737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
Generator Excitation
The APU starter-generator is a three-phase brushless AC
generator. The generator consists of these components:
* Exciter rotor (armature)
* Main generator rotor (field winding)
* Permanent magnet generator (PMG) rotor
* Main generator exciter
* PMG stator
* Neutral current transformer (NCT).
The APU gearbox turns the rotor. As the PMG rotor turns, it
induces an AC voltage in the three-phase windings of the PMG
stator. This AC voltage goes to the start converter unit (SCU). A
voltage regulator (VR) in the SCU transforms and rectifies the
power to DC. Also in the SCU, is a generator control relay
(GCR). With the GCR in the closed position, DC power goes to
the exciter field windings.
The stationary magnetic field caused by the DC voltage in the
windings of the exciter field (stator) makes a three-phase AC
excitation voltage in the windings of the exciter armature (rotor).
This AC voltage rectifies to DC voltage by the rotating rectifier
assembly in the rotor. The DC voltage goes to the field windings
of the main generator. Current flow in the field windings causes
a rotating magnetic field. The rotating magnetic field causes an
AC voltage in the windings of the main generator stator.
The AGCU monitors the voltage and current of the generator.
The SCU monitors and controls the voltage output from the
generator. The APB can close when the voltage and frequency
are within power quality limits. The voltage regulator in the SCU
controls AC power output by control of excitation power.
The generator control relay (GCR) in the APU GCU (AGCU) is
not part of the excitation power circuit for the APU generator.
The AGCU GCR closes to send the voltage regulator (VR)
enable signal to the SCU. The VR enable signal completes the
excitation circuit in the SCU.
APU Starter-Generator Start Mode
The APU starter-generator starter is integral with the generator.
The SCU attaches electrically to the starter-generator through
connections (splices) to the feeder wires at the back of the
power distribution panel (PDP). The SCU voltage regulator uses
these connections as a point of regulation (POR). The SCU also
uses this connection to supply power to the starter-generator
during APU starting. The AGCU protection circuits monitor
power quality at connections to the feeder wires inside of the
PDP.
See the APU ignition/starting section for more information.
(SECTION 49-40)
AC GENERATION - AGCU - PROTECTION AND EXCITATION
EFFECTIVITY
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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AGCU Protection
The AGCU monitors voltage, current, and frequency. The AGCU
opens the APB and removes the excitation from the generator
to protect the electrical channel. The AGCU removes the VR
enable signal to turn off excitation. The AGCU has these
protective functions:
* Overvoltage
* Undervoltage
* Overfrequency
* Underfrequency
* Unbalanced phase current
* Phase sequence
* Overcurrent
* Differential fault
* Underspeed protection
* AGCU fail-safe.
Overvoltage and Undervoltage Protection
The AGCU monitors each of the phase voltages. Overvoltage
protection starts when the highest phase voltages is more than
130 volts. Undervoltage protection starts when the lowest phase
voltage is less than 101 volts for more than 7 seconds. If there is
a voltage fault, the AGCU removes the VR enable signal. The
SCU removes the excitation from the generator and the AGCU
commands the APB to open.
Frequency Protection
The SCU monitors the frequency at the PMG. Proper generator
frequency is 400 Hz. The AGCU receives a conditioned PMG
signal from the APU SCU. The AGCU removes the VR enable
signal to the SCU and opens the APB if the frequency is more
than these limits:
* 425 Hz for more than 1.5 seconds
* 435 Hz for 35 milliseconds.
The AGCU removes the VR enable signal and opens the APB if
the frequency is less than these limits:
* 375 Hz for 1.5 seconds
* 355 Hz for 150 milliseconds.
Unbalanced Phase Protection
The AGCU monitors the three phases of the APU generator line
current transformer. Unbalance phase protection occurs if the
current unbalance between any two phases is more than 140
amps. The AGCU commands the BPCU to open BTB 2. If the
fault remains, the BPCU opens BTB 1 and re-closes BTB 2. If the
fault clears, there is no further action. If the fault remains, the
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