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 -15 AND -16 GENERATOR/CONVERTER AND  ú
 SUBSEQUENT  ú 01 Page 404 ú Jul 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 AC GENERATION SYSTEM - DESCRIPTION AND OPERATION
________________________________________________
 1.  General_______
 A.  The ac generation system consists of generators, manual controls, generator control relays, generator control units, M400 test module, main power circuit breakers, and current transformers.
 2.  AC Generators (on engines with the constant speed drives) (Fig. 1)_____________
 A.  A rotating electromagnetic field causes the output voltage to be induced in the stationary generator armature. This rotating field is excited by an integral ac exciter, in which the output is converted to dc by rectifiers in the generator rotor shaft. The engine-driven generators are driven by CSDs. Generator cooling is by ducting engine fan air from an air outlet on the engine through the generator. During both ground and inflight operation, air which has passed through the generator is directed overboard through an air exit in the engine cowling. A complete generator assembly consists of an ac exciter generator, a rotating rectifier and a main generator. The ac exciter consists of a six-pole stationary dc field and a rotating armature. The operation of the unit is as follows:
 (1)  
The exciter field is supplied dc power from the voltage regulator. This causes a three-phase voltage in the exciter armature. The ac voltage is rectified and fed to the ac generator rotating field. This field generates the useful ac output voltage in the ac generator stator.

 (2)  
The exciter stator has a shunt field winding connected between terminals F and A.

 (3)  
Current supplied by the voltage regulator to the shunt field winding provides excitation for the exciter generator. The current controls the exciter output to the main ac generator. The shunt field winding consists of two wires wound in parallel in six series coils mounted on the six main poles. The coils are alternately reversed in polarity over the six poles, on one end both insulated wires are connected to a common F terminal. On the opposite end, one wire is connected directly to the A terminal and the other wire is connected through a thermister to the A terminal.

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 24-21-00
 ALL  ú ú 02 Page 1 ú Nov 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
AC EXTERNAL POWER
TEST

ENGINE GENERATOR BREAKER 2 (EGB 2) BUS PROTECTION PANEL
GENERATOR CONTROL UNITS (1,2, APU)
APU GENERATOR
BREAKER 2
(APU GB2)
APU GENERATOR
BREAKER 1
(APU GB1) EXTERNAL POWER CONTACTOR 1 (EPC 1)
EXTERNAL POWER CONTACTOR 2 (EPC 2)
ENGINE GENERATOR BREAKER 1 NOSE WHEEL (EGB 1)WELL
J4 J5
AC Power System Component Location Figure 1
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 24-21-00
 ALL  ú ú 01 Page 2 ú Jun 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
 (4)  
The thermistor, 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.
　
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