ELECTRICAL POWER - DESCRIPTION AND OPERATION
1. General
A. The electrical power system is a network consisting of three, 3-phase, 400 cycle 115/200 volt, generating and interconnecting bus arrangements. Electrical power is supplied from isolated generators located on each engine and from an APU generator of the same rating in the tail cone (Fig. 1). Single phase transformers are used to reduce a portion of this power to 28 volts ac, and transformer rectifiers fed from the 3-phase bus connections are used to furnish 28 volts dc. A battery is installed in the lower forward section to furnish emergency power to certain critical loads when the basic sources are de-energized.
B. The generating system consists of the two main ac load busses energized by two isolated brushless generators each driven by a constant speed drive (CSD). In the event of generator failure, selected loads carried by that generator are automatically relayed to the other running generator by means of two transfer bus relays. An identical generator driven by the auxiliary power unit turbine may be used in flight as well as on the ground to supply 3-phase 400-cycle power when necessary. Provisions are made for connecting conventional ground power supplies to external power receptacles. A 3-phase receptacle near the nose wheel well on the lower right side of the body is used for external dc power and those airplanes provided with a external dc receptacle, the receptacle is located below the battery in the lower equipment bay and is used for external dc power (Fig. 1). None of the three generators on the airplane can be operated in parallel with others or with a 3-phase ground supply. The APU generator speed is controlled by a mechanical governor in the turbine fuel line. Engine-driven generator speed is controlled by a mechanical governor in each CSD.
C. A generator control unit (GCU) is provided to regulate and control each of the three generators. They perform the following functions:
(1)
Regulation of the average of the three phase voltages which may exist at the end of the generator feeders, (considering an estimated length of 50 feet).
(2)
Provision of excitation power for normal, overload, and fault conditions by means of a transformer rectifier unit within the GCU and connected to the generator output through boost current transformers used in conjunction with the transformer rectifier.
(3)
Provision of adjustment for generator no-load voltage. (The point of adjustment is marked to show the direction of rotation required to lower or raise the line-to-neutral voltage between 110 and 120.)
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Feb 20/87 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 24-00 Page 1
H20751
Electrical Power System Component Location 500
24-00 Figure 1 Nov 01/74
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(4)
Limitation of power input to the generator shaft to 160 horsepower, by means of a current signal supplied to the regulator in the GCU by three current transformers. (This power limit is based on a generator efficiency of 77%.)
(5)
Protection from abnormal system conditions such as overvoltage, undervoltage, differential current, overcurrent, overfrequency or underfrequency. An anticycling protection feature within the control system prevents cycling of the system when fault conditions exist, and the manual switches in the control cabin are held in the ON position.
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