
• In the event of faulty ground power supplies the busbar protection unit will energize the battery protection relay, interrupting
the supplies to the coils of No.1 and No.2 battery contactors to disconnect the batteries from their charge source.
• If a battery hot situation occurs the battery hot thermostat will energize the battery hot relay, interrupting the supplies to the
coils of No.1 and No.2 battery contactors to disconnect the batteries from their charge source.

BUSBAR PROTECTION/ VOLTAGE SENSING UNIT - SCHEMATICS

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BATTERY

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Warnings related to Battery

WARNING: POTASSIUM HYDROXIDE AND ITS SOLUTION ARE EXTREMELY CAUSTIC. CARE SHOULD BE TA KEN TO AVOID
CONTAMINATION. IF SKIN CONTAMINATION OCCURS, THE AFFECTED AREA SHOULD BE IMMEDIATELY FLUSHED WITH LARGE
AMOUNTS OF WATER AND NEUTRALIZED WITH 3% SOLUTION OF ACETIC ACID, VINEGAR, LEMON JUICE, OR A 10% SOLUTION OF
BORIC ACID.

WARNING: IF THE E YES ARE CONTAMINATED IMMEDIATELY FLUSH WITH LARGE AMOUNTS OF WATER AND REPORT TO A MEDICAL
CENTER.

WARNING: DURING BATTERY CHARGING EXPLOSIVE GASES ARE EMITTED: THEREFORE, GOOD VENTILATION MUST BE MAINTAINED.

WARNING: CELL CONNECTIONS MUST BE TIGHT AND SECURE WITHOUT OVERSTRESSING. LOOSE CONNECTIONS NOT ONLY REDUCE
BATTERY TERMINAL VOLTA GE BUT CAN CAUSE ARCING WITH EXPLOSION HAZARD. CELL TOP (LINK SECURING) NUTS TO BE TORQUE
TIGHTENED TO 87 ±8 lbf.in.

WARNING: BATTERY WEIGHS APPROXIMATELY 55 LBS. USE CARRYING STRAP, DO NOT LIFT OR HANDLE BATTERY BY VENT TUBE.

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Warnings related to Battery

WARNING: WITH BATTERY LID ASSEMBLY REMOVED, BATTERY TERMINALS ARE EXPOSED. WARNING: MAKE SURE INSULATED TOOLS
ARE USED DURING SERVICING AND METALLIC ITEMS
SUCH AS WATCHBANDS AND RINGS ETC. ARE REMOVED AS THESE ITEMS ARE
CONDUCTIVE AND COULD CAUSE INJURY TO OPERATOR OR DAMAGE EQUIPMENT WHEN HANDLING BATTERY.

WARNING: WHEN ADJUSTING ELECTROLYTE LEVELS RUBBER GLOVES, GOGGLES AND A RUBBER APRON MUST BE WORN.

CAUTION: DO NOT ALLOW NICKEL CADMIUM BATTERIES TO COME INTO CONTACT WITH LEAD ACID BATTERIES. KEEP BATTERY
CHARGING ROOMS, DEMINERALIZED WATER, TOOLS AND MATERIALS ETC. SEPARATE. USE DEMINERALIZED WATER FREE OF
CARBONATE AND ACID OTHERWISE CONTAMINATION DAMAGE CAN OCCUR.

CAUTION: NEVER LEAVE THE BATTERY IN A DISCHARGED STATE FOR LONG PERIODS.

