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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
24-20-00-108
Purpose
The start converter unit (SCU) has two functions:
* Regulate the APU starter-generator electrical power output
* Turn the APU during an APU start.
Location
The SCU is on the E2 rack in the EE compartment.
General
The SCU operates with the APU generator control unit (AGCU)
to control and regulate APU starter-generator power. The APU
generator control relay (AGCR) and the starter-generator
voltage regulator are inside the SCU.
The SCU also makes the starter-generator function like a motor
to turn the APU during starting.
See the APU ignition/starting section for more information
about APU starting.
AC GENERATION - START CONVERTER UNIT (SCU)
EFFECTIVITY
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AC GENERATION - START CONVERTER UNIT (SCU)
AC GENERATION - START CONVERTER UNIT (SCU)
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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General
The start converter unit (SCU) has these functions:
* Controls starter-generator voltage output
* Changes 270v dc from the start power unit (SPU) to AC
power to energize the starter-generator during starting
(through SCU internal contactor)
* Sends fault data to the APU electronic control unit (ECU).
See the APU chapter for more information about APU starting
and the APU ECU.
Power Generation
The voltage regulator and APU generator control relay (AGCR)
give excitation to the starter-generator. The voltage regulator
controls the amount of excitation and frequency of excitation to
control generator output voltage. The AGCR completes or
opens the circuit for excitation power.
The APU generator control unit (AGCU) controls the operation
(on/off) of the voltage regulator and AGCR position. The AGCU
sends a voltage regulator enable signal (turn on signal) to the
SCU when you put an APU generator switch to the ON position if
the APU is ready to load. The AGCR in the SCU closes, and the
starter-generator receives excitation.
The starter-generator permanent magnet generator (PMG) is
the power source for the voltage regulator. The AGCU
protection circuits use the PMG output to monitor startergenerator
frequency output.
The voltage regulator in the SCU and the AGCU monitor the
voltage of each phase at a point between the starter-generator
and the auxiliary power breaker (APB). The voltage regulator
uses this information to control excitation. The AGCU uses this
information for system protection.
Protection
The AGCU removes the voltage regulator enable signal and
opens the AGCR when the AGCU protection circuits find an
electrical parameter not in limits. See the AGCU protection and
excitation page in this section for more information.
The AGCU sends a fault signal to the SCU if a starter-generator
overvoltage (high voltage) or undervoltage (low voltage)
conditions happens. The SCU sends this data to the APU
ECU. You can see this data on the control display unit in the
flight compartment.
See the APU chapter for more information about APU starting
and the APU ECU.
AC GENERATION - SCU - INTERFACE
EFFECTIVITY
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AC GENERATION - SCU - INTERFACE
AC GENERATION - SCU - INTERFACE
EFFECTIVITY
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Purpose
The breakers supply 115v ac, 400 Hz, 3 phase power to the
electrical distribution system from the power sources.
The breakers also complete or open 28v dc electrical power
system control and indication circuits.
Location
All breakers are in the power distribution panels. The power
distribution panels are in the EE compartment.
These breakers are in power distribution panel 1 (P91):
* Bus tie breaker 1 (BTB 1)
* Auxiliary power breaker (APB)
* Generator control breaker 1 (GCB 1).
These breakers are in power distribution panel 2 (P92):
* Bus tie breaker 2 (BTB 2)
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