757
MAINTENANCE MANUAL

 D.  Hydraulic Shutoff Valve
 (1)  The hydraulic motor generator shutoff valve is located on the inboard bulkhead of the left wheel well. It controls the left hydraulic system pressure to the hydraulic motor generator, and is operated by a 28-volt dc motor. The valve can be operated by hand using the valve position indicator as a handle.
 E.  AIRPLANES WITH 10 KVA HYDRAULIC MOTOR GENERATOR; Transformer Rectifier Unit
 (1)  
The transformer rectifier unit (TRU) converts power from a nominal 115 volt, 400 Hz, three phase source to an unregulated 28 volt dc output. The TRU has a continuous rating of 50 amperes with convection cooling or 60 amperes with active cooling. No active cooling is provided to the TRU during HMG operation.

 (2)  
The TRU consists of a core type transformer with a three phase, four wire primary and a secondary with multi-phase windings. The secondry windings are connected in series to reactors and series rectifiers to maintain dc output.

 F.  Hydraulic Generator Power Control Panel P71
 (1)  
P71 mounts on the right sidewall aft of E4 rack. It contains all the relays, generator control unit, and control circuitry for the extended range operations system.

 (2)  
AIRPLANES WITH 5 KVA HYDRAULIC MOTOR GENERATOR;
 The time delays are installed in the P71 panel.

 G.  Hydraulic Accumulator
 (1)  
The hydraulic motor generator hydraulic accumulator is located on the inboard bulkhead of the left wheel well. It stabilizes the hydraulic system pressure to the generator hydraulic motor.

 (2)  
One side of the accumulator is continually pressurized by the left hydraulic system. The opposite side of the accumulator is pressurized by nitrogen gas through the ground charging valve. The charging valve and pressure gage are installed at the same location as the accumulator (5 kva HMG) or in the central hydraulic service center (10 kva HMG).

 (3)  
AIRPLANES WITH 5 KVA HYDRAULIC MOTOR GENERATOR; Accumulator nitrogen pre-charge pressure is 1550 to 2050 psi, depending on ambient temperature.

 (4)  
AIRPLANES WITH 10 KVA HYDRAULIC MOTOR GENERATOR; The nominal pre-charge pressure of the accumulator is 1500 psi.

 H.  Hydraulic Flow Limiters
 (1)  The hydraulic motor generator hydraulic system shutoff valve selects one of two hydraulic flow limiters to service the TE flaps/LE slats depressurizations module. When the generator is not operating, the shutoff valve is closed to allow the higher-rated flow limiter to service the depressurization module. When the generator is operating, the shutoff valve opens to allow the lower-rated flow limiter to service the depressurization module, and conserve hydraulic pressure and flow for the hydraulic motor generator system.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH HYDRAULIC MOTOR GENERATOR ú

 24-25-00
 ú ú 03 Page 8 ú May 28/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

 I. Test Switch
 (1)  A momentary on manual toggle switch (GND PROX/HYD GEN) is provided for system checkout. It is located on the right side panel P61 in the flight deck.
 3. Operation (Fig. 2)
 A. Functional Description
 (1)  
When loss of power at both main ac buses is sensed during flight, the standby power system automatically takes over. After a 0.5-second time delay, 28v dc from the battery bus drives the hydraulic motor-driven generator shutoff valve to the open position. Left hydraulic pressure is then applied to the hydraulic motor-driven generator via the shutoff valve and the generator begins to operate.

 (2)  
The open shutoff valve also selects the lower-rated flow limiter of the two flow limiters which service the flap/slat depressurization module. This conserves hydraulic pressure and fluid for the hydraulic motor-driven generator system.

 (3)  
When the generator output voltage and frequency reaches power ready limits, the generator control unit provides a 28 volt dc output. This output closes the bus transfer relays to connect the generator ac and dc outputs to their respective buses.

 (4)  
The dc output is connected to the hot battery bus through the HYD GEN 28v dc power relay to parallel with the standby battery. The ac output is connected to the captains instrument transfer bus through the 115v ac instrument transfer bus relay. The ac output also feeds the left and right transfer buses through the 115v ac L transfer bus relay and the 115v ac R transfer bus relay respectively.
　
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