(10)
The L and R UTILITY BUS switches control their respective utility bus relay (UBR). The amber OFF light is on when: 1) The utility bus switch is off (out position); and 2) the main bus supplying the utility bus is powered. The amber OFF light is also on when: 1) The utility bus switch is on, 2) the main bus supplying the utility bus is powered, and 3) the UBR has been opened by the load shed system. Along with the OFF light a (L or R) UTIL BUS OFF level C message appears on the EICAS display.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757 MAINTENANCE MANUAL
AC METERS - DESCRIPTION AND OPERATION
_____________________________________
1. General_______
A. The ac electrical power system values are shown on the EICAS display.
2. Operation (Fig. 1)_________
A. Functional Description
______________________
(1)
AC power system voltage, frequency, and load are displayed on the EICAS screen. (The EICAS display is on pilots' center panel P2.) These values only appear on the maintenance page of the EICAS system. Therefore, the electrical system values can only be viewed on the ground not inflight.
(2)
The output voltage and frequency of the left and right integrated drive generators (IDGs), APU generator, external power, and static inverter, are monitored by the EICAS system. The voltage and frequency of the IDGs is measured on the generator side of the generator circuit breakers. APU generator power is measured on the generator side of the auxiliary power breaker. External power is measured on the receptacle side of the external power contactor. (One phase of the three-phase power channels is monitored.) The output of the static inverter is taken directly for measurement. Each of the measuring points provide inputs to the right and left EICAS computers. The values are then shown on the EICAS display.
(3)
The EICAS display shows the load supplied by the left, right, and APU generators; as well as the external power source. Each generator control unit (GCU) monitors the load of its associated generator. The external power load is monitored by the bus power control unit (BPCU). The GCUs and BPCU send a dc current proportional to the load each senses to the left and right EICAS computers. The values are then shown on the EICAS display. The EICAS displays the load in per unit values. A 1.0 reading is the rated generator load of 90-kva. (A 1.5 reading indicates the generator is supplying 150 per cent rated load.) A 1.0 external power load equals 90-kva.
(4)
AIRPLANES WITH HYDRAULIC MOTOR GENERATOR; The hydraulic motor generator frequency, and ac voltage are also displayed on EICAS maintenance page.
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ALL ú ú 01 Page 1 ú Jan 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
L/R EICAS COMPUTER
EQUIPMENT RACK EQUIPMENT RACK
AC Meters SchematicFigure 1
A
757
MAINTENANCE MANUAL
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
24-28-00
ALL ú ú 02 Page 2 ú Jan 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
H12736
A
757 MAINTENANCE MANUAL
DC GENERATION - DESCRIPTION AND OPERATION
_________________________________________
1. General (Fig. 1)_______
A. The DC generation system consists of three subsystems: primary dc system, standby system, and the battery/battery charger system.
B. Primary DC System (24-32-00)
(1)
The primary DC system consists of two transformer-rectifier (T-R) units. The T-R units convert 115v ac power to 28v dc power for the DC buses.
(2)
The APU start TRU is the primary source of power for starting the APU. The TRU receives input power from the right main ac bus, and converts it to dc power for the APU starter motor. Main system components are the APU start TRU and the TRU APU start relay.
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