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MAINTENANCE MANUAL
The speed control circuitry in the GCU maintains generator speed at 12,000 revolutions per minute (rpm). The GCU senses permanent magnet generator (PMG) frequency and compares this frequency to a reference frequency. If a difference exists, the GCU adjusts the trim head current as required to maintain constant generator speed. 1) In the absence of trim head current, the flyball governor
within the IDG maintains the output frequency at 400 |5 Hz.Overvoltage (OV) protection is accomplished by sampling the three generator point of regulation (POR) phases. An OV condition exists when the highest of the three POR phase voltages is greater than the OV threshold. The 3-phase minimum actuating level for an OV protective trip is 131 |2 volts ac for POR frequency between 350 and 440 Hz. When OV is detected, a timer function is implemented based on the amount of OV. As the amount of OV increases, the time between the fault occurrence and the protective trip decreases. An OV condition shuts down the generator by tripping the GCR. 1) The OV time delay and GCR trip are inhibited when the GCB
and the BTB of the associated IDG are closed and the
channel is in parallel operation with another IDG channel.Undervoltage (UV) protection is accomplished by sampling the average of 3-phase POR voltages. The UV condition exists when 3-phase average POR voltage is less than 104.5 |1.5 volts ac. When this condition exists, UV protective circuitry inhibits GCB closure and trips the GCR and GCB in 9.75 |.25 seconds. 1) The UV function is inhibited while the underspeed
protection is active or when the GCR is open.Overfrequency (OF) protection, when activated, trips the GCR and GCB after the BTB has tripped. Generator frequency is monitored by the PMG circuit. The PMG and main rotor are located on the same shaft. The PMG frequency is exactly three times the POR frequency. There are two overfrequency setpoints: overfrequency 1 (OF1) and overfrequency 2 (OF2). 1) The OF1 setpoint is 427.5 |2.5 Hz. When an OF condition is
detedted, and persists for 470 +/- 30 milliseconds, the
associated BTB trips. If the OF condition persists after
the BTB trips, the GCR and GCB will trip in 1.2 |0.4
seconds after the fault first occurred. The OF1 has a
built in time delay to prevent against false trips caused
by transient overfrequency conditions.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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MAINTENANCE MANUAL
2) The OF2 setpoint is 437.5 |2.5 Hz. If a fault condition persists for 30 |10 milliseconds, OF2 protective circuitry trips the GCR and GCB.
3) AIRPLANES WITH -10 GCU'S; When an OF/UF condition occurs that causes the GCR/GCB to trip for the second time in a single flight, the associated IDG DRIVE light will be illuminated. This indicates the IDG should be disconnected to prevent further damage from occuring.
Underfrequency (UF) protections senses PMG frequency and is accomplished using the same sensing hardware that is used for OF. The UF function is locked out during underspeed conditions. There are two UF setpoints: UF1 and UF2. 1) The UF1 setpoint is 372.5 |2.5 Hz (generator frequency).
When a UF condition is detected and persists for 470 +/- 30 milliseconds, the associated BTB trips. If the UF condition perists after the BTB trips, the GCR and GCB will trip in 1.2 | 0.4 seconds after the condition was first detected. The BTB will then reclose automatically. The UF1 has a built in time delay to prevent against false trips caused by transients overfrequency conditions.
2) The UF2 setpoint is 352.5 |2.5 Hz (generator frequency). If this condition persists for 150 |50 milliseconds, a UF2 trip occurs. The GCR and GCB trip as a result of a UF2 trip.
3) AIRPLANES WITH -10 GCU'S; When an OF/UF condition occurs that causes the GCR/GCB to trip for the second time in a single flight, the associated IDG DRIVE light will be illuminated. This indicates the IDG should be disconnected to prevent further damage from occuring.
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