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(c) External Power Operation to IDG Power Operation - One External Power Source Connected and Being Used (SSB Closed) 1) Generator Control Switches ON and Engines Not in Operation a) When an engine is started and up to normal speed with the associated GEN CONT switch initially in the ON position, the IDG will automatically synchronize with the external power source. When the frequency and phase angle are within acceptable limits, the GCB automatically closes, momentarily paralleling the IDG with external power until the XPC automatically opens. Starting the remaining engines causes the additional IDGs to synchronize and auto-parallel through their GCBs. 2) Generator Control Switches ON and Engines in Operation a) In this configuration, if the external power source is lost for any reason, the IDGs will auto-parallel through their GCBs and supply the buses. Cycling a GEN CONT Switch from ON to OFF and back to ON with the associated engine in operation at normal speed will trip and reclose the GCR. System operation will then be the same as described above for engines not in operation (where cycling GEN CONT switches replaces starting engines). 3) Generator Control Switches OFF and Engines in Operation a) When a GEN CONT switch is placed in the ON position with the associated engine in operation at normal speed, the GCR closes and system operation then is identical to that of an engine not in operation (where placing GEN CONT switches in the ON position replaces starting engines). 4) Generator Control Switches OFF and Engines Not in Operation
a) Starting an engine with the associated GEN CONT switch in the OFF position causes no change in system status. Placing a GEN CONT switch in the ON position with the associated engine not in operation has no effect other than to cause the associated GCR to close.
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747-400
MAINTENANCE MANUAL
(d) External Power Operation to IDG Power Operation - BTB Open 1) If the BTB on the IDG channel being started is open (with the GEN CONT switch in the ON position), the GCB will close to energize that bus when acceptable power quality is available. The other buses and sources shall not change state as long as this bus remains isolated by its open BTB. Placing the associated alternate action BUS TIE switch in the latched (AUTO) position will cause a power transfer to occur as follows: a) The IDG will auto-parallel with the source on the sync bus, or close due to dead bus sensing circuitry.
b) If the sync bus is presently energized, a power transfer occurs in the direction of the last source energizing that sync bus half. For example if the GCB was most recently closed, then external power will be tripped off. If an external power source was most recently placed on that sync bus half, then the GCB will be tripped off.
(e) Auxiliary Power Operation to IDG Power Operation (SSB Open or Closed) 1) The transfer method from auxiliary power to IDG power is
the same as that for the transfer from external power to IDG power with the SSB either closed or open. The first IDG is synchronized and paralleled with auxiliary power before the APB is automatically tripped.
(f) IDG Power Operation to External Power Operation 1) All engine generator power to external power transfers described below are inhibited for each of the following conditions: takeoff, landing rolls, and air mode.
2) If the engines are operating after an XPC trip and the GEN CONT switches are ON, the IDGs will auto-parallel through their GCBs and BTBs. When IDG power is established on both halves of the sync bus, the two halves will automatically be brought within acceptable frequency and phase angle limits and auto-paralleled thorugh the SSB.
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