(2)
At 45 to 50% N2, the GCCU sees a generator underspeed condition. Then, it disconnects the generator field and stops the inverter.
(3)
At 20 to 30% N2, the PMG voltage is too small to operate the GCCU
and the GCCU stops.AIRPLANES WITH TWO LIGHT BIT MODULE VSCF; Operation During a Failure Condition (Table 1)
(1)
The VSCF has built-in test (BIT) circuits. The microprocessor in the GCCU always monitors the operation of the VSCF.
(2)
If a failure occurs in the VSCF, the GCCU stores data in the non-volatile memory (NVM). You can use this data in the shop to do trouble-shooting to the VSCF.
(a) The VSCF has a RS-232 serial port on the main connector. This permits you to use an "IBM compatible PC" to get failure data from the NVM in the GCCU.
(3)
The GCCU has protective functions that are also in the GCU. Thus, the GCU protective functions are back-ups for the GCCU protective functions. These protective functions are:
(a)
Overvoltage (130 | 3 volts)
(b)
Undervoltage (100 | 3 volts)
(c)
Overfrequency (430 | 5 Hz)
(d)
Underfrequency (365 | 5 Hz)
(e)
Overcurrent (180.5 | 13.5 Amps)
(f)
Differential Protection (25 | 5 Amps)
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(4)
(5)
(6)
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737-300/400/500MAINTENANCE MANUAL
If a failure occurs in the VSCF, these things occur:
(a)
A bad VSCF will put a signal into the differential protection circuits. This will cause the GCU to energize the feeder fault (FF) light on the M238 annunciator panel. Then, the GCU will open the GCR and the GB. This disconnects the VSCF from the ac buses.
(b)
The GCCU disconnects the generator field. This will make the VSCF output voltage go to 0 volts.
(c) The GCCU sets a latch relay to control the Bit Module display. 1) When you push the BIT switch to INDICATE, the latch relay will make the VSCF FAULT light comes ON. a) AIRPLANES WITH VSCF-19/-20 DASH NUMBERS;
If after 20 flight cycles, a fault does not reoccur the VSCF FAULT indication light will not come on when the INDICATE button is pushed. This is a reset feature for erroneous VSCF FAULT detection. (Hamilton Sundstrand CMM 24-20-98 for manual reset instructions of the VSCF fault relay).
(d) After 2 to 3 seconds, the GCCU will try to set reset. 1) If the VSCF can operate, it will set the output voltage to 5-10 volts. a) Now, you can push the GEN switch to ON. The VSCF voltage will go to 115 volts and the GCR and the GB will close. b) Subsequently, after you push the GEN switch to OFF or do a transfer to a different power source, the VSCF voltage will go to 5-10 volts and stay there. 2) If the VSCF can not operate, the output voltage will stay at 0 volts. When you push the GEN switch to ON, the GCR and the GB will not close.
If the voltage regulator in the GCU has a failure to the "full off" condition, the VSCF will keep the output voltage between 115 and 120 volts. The VSCF will not disconnect in this case.
(a)
When you open the GB by transferring the bus to another source, the output voltage will go to residual volts (Usually, it would stay at 115 volts). The GCU will see an undervoltage condition and energize the "LV" light.
The GCCU also has open phase protection. The open phase protection operates when the low phase has less than 5 | 1 Amps and the next lowest phase has more than 10 | 2 Amps.
(a)
If the GCCU finds an open phase, it sets a latch relay. When you push the BIT switch to INDICATE, the AIRCRAFT OPEN PHASE light will come on.
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737-300/400/500MAINTENANCE MANUAL
(b) An open phase will cause the feeder fault (FF) light on the M238 annunciator panel to come on. The GB will also open and the VSCF becomes disconnected from the ac buses.
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