GENERATOR CONTROL
UNITS
Left and right generator control units (GCUs)
are provided to monitor and control the engine-
driven starter/generators. They incorporate
the normal control and protective functions re-
quired for the electric power system. They
regulate the voltage of the generators to ap-
proximately 28 volts and limit the output on
the ground and in-flight cross start. The GCUs
will automatically disconnect the generators
from the electrical system if a generator mal-
function is detected. If the generator fault was
momentary or has cleared, generator operation
may be restored by momentarily depressing the
generator switch. If the fault has cleared, this
will reset the field relay allowing the genera-
tor 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.
Other GCU functions are as follows:
VOLTAGE REGULATION – To maintain a pre-
set constant voltage at the generator output ter-
minal, the GCU controls the shunt field current
when the generator is rotating within its nor-
mal speed range. With both generators on-
line, the GCUs also perform a paralleling
function in the unlikely event of an inadver-
tent bus-tie condition between the generator
busses with both generators operating.
GENERATOR CURRENT LIMITING – When
activated by a squat switch or starter selection,
the generator output is restricted. This prevents
excessive generator heating when charging
depleted batteries and when assisting opposite
engine starting.
AUTOMATIC STARTER CUTOFF – As the
engine start cycle progresses, the starter/gen-
erator shunt field is weakened to enhance
torque performance. At 50% N
2, the start cycle
is terminated automatically when starter cut-
off speed is sensed by a magnetic pickup in the
starter. This is a back-up to the normal cutoff
at 50% N
2 input to the GCU from the engine
computer.
LINE CONTACTOR CONTROL – In the
generating mode, power is automatically pro-
vided to the line contactor in order to connect
the generator to the generator bus when the out-
put voltage is at an adequate level (engine
computer ON). During engine shut-down, 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 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 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 fur-
ther 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 auto-
matically disengage.
UNDERVOLTAGE PROTECTION – Should a
voltage regulator fail, causing a generator un-
dervoltage condition (less than 10 volts for 5
seconds), the generator will disconnect from
the system by de-energizing the field relay,
causing the line contactor to open.
DIFFERENTIAL CURRENT PROTECTION –
Should the output current at the generator dif-
fer significantly from the sensed load within
the power distribution panel the generator will
be de-energized and disconnected from the
system by the differential current protection
circuit.
LEARJET 45 PILOT TRAINING MANUAL
2-8
FOR TRAINING PURPOSES ONLY
FlightSafety
international
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