757 
MAINTENANCE MANUAL 
P11 PANEL 
P31 
PANEL 
P32 
PANEL 
P6 PANEL 
. DC TIE CONTROL UNIT 
. DC TIE RELAY 
FLIGHT COMPARTMENT 
MAIN EQUIPMENT 
CENTER (E1 RACK) 
SEE  A 

E1 RACK
1

RIGHT TRU
FWD LEFT TRU
MAIN EQUIPMENT CENTER A
1 TOP SHELF NOT ON ALL AIPLANES
Transformer-Rectifier - Component Location Figure 1 (Sheet 1)
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MAINTENANCE MANUAL

Transformer-Rectifier - Component Location
Figure 1 (Sheet 2)

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MAINTENANCE MANUAL

 D. APU Start TRU Fan Control Relay
_______________________________
 (1)  The APU start TRU fan control relay turns on the fan in the TRU when starting the APU. The fan will then keep running until TRU temperature drops below the TRU fan control thermal switch closing temperature (See Fig. 3 for temperature). The relay is in the E6 panel.
 E. APU TRU Start Relay
___________________
 (1)  The APU TRU start relay connects 3 phase AC power from the right main AC bus to the APU start TRU. The relay is in E6 panel.
 3. Operation
_________
 A. Functional Description
______________________
 (1)  
Main TRU's (Fig. 2)

 (a)  
In normal operation, the BUS TIE switches on M10063 electrical system control panel are in AUTO position. The autoland bus isolate and DC tie relays are de-energized (airplane not in autoland mode).

 (b)  
If either bus loses power for longer than 11.5 seconds, the DC tie control unit provides a ground for the DC tie relay. The DC tie relay energizes, tieing the buses together. The EICAS computers display a TRU failure message.

 (c)  
The IBVSU's monitor power on the main AC buses. In the autoland mode, when the IBVSU's sense a loss of power on either main AC bus, a ground is supplied through the center bus isolation relay to energize the DC tie relay. This ties the DC buses together and restores the dead DC bus.

 (d)  
EICAS computers monitor the output voltage and current of each TRU, and EICAS will display the status/maintenance message TR UNIT when the dc buses and tied/dc tie relay energized.

 (2)  
APU Start TRU (Fig. 3)

 (a)  
The APU is normally started by the TRU, but may also be started by the APU battery.

 (b)  
When an APU start command is given, the APU TRU start relay is energized by the APU start enable relay via the APU start relay (AMM 49-11-00). The APU TRU start relay provides AC input power to the TRU, and energizes the fan control relay. The fan will be on while the APU is being started, and until TRU temperature drops below the fan control thermal switches closing temperature (Ref Fig. 3 for temperature). The APU crank contactor is inhibited from closing provided the APU start enable relay is energized. The APU start enable relay will be de-energized if the TRU overheats, or power is not available from the right AC bus. Under these conditions the APU crank contactor will close and the APU starter motor receives power from the APU battery.

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Transformer Rectifier Schematic
Figure 2

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