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5. APU Transformer Rectifier Unit (Fig. 1)
______________________________
A. The APU TRU is a static device which converts 3-phase 115/200 volt, 400 Hz ac input power into 28 volts dc output power for the APU starter motor.
B. The TRU employs a transformer with a wye connected primary and paralleled wye-delta secondary connections. The 6-phase output from the secondaries is connected to two full wave rectifier sets to produce the dc output. The TRU has an output voltage of 28 volts and is rated for an output load of 200 amps. The TRU may be operated at 400 amps for 30 seconds and at 1200 amps for very short time intervals.
C. Forced air cooling is provided by the cooling fan inside the TRU. The cooling fan is used to keep the TRU from overheating from the large current demand.
D. The APU TRU is located on the E-33 aft equipment rack in the aft-left side of the airplane.
Electrical Power Generation System DC Current Sensors
Figure 3
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6. Main Deck Cargo Handling Transformer Rectifier Unit (Fig. 1)
___________________________________________________
A. The main deck cargo handling TRU is a static device which converts 3-phase 115/200 volt, 400 Hz ac input power from the main deck cargo handling ac bus into unregulated 28 volts dc output power for main deck cargo handling control.
B. The TRU employs a transformer with a wye connected primary and paralleled wye secondary connections. The 6-phase output from the secondary is connected to a 6-phase full wave rectifier and filtered to produce the desired output. The TRU has an output voltage of 29 volts at no load and approximately 26 volts at rated load. The TRU is rated for a continuous output load of 20 amps.
C. The TRU is operated with convection cooling only.
D. The TRU is mounted to the bulkhead in the forward-right main equipment center.
7. Operation
_________
A. Functional Description of Main TRU (Fig. 4)
(1)
A nominal 28 volt dc power system is provided to supply load buses requiring dc power.
(2)
The main dc system is a 2-wire system which uses the airframe structure as the ground return circuit. DC power is supplied to the dc loads, during normal operation, by four 75 ampere Transformer Rectifier Units (TRUs), which are energized from the No. 1, 2, 3, and 4 ac buses, respectively, through thermal circuit breakers.
(3)
Each TRU powers an associated main 28 volt dc load bus. In addition, under normal operation, the battery buses are supplied from dc bus No. 3.
(4)
The TRUs are normally operated in parallel through the DCIRs, 75 amp thermal circuit breakers, and a dc tie bus.
(5)
A control relay in the GCU provides manual and automatic control of the DCIR in conjunction with the BUS TIE alternate action switch located on the overhead panel in the flight deck. Switch operation to the released position causes the DCIR to open. Operation of the switch to the latched position will close the DCIR. The DCIR also operates as described for autoland operation (Ref 22-10-00/001.
(6)
The maximum continuous total demand of airplane loads connected to the main dc system is approximately 120 amps. Each of the four TRUs are capable of continuously supplying 75 amps.
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ENERGIZES WHEN APU BATTERY IS USED TO START APU (REF 49-11-00)
DC Electrical Power Simplified Schematic
Figure 4
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