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 C.  AC standby power is provided by the airplane static inverter for flight
 critical loads if there is a complete loss of primary ac power.115-Volt AC Power Generation and Protection (AMM 24-21-00)
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 A.  The integrated drive generator (IDG), which is a single unit assembly combining a constant speed drive and a generator, provides electrical power to the airplane when selected.
 B.  The APU generators allow the airplane to be self-sufficient by providing on ground electrical power.
 C.  Lightning protectors are installed on the ac load buses to protect the
 system from high voltage surges.Manual Control (AMM 24-22-00)
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 A.  Switches located on the flight deck control panel enable the flight crew to select the function to be performed by directing the automatic operation.
Automatic Control, Protection, and Regulation (AMM 24-23-00)
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 A.  Automatic features include on-line power reconfiguration after system faults, load management, autoland configuration, no break power transfer, master frequency reference, APU speed trim, ac system status monitoring, and ARINC 429 digital data communications.
115-Volt AC Standby Power Generation (AMM 24-29-00)
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 A.  115-volt ac requirements for flight instruments, communications and navigation (flight critical loads) are provided by the static inverter, which converts 24-volt dc power from the main battery into 115-volt ac power when the primary sources of power are inoperative.
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 24-20-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 6. Operation
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 A. Functional Description (Fig. 1)
 (1)  
The Electric Power Generating System (EPGS) can operate in several split or parallel configurations, powered by engine generator power, Auxiliary Power Unit (APU) generator power, external power, or a combination of the three. In the description that follows, all Generator Control Relays (GCR) (internal to the GCU) and Bus Tie Breakers (BTB) are assumed to be closed unless otherwise specified.

 (2)  
Before the engines are started, external power or APU generator power is used to operate the airplane electrical systems. Start up of an engine results in a mechanical input from the engine gearbox to the IDG input shaft. The CSD portion of the IDG converts the variable mechanical input through a hydromechanical system to a constant output speed. The generator portion of the IDG is driven at a constant speed of 12,000 revolutions per minute (rpm) to produce alternating current electrical power with a frequency of 400 |5 Hz.

 (3)  
When the engine reaches idle rpm, a power "ready" signal is sent to the flight deck control panel. If, at the time the main generator is switched on line, power is being supplied to the buses by external power or the APU generator, a no-break power transfer occurs between external power or APU generator power and main generator power.

 (4)  
Power from the main generator flows through the GCB to the appropriate load bus. If the SSB and BTBs on other channel are closed, the generator also supplies power to those load buses through the sync bus.

 (5)  
Start up of the second engine follows a similar operational procedure. If the second generator brought on line is on the same side of the SSB as the first generator, and the GEN CONT switch on the P5 pilots overhead panel is in the AUTO position, the second GCB will automatically close as the engine reaches idle rpm, and power characteristics are within acceptable limits. If the second generator brought on line is not on the same half of the SSB as the first, a no-break power transfer will occur between the generator coming on line and the external or APU generator power source going off-line. The SSB auto-closes as the IDG(s) parallels with the IDG(s) on the other side of the sync bus causing the two sync buses to be connected. Successive engine startups and generator power transfers follow the same operational sequence.

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