(4)
The thermister, mounted on the exciter frame, has an inverse temperature resistance characteristic. The high resistance at low or normal ambient temperature blocks current flow in one of the parallel wires and causes the overall shunt field resistance to be about that of the remaining single wire. At the higher temperature resulting from normal operation, the resistance of each single wire increases to approximately double. At the same time, the resistance of the thermister drops to a negligible value permitting approximately equal current to flow in each wire. The combined resistance of the two parallel wires at higher temperatures is approximately equal to that of the single wire at low temperature, temperature compensation is thereby provided.
(5)
Six permanent magnets are mounted on the exciter frame between the six stator poles. These magnets, alternately reversed in polarity, provide a built-in residual voltage which results in main generator voltage build-up and eliminates the need for field flashing or for a starting relay.
(6)
A stabilizing winding is wound directly over the shunt field coils. However, the winding is not used since a feed back circuit in the transistorized-type voltage regulator senses and damps out any voltage fluctuations or transients due to load switching or faults.
3. Generator Manual Controls
A. Manual controls and monitoring devices are provided on the forward overhead panel (P5) for the electrical power system. (See figure 3.) These switches, knobs, indicators, and lights are accessible to the flight crew. Where latching-type contactors are used, the control switches are the momentary type. Three ammeters provide for monitoring output current in the three phases of both engine generators and the APU generator. The phase selector switch is located on the Module M400 generator test panel. (See figure 1.) A dc ammeter, a voltmeter and a selector switch on the forward overhead panel provide for monitoring dc power as indicated on figure 3.
500
Aug 15/69 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 24-21-0 Page 7
AC/DC Power and APU/CSD Temp Indicators 5C8
24-21-0 Figure 3 (Sheet 1) Feb 20/85
Page 8
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
H20948
501 AC/DC Power and APU/CSD Temp Indicators
Nov 01/74 Figure 3 (Sheet 2) 24-21-0
Page 9
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
4. Generator Control Relay (GCR)
A. The generator (field) control relay (GCR), located in the generator control unit, energizes the generator field. The generator control relay and therefore, the generator field circuit are normally closed. The GCR's may be opened or closed by the generator switches on the pilot's forward overhead panel or opened by signals from the generator control unit protective circuits.
B. With the GCR closed, the generator voltage will build up with engine (or APU) speed and the voltmeter and frequency meter will read approximately l15 volts and 400 Hz at normal speed. The GCR trips with the generator breaker from differential fault or abnormal voltage and any time the generator switch is moved to OFF.
C. If an engine or the APU is started with the GCR tripped or open, the generator voltage will not build up with engine speed, the voltmeter wil1 not indicate at normal engine speed, and, since the frequency meter is voltage sensitive, the frequency meter will read low. The GCR can be closed by moving the generator switch to ON whether the engine is running or not. Therefore, moving the generator switch momentarily to ON before engine start will close a tripped GCR and ensure voltage build up during engine acceleration. Moving the generator switch to ON after the engine is running will close a tripped GCR causing the voltage to build up and the generator breaker to close connecting the generator to the bus.
5. Generator Control Unit
A. The generator control unit (GCU) is designed for use in single generator operation only. The GCU provides automatic operation of the control functions for each generator system and provides signals to open the generator field by tripping the generator (field) control relay, to trip the generator breaker, and to trip the bus-tie breaker when a system fault occurs. The panel utilizes transistorized circuitry interconnected with hermetically sealed relays. These relays perform switching functions only and do not require adjustment.
B. There are several control and protective functions provided in the generator control unit. (See figure
(1)
The control functions are those of dc voltage indicator, generator breaker, field relay, bus-tie breaker, and the control of bus voltage and underspeed conditions.
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