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Batteries

• Two 24-volt batteries (main, No.1 and No.2) are installed in the rear equipment bay, and maintain d.c. supplies to the essential
services busbar PE, in the event of generated power failure. The batteries also supply power tor engine starting in the absence of
a ground supply.
• The minimum permissible battery temperature for take-off in order to meet emergency flight time is -10 degree C. The
emergency flight time for batteries is calculated at-20 degree C, the -10 degree C difference is due to further cooling in flight.
• Individual battery isolation is achieved by operation of the BATT ISOLATE switch on the pilots' roof panel CG. The switch has
positions BATT 1, centre OFF and BATT 2.
• The batteries are 24 volt 23 Ah nickel cadmium, and are vented and monitored as follows:
• Ventilation of battery gas to atmosphere is accomplished by plastic pipes extending from the front of the battery to an
outlet position on the fuselage right side forward of frame 21.
• Each main battery has integral hot (57°C) and overheat (71°C) thermal switches, which, when operated, light associated
BATT 1 HOT (amber), BATT 2 HOT (amber), BATT 1 OVHT (red) and BATT 2 OVHT (red) annunciators on roof panel CG;
additional warning is also provided on the MWS (Chapter 31 - MASTER WARNING SYSTEM). The main batteries can be
charged in-situ when busbars are energized from a ground power supply. If a battery 'hot' condition arises, the 'hot'
thermal switch completes a circuit to energize the battery hot relay to disconnect the battery charge circuits; protection is
also provided against an overvoltage supply (see paragraph L. Ground power).

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• A platinum wire wound temperature bulb is incorporated in each main battery and, in conjunction with an indicator on
pedestal BA, provides indication of battery temperature. The indicator also displays outside air temperature (OAT) and
fuel temperature. OAT is normally displayed on the indicator and by using the rotary selector switch and momentary
action push switch an indication of FUEL, BATT 1 and BATT 2 temperature can be obtained as required.
• Selection of the BATT switch to the EMERG position, connects direct supplies from No.1 and No.2 batteries, via diodes, to
energize the emergency contactors and connect supplies as previously described.

STARTER GENERATOR
• The Lear Siegler starter/generator provides torque for engine starting from a 28 volts d.c. supply and in the generating mode,
when driven within its speed range, supplies 28 volts d.c. (regulated) 330 amps, to the airplane d.c. busbar system.
• The starter/generator is a self-excited, compound-wound machine and its stator incorporates a shunt field, compensating
windings and four interpoles in relation to its four main poles. The stator houses an armature supported by two ball bearings,
one at each end of the housing.
• Four sets of brushes are installed in brush holders. Brush contact pressure on armature commutator is maintained by coil
springs. The brush assembly is secured to an end bell plate.
• Also secured at commutator end of the housing, is an integral cooling fan on the drive shaft. The machine is also cooled when
airborne, by ram air passing through the inlet and being exhausted at the drive end quick attach/detach (QAD) adaptor.

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IDENTIFICATION OF STARTER GENERATOR TERMINAL BLOCK CONNECTORS

• CAUTION: DO NOT CONDUCT HIGH POTENTIAL TESTS AT TERMINAL BLOCK UNLESS STATOR LEADS TO TERMINAL BLOCK ARE
DISCONNECTED, OTHERWISE RADIO NOISE FILTERS IN BLOCK WILL BE DAMAGED.
• Located on the stator is a speed sensor pick-up, which through its connector supplies a signal to the generator control unit (GCU)
to effect starter cut-off. Electrical power output to the aircraft system is provided from a terminal block, which includes a potted
radio noise filter and which is enclosed by a rubber cover. Each terminal connection is coded alphabetically.

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IDENTIFICATION OF STARTER GENERATOR TERMINAL BLOCK CONNECTORS

• Starter
28 volts 300 amps
• Generator
• 30 volts 400 amps
• Weight
• 36.5 lbs (16.6 kg}
• Speed
• 6800 - 12000 rpm
• Range cooling
• integral fan and ram air

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Location of Starter/Generator Heavy Duty Cables

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POWER SUPPLY UNIT (J.E.T. Type PS835D)

• The Jet Electronics and Technology Inc. unit type PS835D is an emergency power supply unit (PSU).
• It consist of battery pack containing sealed LEAD ACID CELLS providing a nominal 24 volts d.c. 5.0 ampere/ hour output.
• The PSU is charged from busbar PS2. To energize it turn on BATTERY MASTER SWITCH.
• To charge PSU when external power is ON. Select EXT POWER CHG.
• A plug in DC/DC converter (J.E.T. type SC841A) can be installed to the PSU to supply 5 volts d.c. for emergency lighting
Important Notes
• Aircraft with two Emergency Power Supply units installed will have a plug in DC/DC converter installed to No. 1.PSU. It is
essential that these two PSU's are not cross installed. For information concerning the correct coding and installation
position for both of these units refer to aircraft electrical routing chart No.25-8NA5/"36.
• For maintenance practices refer to Chapter 34, STANDBY ATTITUDE INDICATOR.

