FLIGHT CONTROLS DC POWER -POWER SUPPLY ASSEMBLY
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FLIGHT CONTROLS DC POWER -FUNCTIONAL DESCRIPTION
General
The center PSA can get power from these sources:
*
PMG L2
*
PMG R2
*
Captain’s flight instrument bus
*
Hot battery bus.
The PSA has a controller that selects which power source to use. The controller supplies grounds to relays in the PSA to select the different power sources.
PSA Operation Modes
The PSA has three modes of operation:
*
Primary
*
Secondary
*
Backup.
Primary mode is the normal mode when the engines are running. The PSA uses power from one or both PMGs when it is in primary mode. The PMGs send ac power to a power converter. The power converter changes the ac power to 28v dc power.
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When there is no power from either PMG, the PSA goes into secondary mode. Secondary mode is the normal mode when the airplane is on the ground and the engines are off. The K2 relay relaxes when the PSA is in secondary mode. This permits the PSA to use power from the captain’s flight instrument bus.
When there is no power from either PMG or from the captain’s flight instrument bus, the PSA goes into backup mode. The K1 relay energizes when the PSA is in backup mode. This permits the PSA to use power from the hot battery bus.
NOTE: The right PSA does not use backup mode because it does not connect to the hot battery bus.
FCDC Battery
Normally, the controller energizes the K3 relay to connect the FCDC battery to the FCDC bus. A diode prevents uncontrolled charging of the battery from the bus. When the bus loses power, the battery supplies power until the controller connects another power source.
The PSA monitors the voltage of the FCDC battery. When the battery voltage is below 25 volts, the PSA energizes the K4 relay. This permits the FCDC bus to charge the battery. The battery charge cycle lasts for 300 minutes.
The PSA will not charge the battery if the battery voltage is below 21 volts or if the PSA is in backup mode.
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FLIGHT CONTROLS DC POWER -FUNCTIONAL DESCRIPTION
Training Information Point
If you replace a primary flight computer (PFC), you must open the battery interlock circuit breaker of the related PSA before you remove power from the airplane. The battery interlock circuit breaker removes power from the K1 and the K3 relays. This prevents the FCDC loads from draining the main battery and the related FCDC battery. But, the PSA does automatically open K1 and K3 if the FCDC bus voltage drops below 17.5.
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FLIGHT CONTROLS DC POWER -FUNCTIONAL DESCRIPTION
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EXTERNAL POWER -INTRODUCTION
General
The external power system controls the electrical power on the airplane from ground power sources. The external power system has these electrical power system functions:
*
Bus power control
*
Ground handling power
*
Ground service power
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Primary external power
*
Secondary external power.
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EXTERNAL POWER -INTRODUCTION
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EXTERNAL POWER -GENERAL DESCRIPTION
General
You use external power to supply the airplane with electrical power from a ground power source. You can supply the power directly to these electrical buses:
*
Ground Handling
*
Ground Service
*
Main ac.
External Power Panel
The external power panel has two external power receptacles that have a continuous supply capacity of 90 kva each. The forward receptacle is for primary external power. The aft receptacle is for secondary external power.
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