VOLTAGE
O
GCR
DIODE
OFF

I SENSING
U
AUX
DETECTFIELD
N T
P MAIN
SW
W
P MD&T
U EXCITER
U
T FIELD
T
POWER VOLTAGE I
SUPPLY REGULATOR
OFFI
N N
ON
T T
RESET E DISCE
1R R
L/R/APU GEN FIELDCLOSE
F INTERNAL
F
FIRE PROTECTION
CONTROL SW LTA DC
5 SEC
A
SYSTEM
C GEN CONT
C
GEN FIELD AND HYD PANEL  E RELAY
SOFTWAREE HARDWARE
TO L GCB TRIP
ISOL DIODE L/R/APU GEN CONT UNIT

1 L/R GENERATOR FEILD CONTROL ONLY
Left/Right/APU Generator Field Control Schematic
Figure 2

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757
MAINTENANCE MANUAL

The right and left GEN CONT switches control their respective GCR and generator circuit breaker (GCB). When a GEN CONT switch is off (out), its GCR and GCB are open. The GCB opens the circuit connecting generator output power to its main ac bus. To close the GCR and GCB, the GEN CONT switch is pressed ON (in). The GCU first energizes the close coil of the GCR. The GCR closes, allowing dc current to energize the exciter field windings of the generator. The GCU monitors generator output power until the quality is acceptable. The GCU then sends 28-volts dc to the close coil of the GCB. This allows the generator to supply ac power to its main bus. The OFF light in the GEN CONT switch goes off when the GCB closes.Four conditions can trip (open) a generator GCR and GCB: 1) Pressing the GEN CONT switch off (out); 2) pressing the appropriate guarded GEN DRIVE DISC switch on M10063; 3) pulling the engine fire switch on aft pilots' control stand P8; or 4) a protective trip by the GCU (Ref 24-23-00). Any of these conditions cause the GCU to energize the trip coils of the GCR and GCB. This stops generator operation, and isolates the generator from its bus. An integrated drive generator under-speed protection function only trips the GCB, not the GCR. All other protection functions trip the GCR and GCB.The field switch operates only if its corresponding GEN CONT switch is off. The field switch controls the GCR, but has no effect on the GCB. The GCB remains open. With the field switch on, the associated GCR is energized to close. The generator produces ac power. The field switch allows a check of the generator voltage and frequency without connecting it to the bus. Pushing the field switch again, opens the GCR. The GCR may also be tripped by: 1) Pressing the appropriate guarded GEN DRIVE DISC switch; 2) pulling the engine fire switch on aft pilots' control stand P8; or 3) a protective trip by the GCU. When a GCR is open, the white FIELD OFF light in its field switch is on.The APU field control switch is also located on generator field and hydraulic control panel M10191 on right side panel P61. This is a momentary action switch. Pushing the switch once turns it on, pushing it again turns it off. The field switch only operates if the APU GEN CONT switch is off. The field switch controls the APU GCR, but has no affect on the auxiliary power breaker (APB). The APB remains open. With the field switch on, the APU GCR is energized to close. The generator produces ac power. The field switch allows a check of the generator voltage and frequency without connecting power to a bus. The APU GCR is tripped by: 1) Pushing the field switch off; 2) pulling the APU fire switch on P8; or 3) a protective trip by the APU GCU. When the APU GCR is open, the white FIELD OFF light in the switch is on.
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MAINTENANCE MANUAL

 (2)  
Generator Circuit Breaker (GCB) Control (Fig. 3)

 (a)  
The left and right GEN CONT switches on M10063 control their respective GCBs. The switches work with their generator control units (GCUs), to open and close the GCBs. In the closed position, the GCB connects generator output to the generator main ac bus. With the GCB in the trip position, the circuit is open. The amber OFF light in the GEN CONT switch is on when its GCB is open. Also, the EICAS display provides the R (or L) GEN OFF level C EICAS message when the corresponding GCB is open, and the corresponding engine is on. Each GCU contains separate GCB close and trip relays. To close a GCB, its GEN CONT switch is set to ON (in). The generator field control relay (GCR) closes, allowing the generator to operate. The GCU checks power quality. When the quality is acceptable, the GCU sends 28-volts dc to the GCB close coil. The GCB closes. The GEN CONT switches are normally ON.
　
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