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A 737-300/400/500MAINTENANCE MANUAL
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A
737-300/400/500MAINTENANCE MANUAL
B. With the GCR closed, the generator voltage will build up with engine or APU speed and the voltmeter and frequency meter will read approximately 115 volts and 400 Hz at normal speed. The GCR trips with the generator breaker (GB) from differential fault or abnormal voltage and any time the generator switch is moved to OFF.
C. If an engine or APU is started with the GCR tripped or open, the generator voltage will not build up with engine speed, the voltmeter will not indicate at normal engine speed, and the frequency meter will read low. Moving the generator switch to ON before engine start will close a tripped GCR and ensure voltage buildup during engine acceleration. Moving the generator switch to ON after the engine is running will close a tripped GCR, causing the voltage to build up and the generator breaker to close, connecting the generator to the bus.
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A 737-300/400/500MAINTENANCE MANUAL
6. Generator Control Unit (GCU) (Fig. 3)
____________________________
A. The generator control unit (GCU) provides automatic operation of the control functions for each generator system and provides signals to open the generator field by tripping the GCR and to trip the generator breaker.
(1)
The GCU controls dc voltage indicator, generator breaker, field relay, bus voltage, and underspeed conditions.
(2)
The GCU protects the generator system from overvoltage, undervoltage, overfrequency, underfrequency, and feeder faults (differential and overcurrent).
(3)
Overvoltage, undervoltage, and feeder faults are indicated on the M238 annunciator panel.
B. The voltage regulator is in the GCU. It regulates the average of the three-phase voltages at the generator feeder ends and limits the amplitude of phase voltages under unbalanced load or fault conditions. DC power supplied to the generator exciter is controlled by the voltage regulator. This dc power is furnished by a T-R unit within the GCU. The voltage regulator limits the input power to the generator to 160 horsepower by means of power limiting current transformers on the generator feeder lines. The voltage regulator senses system voltage by voltage transformers and line current by means of the power limiting current transformers. These two signals are then combined to obtain a signal which is approximately proportional to the real load on the system. The output of the power control circuit is connected to the voltage regulator circuit. When system real load approaches a value sufficient to cause excessive generator input torque, the signal from the power control circuit takes over and reduces the system voltage. In this manner, the system real load is limited so that the critical torque is not exceeded.
C. The GCU sends a trip signal to the GCR or GB when faults are detected.
(1)
When the average three-phase voltage exceeds 127 to 133 volts line-to-neutral, overvoltage is detected, and the GCR is tripped.
(2)
Within 5 to 9 seconds after the voltage drops and remains below 97 to 103 volts line-to-neutral on all three phases of the generator, undervoltage is detected, and the GCR is tripped.
(3)
When the differential current of the generator, generator main bus, and interconnecting feeders is 20 amps or more, differential faults are detected, and the GCR is tripped.
(4)
When the load on a generator exceeds 170 to 175 amps, overcurrent is detected, and after a time delay, the GCR is tripped. Overcurrent protection inhibits underfrequency protection.
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