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During normal operation, both generators operate independently
unless the bus tie is closed. When both generators are on line, the bustie
will normally be open and the left generator will recharge the left
main battery and the right generator will recharge the right main
battery and the emergency battery. If the bus-tie is closed, both
generators will recharge the main batteries and the emergency battery.
The generators supply dc power to all dc powered equipment on the
airplane under normal conditions.
GENERATOR CONTROL UNITS
Left and right Generator Control Units (GCUs) are provided to monitor
and control the engine driven starter/generators. They regulate the
voltage of the generators to approximately 28-vdc and limit the output
on the ground and for in-flight cross starting. The GCUs will
automatically disconnect the generators from the electrical system if a
generator malfunction is detected. If the generator fault was
momentary or has cleared, generator operation may be restored by
depressing the generator switch for one second. If the fault has cleared,
this will reset the field relay allowing the generator to be energized and
the line contactor to close. The field flashing relay and associated
circuitry ensures that the generator can be built up from residual
voltage without any other power source required. The GCUs also
provide several engine starting functions.
GCU functions are as follows:
VOLTAGE REGULATION — To maintain a preset constant voltage at
the generator output terminal, the GCU controls the shunt field current
when the generator is rotating within its normal speed range. With both
generators on line, the GCUs also perform a paralleling function in the
unlikely event of an inadvertent bus-tie condition between the
generator buses.
GENERATOR CURRENT LIMITING — When activated by a main
gear weight-on-wheels switch/starter selection, the generator output is
restricted by appropriate suppression of generator voltage. This action
prevents excessive generator heating when charging depleted batteries
and when assisting opposite engine starting.
Pilot’s Manual
4-6 PM-126A
AUTOMATIC STARTER CUTOFF — As the engine start cycle
progresses, the starter/generator shunt field is weakened to enhance
torque performance. At 50% N2, the start cycle is terminated
automatically when starter cutoff speed is sensed by a magnetic pickup
in the starter. This is a back-up starter cutoff to the normal cutoff at 50%
N2 input to the GCU from the DEEC.
LINE CONTACTOR CONTROL — In the generating mode, power is
automatically provided to the line contactor in order to connect the
generator to the generator bus when the output voltage is at an
adequate level. During an engine shutdown, as the generator runs
down, reverse current is sensed and signals the line contactor to open,
disconnecting the starter/generator from its bus.
OVERVOLTAGE PROTECTION — In the event of a failure of normal
voltage regulation, and with due allowances for surges and transients,
an independent circuit causes the line contactor to open, disconnecting
the starter/generator from the generator bus if the voltage exceeds
approximately 32 volts.
OVERSPEED (RUNAWAY) PROTECTION — Should a starter shaft
shear during the starting mode, the starter/generator is de-energized
as the speed passes the starter/cutoff point, preventing further damage
by the overspeed condition. It is the starter speed, rather than engine
speed that is sensed to provide this function.
STARTER ABORT OPERATION — If during a start cycle, the
corresponding thrust lever is selected to CUTOFF, the starter will
automatically disengage.
UNDERVOLTAGE PROTECTION — Should voltage regulation fail,
causing a generator undervoltage condition (less than 10 volts for 5
seconds), the generator will disconnect from the system by deenergizing
the field relay, causing the line contactor to open.
DIFFERENTIAL CURRENT PROTECTION — Should the output
current at the generator differ significantly from the sensed load within
the power distribution panel (due to a generator feeder line fault) the
generator will be deenergized and disconnected from the system by the
differential current protection circuit.
Pilot’s Manual
PM-126A 4-7
GROUND POWER UNIT (GPU)
Ground power can be connected to the airplane through a receptacle
located on the lower left side of the fuselage just aft of the tailcone
baggage door. The anti-flash contactor that connects the output of the
GPU to the aircraft electrical system will only close if the voltage and
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