(8)
The 115 volt ac three phase power from ac bus 1, 2, 3, and 4 is supplied to the four TRUs, which supply the inputs of the main dc buses; dc bus 1, 2, 3, and 4. The main dc buses are normally tied together by the dc isolation relays (DCIRs) and the dc tie bus.
(9)
Ground Handling and Ground Service Systems
(a)
The ac ground handling bus is energized automatically through the ground handling relays (GHRs), provided BCU No. 1 determines the power from either external power No. 1 or APU generator No. 1 is of good quality. External power has priority over auxiliary power if both are connected.
(b)
The external power TRUs are supplied with 115 volt ac three phase power from the ac ground handling bus. The 28 volt dc output of the external power TRUs will power the dc ground handling bus.
(c)
The ac main deck cargo handling bus is energized automatically through the main deck cargo handling relays, provided BCU No. 2 determines the power from either external power No. 2 or APU generator No. 2 is of good quality. External power has priority over auxiliary power if both are connected.
(d)
The main deck cargo TRU is supplied with 115 volt ac three phase power from the ac main deck cargo handling bus. The 28 volt dc output of the main deck cargo TRU will provide power for control during main deck cargo operations.
(e)
The ground service bus receives 115 volt ac three phase power from either ac bus 1, external power No. 1, or APU generator No. 1 through the ground service select relays (GSSRs) and the ground service transfer relays (GSTRs), depending on the source selected by BCU No. 1. External power has priority when connected for ground operations. AC bus 1 powers the ground service bus for electrical loads necessary during normal flight operations.
(f)
An autotransformer, powered by the ground service bus, supplies 28 volt ac single phase power to the 28 volt ac ground service bus.
(g)
An autotransformer, powered by the ac bus 3, supplies 28 volt ac single phase power to the main 28 volt ac bus.
(h)
The main and APU battery chargers receive power from the ground service bus. The output of the battery chargers power the main and APU hot battery buses, also maintaining the batteries at full charge condition.
(10)
Main Standby System
(a)
During normal power operations, dc bus 3 powers the main and APU battery buses through the battery transfer relay. The ac standby bus is powered by ac bus 3 through the ac standby power transfer relay.
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747-400 MAINTENANCE MANUAL
(b)
If power is lost on ac and dc bus 3, the system switches to standby power, energizing the main battery relay, and also de-energizing the battery transfer relay and the ac standby power transfer relay. The main and APU battery buses will now be powered by the main and APU hot battery buses. The static inverter will also receive 28 volt dc power from the main battery hot bus. The 115 volt ac single phase power from the static inverter will power the ac standby bus.
(c)
The captain's transfer bus receives power from ac bus 3 through the instrument bus voltage sense unit (IBVSU), and the first officer's transfer bus receives power from ac bus 2 through another IBVSU.
(d)
If the voltage is lost on ac bus 3, the IBVSU will automatically switch to ac bus 1 to power the captain's transfer bus. If ac bus 2 has a power loss, the IBVSU will switch the first officer's transfer bus to ac bus 1.
(11)
APU Standby Power System
(a)
The primary flight displays, navigation displays, and the flight management computers normally receive 115 volt ac power from the captain's and first officer's transfer buses. With a loss of the captain's transfer bus, the APU static inverter will energize and supply power to the left flight display, left navigation display, and the left flight management computer.
(b)
The APU static inverter receives 24 volt dc power input from the APU inverter transfer relay when the system switches to standby power.
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