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TURBOMACH APU GENERATION – LOCATION OF COMPONENTS

• Auxiliary Batteries
• The three auxiliary batteries are nickel-cadmium vented batteries and are designated No. 3, No. 4 and No. 5 batteries.
Each battery is connected to the battery vent system through tee-pieces and flexible tubes.
• The No. 3 battery supplies a limited time power source for the standby horizon and the standby altimeter vibrator. This
battery also supplies instrument lighting for the standby airspeed indicator and the standby altimeter indicator.
• The No. 4 battery supplies a limited time power source for the following systems in the event of a total power failure: ATC
1, NAV 1, clearance delivery unit, RMU 1, and COMM 1. The radio emergency power is controlled by a guarded switch
located above the DA panel. This battery also supplies power to the N1 gauge through the standby horizon switch and to
the No. 1 AHS for three to four minutes.
• The No. 5 battery supplies two light bulbs at the escape hatch handle, two reading lights near the escape hatch, three aisle
lights, opposite the escape hatch - plus two external lights. The external lights supply lighting to the upper wing surface
and the ground area below the wing near the hatch.

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TURBOMACH APU GENERATION – LOCATION OF COMPONENTS

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TURBOMACH APU GENERATION

• A dual purpose starter/generator is used for both starting the APU and power generation.
• In the generating mode the starter/generator output is connected to PS2 busbar.
• PS1 busbar can also be energized by selecting the BU STIE switch on the pilot's roof panel to CLOSE.
• APU can be started using external d.c. ground power.
• The APU generator can be operated in parallel with the main generators.
• Generator inhibit relay prevents the APU generator line contactor (GLC) from closing when ground power is applied to the
busbars.
• The APU control circuits are energized on the ground by a squat switch operated 'weight-on-wheels relay'. When the aircraft
becomes airborne the 'weight-on-wheels' relay relaxes and de-energizes the 'auto shut off' relay and the APU shuts down.

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TURBOMACH APU GENERATION

• When the APU generator comes 'on-line' PS2 and (via diodes) PE busbars are energized and No.2 battery contactor is closed to
connect No.2 battery to PS2 busbar.
• PS1 busbar can be energized by selecting the BUSTIE switch to CLOSE and, consequently, No.1 battery contactor closes to
connect No.1 battery to PS1 busbar.
• To prevent the APU being used to assist main engine starting, the supply to the coils of the battery contactors is routed via
normally closed contacts of a cross start inhibit relay. This relay is energized when internal starting of the main engines or APU is
initiated.
• To de-energize the battery contactors disconnect battery from PS1 and PS2.
• The APU generator system is monitored by the APU GEN FAIL and APU GEN OVHT annunciators and an ammeter. With neither
main generator 'on-line' the APU generator output may also be monitored on the
• d.c. busbars via the d.c. voltmeter and its switching circuit on roof panel CG.
• The starter generator has an integral shaft-driven cooling fan and is interchangeable with the main engine starter generators.

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OPERATION
• When the GCU speed sensor detects sufficient speed from the starter generator speed pick-up, the GCU speed switch will open
to remove the output from pin Z to de-energize the field control relay and start contactor.
• The voltage regulator circuit now assumes control of the generator field via the relaxed field control relay contacts.
• If the APU is running within its parameters, as determined by the electronic sequence unit (ESU}, an output from ESU pin L will
energize the 'ready' relay to arm the bustie, APU GLC and battery charging circuits, connect the APU equalizing circuit to the
main generators equalizing circuits and light the APU GEN FAIL annunciator.
• When the generator/battery voltage differential measured at GCU pins D and E is greater than approximately 0.3 volt an output
from GCU pin H will energize the GLC and the No.2 battery charging circuit (battery contactor} and the APU GEN FAIL
annunciator will go out provided the following conditions are met:
• The APU is running within its parameters, as determined by the ESU.
• Generator satisfactory output quality is, as sensed by the GCU.
• No ground power is applied to the aircraft.
• No other engine is being started (cross start inhibit relay relaxed).

