ELECTRICAL LOAD DISTRIBUTION - DESCRIPTION AND OPERATION
1. General
A. The electrical loads on the airplane are distributed through three kinds of bus arrangements, 115 volt ac 3-phase 400 cycle, 28 volt ac single phase 400 cycle, and 28 volt dc. The airplane system begins with the 115 volt ac 3-phase 400 cycle network, and branches out to 28 volt ac and dc bus systems. (See figure 1.)
2. 115 Volt AC System
A. The first of the three considered is the 115 volt ac system fed by the four 400 cycle, 3-phase sources. This system terminates in 3-phase 115 volt ac busses connected directly to the No. 1 and No. 2 generator busses, and includes two additional 3-phase 115 volt transfer busses connected to the No. 1 and No. 2 transfer relays. Since no two power sources of the electrical syste m can be operated in parallel a means of load transfer is provided. (See figure 1.) The No. 1 and No. 2 transfer busses can be energized, through a transfer relay, from either generator main bus No. 1 or No. 2. Normally, each transfer bus is energized by its associated generator; however, in the event a generator stops delivering power causing the generator main bus to go dead, the transfer bus transfers automatically to the other generator main bus. As soon as power is restored, the transfer bus transfers back to its original position. Control logic is provided to ensure the transfer busses are energized any time a main generator bus is energized and the bus transfer switch on the pilots' forward overhead panel is in the AUTO or normal position. The control of the transfer relay is accomplished by auxiliary contacts on the generator and bus tie breakers. With the bus transfer switch in the OFF position, the transfer bus is dead when the generator main bus is de-energized. Except for the external power contactor and the generator No. 1 and No. 2 circuit breakers which are located in the right hand forward compartment outboard of the nose wheel well, the contactors, relays, main circuit breakers, and other major components necessary for electrical load distribution are located in the load control center panels (P6). (See figure 2, sheet 2.) The fronts of the panels contain circuit breakers relating to airplane systems as indicated along the tops of each separate panel, with the individual circuit breakers marked as they pertain to that system. The separate panels are hinged along the edge which permits swinging the panel forward. When the panels are opened in this manner switching components are accessible as indicated on figure 2 (sheet 2). The control units for the three airplane generators, a receptacle panel to 115 volt ac and 28 volt dc (P6-6) and the dc system circuit breaker panel P6-5 are located on the left side of load control center (P6).
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May 15/68 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 24-50-0 Page 1
3. 28-Volt AC System (Fig. 1)
A. Additional loads are connected to the above buses at a reduced voltage through 28-volt autotransformers. Such transformers are located at various points in the airplane and are connected to both the main and transfer buses mentioned above. The loads connected to 28-volt ac buses are mostly used for airplane lighting, and for small amounts of power required for various indicators, showing temperature, pressure or position of actuators. For detailed information on the loads supplied by such transformers through 28-volt ac buses, see wiring diagrams.
4. 28-Volt DC System (Fig. 1)
A. A network of 28-volt dc buses, supplied by various transformer-rectifiers (T-R units) are energized with three-phase 115- volt, 400-Hz power from the first system. T-R units No. 1 and 2 are connected to transfer buses and have a changeable input source. T-R unit No. 3 is supplied from section X of the No.2 115-volt ac three-phase bus. The loads connected to T-R unit output buses consist mostly of control actuators and operating coils for contactors and relays in the electrical generating and distribution systems. In addition to the above three T-R units the system contains a T-R unit used to charge the airplane battery. The dc buses used in distributing the above dc loads are the battery bus, the hot battery bus, the No. 1 and 2 dc buses and the 28-volt dc standby bus. A fifth or external power T-R unit located within the bus protection panel provides a limited source of dc when external 115-volt ac three-phase power is connected to the external power receptacle. The external power T-R unit provides a dc supply necessary to operate contactors and relays with only external power available. For a detailed utilization of dc power or load distribution for dc buses, see wiring diagrams. Loads supplied from the battery bus are shown on a combined circuit breaker and wiring diagram. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：737 AMM 飞机维护手册 电源 ELECTRICAL POWER(63)
