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Electrical Power
These are the electrical power sources for the ignition and start
system function:
* 24/28v dc battery
* 24/28v dc switched hot battery bus
* 115v ac standby bus
* 115v ac transfer bus 1.
Ignition/Starting Function Sequence
This is the APU start sequence:
* The start sequence starts when the APU switch is put to the
START position and released to the ON position.
* The ECU turns on the oil pressure light on the P5 panel when
the ECU receives the start signal. The low oil pressure light
goes out at 30-40 psi oil pressure.
* The ECU opens the APU fuel shutoff valve and air inlet door.
The inlet door switch sends a door open signal to the ECU
when the air inlet door is open.
* At 0 percent speed and before the start system is energized,
the ECU energizes the ignition unit.
* If the air inlet door is open and the APU speed is less than 7
percent, the ECU sends a start signal to the SCU. If the APU
speed is more than 7 percent, the ECU waits until the speed
is less than 7 percent.
* If 115v ac is available on transfer bus 1, the SCU sends a
start signal to the ac to dc converter in the SPU. The ac to dc
converter gives 270v dc power to the SCU.
* If 115v ac is not available on transfer bus 1, the SCU sends a
start signal to the dc to dc converter in the SPU. The dc to dc
converter gives 270v dc to the SCU. The SCU also has a
control function to prevent the DC to DC converter from
depleting the battery below limits (18v dc on ground, 20v dc
in air) during a DC start attempt. The proximity switch
electronics unit (PSEU) provides the air/ground input.
* The dc to ac converter in the SCU changes the 270v dc power
from the SPU into three-phase start power. This power goes
to the starter-generator. The starter-generator turns the APU
turbine shaft. The SCU receives starter-generator rotor
position from the starter-generator resolver. The SCU uses
this signal to synchronize the three-phase start power to the
starter-generator rotor position.
* At 7 percent speed, the ECU energizes the fuel solenoid
which supplies fuel for combustion.
* At approximately 30 percent speed, oil pressure goes above
30-40 psi. The oil pressure switch removes the low oil
pressure signal. The ECU turns off the low oil pressure light.
* At 60 percent speed, the ECU deenergizes the ignition unit.
* At 70 percent speed, the ECU removes the start signal from
the SCU. The SCU removes the start signal from the SPU ac
to dc and dc to dc converters. This deenergizes the startergenerator.
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* At 95 percent speed, plus two seconds, the ECU gives the
ready to load (RTL) signal to other airplane systems. This
signals the airplane systems that the APU is ready to accept
pnuematic and electrical loads.
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
Purpose
The APU bleed air system supplies pressurized air for these
airplane pneumatic operations:
* Main engine start
* Air conditioning
* Pressurization.
Components
These are the APU bleed air system components:
* Load compressor
* Inlet guide vanes (IGV)
* Inlet guide vane actuator (IGVA)
* Bleed air valve (BAV)
* Pressure sensors (PT, DP, P2)
* Surge control valve (SCV).
All bleed air system components, except the load compressor
and inlet guide vanes, are line replaceable.
Controls
The ECU sends signals to the IGV actuator to control the IGV
position. With the APU bleed switch in the ON position and APU
speed above 95 percent, the ECU sends a signal to open the
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