24-32-00
 ALL  ú ú 02 Page 7 ú Jun 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (7)  
Should two TRUs become inoperative, the remaining two TRUs can supply the total dc system load demand. Should three TRUs become inoperative, main battery bus transfers to the main hot battery bus which is powered by the main battery/battery charger (in the TRU mode of operation), and the APU battery bus transfers to the APU hot battery bus which is powered by the APU battery/battery charger (in the TRU mode of operation). The remaining TRU continues to power all four main dc buses.

 (8)  
The primary dc power system provides 23.0 to 29.5 volt power at dc load equipment terminals under normal operation with all four TRUs operating or under abnormal operation with up to three TRUs inoperative.

 (9)  
The TRU output voltage is unregulated. The TRU output voltage ripple, with a balanced and sinusoidal ac input condition, does not exceed 1.5 volts peak-to-average over the load range of 10 to 75 amps.

 (10)
 Each TRU is capable of delivering an overload of 112.5 amps (150 percent of rated) for 5 minutes, 187.5 amps (250 percent of rated) for 1 minute, and 375 amps (500 percent of rated) for 1 second. The unit efficiency and power factor exceed 82 percent and 0.85, respectively, for all loads between 5 and 100 percent full load.

 (11)
 The three-phase input wiring to each TRU is protected by a 3-phase thermal circuit breaker.

 (12)
 The 3-phase input thermal circuit breakers to the TRU provide protection of the dc output feeder between the TRU and dc bus. This feeder is adequately sized to carry dc fault currents up to a level that will cause the 3-phase input breakers to trip.

 (13)
 Each TRU Is designed to deliver a short circuit current to clear a circuit breaker (75 amps) in the dc distribution system without damage to the unit and without tripping the 3-phase input breaker.

 (14)
 The dc current sensors (DCCS) senses dc current flowing in a phase lead that passes through the sensor. Four DCCSs sense transformer rectifier unit (TRU) outputs in each IDG channel and two sense battery currents for the main and APU batteries.

 (15)
 Two output ranges are provided by the DCCS.  Low range is -10 to +10 amps and high range is -180 to +180 amps. For TRU applications, the low range is used to determine when the TRU output is greater than 2 amps. The high range is used to indicate the magnitude of TRU output. Both low range and high range TRU sensor outputs go to the GCU and are processed. This information is then sent to each BCU.

 (16)
 For the main and APU battery applications, the low range is used to indicate a 2 amp or greater discharging condition. The high range is used to indicate the magnitude and direction of battery current. Main battery sensing is done by BCU 1 and APU battery sensing by BCU 2 (Ref 24-31-00/001).

 (17)
 The DCCS requires an input voltage of 15.5 |1 volts dc and a maximum steady-state current of 100 milliamps dc. This operting power is supplied from the GCUs and BCUs.

 (18)
 The DCCS can withstand a reversed input voltage polarity of 15 volts dc without damage.

 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 24-32-00
 ALL  ú ú 09 Page 8 ú Oct 10/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 (19)
 TRU current is monitored by the GCUs using their respective DCCS. If the GCU senses a dc current flow of less than 2.5 amps, a digital TRU fail signal is generated and sent to BCU 1 and 2. The BCUs determine if the failure was caused by a loss of ac bus power. If the respective ac bus is powered, then BCU 1 (BCU 2 if BCU 1 fails) causes the applicable ELECT TR UNIT EICAS status message to be generated. This message indicates a failure of the TRU. When either BCU determines that only one TRU is operating, the single TRU operating relay is energized.

 B.
Functional Description of APU TRU (Fig. 4)

 (1)  
The APU TRU allows the starting of the APU using the power from galley number 2. The system redirects this galley power to the APU TRU, which will rectify the ac input power into the needed dc output power for the APU TRU.

 (2)  
The following conditions must be met in order to start the APU using the TRU: