13. Operation
_________
A. Functional Description
(1)
Main Standby Power System
(a)
During normal operation the battery buses are powered by dc bus
3. The main static inverter is also continuously energized by this source, but is turned off and supplies no load. The main ac standby bus is normally powered from ac bus 3 (Fig. 4).
(b)
With the ac bus 3 powered and the standby power selector in the AUTO position, the standby power transfer relay is energized. This connects the ac bus 3 to the main ac standby bus.
(c)
Another set of contacts in the relay places a ground on the remote control input to the main static inverter keeping it turned off.
(d)
If the ac bus 3 loses power, the standby power transfer relay will de-energize, which connects the main ac standby bus to the output of the main static inverter and removes the ground from the remote control lead. The main static inverter becomes activated and supplies 115-volt ac power to the main ac standby bus.
(e)
Switching the standby power selector to the OFF position places a ground on the remote control lead, therefore turning off the main static inverter. The standby power transfer relay de-energizes, which removes the 115-volt ac bus 3 from the main ac standby bus.
(TYPICAL) RELAY B
RELAY A
Battery and Transfer Relays Figure 3
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A
747-400
MAINTENANCE MANUAL
STANDBY POWER
AUTO
OFF BAT
SINGLE TRU RELAY ENERGIZES WHEN IN SINGLE TRU OPERATION
CAPTAIN IBVSU CONTACT OPEN WHEN POWER ON AC BUS 3 VOLTAGE IS BELOW 97 |2V AC
Main AC Standby Power Simplified Schematic
Figure 4
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747-400 MAINTENANCE MANUAL
(f)
Switching the standby power selector to the BAT position de-energizes the standby power transfer relay, turns on the main static inverter, and connects the output of the main static inverter to the main ac standby bus.
(g)
Under normal conditions, the battery transfer relay is energized supplying the main static inverter from the dc bus 3. With the standby power selector in the BAT position, the battery transfer relay will de-energize and the main battery relay will energize, connecting the input of the main static inverter to the main hot battery bus. Battery power will then power the main static inverter and the main static inverter will power the main ac standby bus.
(h)
A standby power EICAS advisory and/or status message will be generated whenever power is lost on the main ac standby bus with the battery switch ON. In addition, placing the standby power switch to OFF position will also cause an EICAS message if the battery switch is in the ON position.
(2)
APU Standby Power System
(a)
The APU standby power system supplies 115-volt ac to the APU AC standby bus (captain's primary flight and navigation displays and the left flight management computer (Fig. 5)).
(b)
With the standby power selector in the normal (AUTO) position, the output of the captain's transfer bus supplies 115-volt ac to the APU AC standby bus. If power is lost on the captain's transfer bus, the APU standby bus transfer relay will de-energize, switching to the APU static inverter output.
(c)
With the battery switch on, and the standby power selector in the AUTO position, a ground is supplied to the APU inverter transfer relay through the de-energized contacts of the APU standby bus transfer relay.
(d)
When the APU inverter transfer relay is energized, 24-volts dc is supplied from the APU battery hot bus to the APU static inverter, causing the 115-volt ac output of the APU static inverter to feed the APU AC standby bus.
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