3. Variable Speed Constant Frequency Generator/Converter_____________________________________________________ (on engines with the VSCF) (Fig. 1A)
A. The variable speed constant frequency generator/converter (VSCF) is interchangeable with the CSD and generator assembly. The VSCF gets power from a variable speed shaft on the accessory gearbox on the engine. It converts this to three phase electrical power at 115 volts and a constant frequency of 400 Hz.
B. These are the components of the VSCF:
(1)
The generator converts shaft energy at variable speeds to three phase electrical power at 115 volts. The output frequency of the generator depends on the engine speed; it varies from 1370 to 2545 Hz.
(a) For more details about the mechanical operation of the VSCF, refer to 24-11-00/001.
(2)
The dc filter rectifies the generator output to 270 volts dc. The dc filter also uses large capacitors to remove the ripple from the dc link voltage.
(3)
The inverter uses six large transistors to convert the dc link voltage to a three phase, pulse-width modulated (PWM) waveform. Inside the inverter is the neutral forming transformer. This adds a neutral lead to the three-wire output of the transistors. The neutral forming transformer permits the transistors to equally share a load that is not balanced.
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24-21-00
ALL ú ú 02 Page 3 ú Nov 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
INPUT:VARIABLE SPEED SHAFT POWER FROM THE ACCESSORY GEARBOX ON
OUTPUT:
3-PHASE, 4-WIRE,
115 VOLT, 400 HZ
50 KVA ELECTRICAL
A B
N
POWER
SUPPLY
FROM
PMG
MONITORS, AND HAS BIT
Variable Speed Constant Frequency Generator/Converter - Block Diagram
Figure 1A
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VARIABLE SPEED CONSTANT FREQUENCY ú 24-21-00
GENERATOR/CONVERTERS ú ú 04 Page 4 ú Nov 12/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
(4)
The ac filter uses capacitors to change the PWM waveform from the inverter to a three phase sinusoid at 115 volts and 400 Hz.
(5)
The current transformer/electromagnetic interference (CT/EMI) filter monitors the output current of the VSCF. It also removes unwanted signals from the VSCF output.
(6)
The generator/converter control unit (GCCU) uses a microprocessor to operate. It gets power from the permanent magnet generator (PMG) in the generator housing. The GCCU has two important functions:
(a) The GCCU controls the generator and the inverter.
1) The voltage regulator in the GCCU sends a signal to the field of the generator to control the generator output voltage.
2) The GCCU also controls the output transistors in the inverter. It makes them come on and go off at the correct times to make the output voltage waveform.
(b) The GCCU also monitors the operation of the VSCF.
1) The GCCU monitors the dc link voltage from the dc filter. It uses this signal to control the generator output voltage.
2) The GCCU monitors the generator output voltage. It uses this signal as feedback for the voltage regulator.
3) The GCCU monitors the operation of the internal circuitry of the VSCF. The GCCU looks for failures and stores failure data for use during BIT testing.
4) The GCCU monitors the output of the VSCF through the CT/EMI filter. This information helps the GCCU find failures that are external to the VSCF.
4. Generator Manual Controls (Fig. 2)_________________________
A. Manual controls and monitoring devices for the electrical power system are on the forward overhead panel P5. The switches for latching contactors are momentary. There is an ac ammeter for each engine. The dc ammeter, dc voltmeter, ac voltmeter, and frequency meter monitor various systems as determined by the selector switches below the meters.
5. Generator Control Relay (GCR)_______________________
A. The generator field control relay (GCR), in the generator control unit (GCU), energizes the generator field. The GCR and the generator field circuit are normally closed. The GCR's may be opened or closed by the GEN switches on the pilot's forward overhead panel or opened by signals from the GCU protective circuits.
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