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APU STARTER/GENERATOR CONTROL - SCHEMATICS

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DISTRIBUTION SYSTEM

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GENERAL
• Electrical power is fed from the primary supply and control panels GA, ZK and ZL in the rear equipment bay, to panel DA in the
forward vestibule for distribution to the electrical service.
• Panel DA houses the majority of aircraft consumer services circuit breakers on two sub-panels, coded A and D.
• Sub-panel A is located on the upper forward face of panel DA and contains circuit breakers for avionics systems.
• Sub-panel D located on the inboard face of panel DA. All wiring passing into panel DA passes through plug-breaks behind sub-
panel B.
• The circuit breakers are identified functionally, by a label, and a grid reference on the sub-panels is provided for the operator's
domestic use.
• The circuit breakers have their ampere rating marked on the button, and can be manually tripped and reset by operating the
'push-pull' button.
• 115 volts a.c. and 28 volts d.c. test equipment supply sockets are mounted on panel 'DA-P' in the cockpit and panel 'TY' in the
rear equipment bay.

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DC distribution
• Power supplies from the generators, the aircraft batteries, or an external ground supply are supplied to the aircraft services
through a split busbar distribution system.
• The essential busbar PE is primarily located at panel ZL, and can be energized from the aircraft batteries, generators or ground
power supplies.
• Panel ZL busbar PE supplies are extended via duplicated fuse protected main feeder cables, to distribution busbars PE (a) on
panel ZK, and through the pressure dome bung DJ to the main distribution panel DA.
• PS1 and PS2 busbars link to PE busbar at panel ZL through power diodes on panel ZL which prevent the batteries feeding to PS1
and PS2 busbars.
• The d.c. busbar assembly in the main distribution panel DA, consists of busbars PS1, PS2 and PE located on the panel forward
face in its lower section.
• Busbars PS1 and PS2 are linked together at panel GA when the bustie contactor is energized; the contactor is controlled from the
roof panel CG BUSTIE switch when a generator is 'on-line'.

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DC distribution
• When an external ground power supply is connected to the ground plug on the rear fuselage right side, selection of the EXT
POWER switch on the roof panel CG to its ON position will operate the ground power contactor and connect ground power via
panel GA to energize busbars PS1, PS2, PE and PE2. With ground power connected each generator output is isolated from the
d.c. busbar system.
• If only batteries are available to provide electrical power, BATTERY switch selection on the roof panel CG to its ON or EMERG
position will energize the emergency contactors and connect No.1 and No.2 batteries to power busbar PE at panel ZL, via power
diodes.
• Panels ZL, ZK and DA busbar supplies to the majority of aircraft services.
• Non-essential services which are duplicated (twin system) or where the load is divisible (lighting etc.) are supplied from busbars
PS1 and PS2, so that power failure on either busbar will not result in complete failure of these services.

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Main distribution panel and busbar arrangement

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AC distribution
• Constant frequency a.c. outputs from the inverters are fed to the aircraft services via distribution terminal block (busbars) XS1,
XS2 and XE. The busbars are located in panel DA, above sub-panel C.
• The non essential a.c. aircraft services are connected to busbars XS1 and XS2, with XE providing the essential supplies should XS1
and XS2 supplies fail.
• The majority of fuses and circuit breakers are located in and on panel DA, the remainder being divided between panel XZ, and
the radio junction box, SP.

Avionic supplies
• Panel DA is the primary distribution panel for a.c and d.c. avionic supplies.
• The d.c. supplies are fed directly from the distribution busbars at the base of panel DA to the avionic system circuit breakers,
located on panel DA, sub-panel A.
• No master radio switching is provided.
• Single-phase 115 volts400Hz a.c. supplies are fed directly from panel DA XS1 and XE distribution terminal blocks, to supply the
avionic systems via the relevant circuit breakers.
• Single-phase 26 volts400Hz a.c. supplies utilized in the various navigation systems are provided by two auto-transformers
located under the Co-pilot's seat. No.1 auto-transformer input is supplied from XE busbar, and No.2 auto-transformer from XS1.
Both transformer outputs are fed to banks of ring-connected fuses, through which the services are supplied.

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