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E.  The voltage regulator, contained with in the GCU, supplies and controls the direct current required by the exciter field to maintain the proper exitation of the main generator field. The exitation requirement is determined by the voltage regulator as a function of generator output voltage. The voltage regulator is the static type specifically designed for this particular type of ac generator. 
F.  The current transformers in each generator system are so located that inputs for various control and protective functions such as overcurrent, power limiting, generator excitation boost, load differential protection, neutral differential protection, and metering are obtained from the generating and distributing system 3-phase power leads and/or bus connections. Three separate 3-phase current transformer (CT) assemblies are used in the aircraft to sense current for different purposes.
(1)  
The differential protection neutral side current transformers, mounted on the power plant near the generator, sense current in the grounded feeders from generator terminals T4, T5, and T6. This arrangement is essentially the same for the three generators.

(2)  
The differential protection load side assembly contains two sets of CT's identical to the neutral side. One set functions as the differential protection for the engine generator and the other for the APU generator. Both sets are capable of supplying the main load bus and are identified as T4 and T5. Each set contains six separate current coils. Switching of the outputs from the coils is done inside the generator control units by the differential protection relays.

(3)  
The functions of measuring overcurrent, power limiting, boost, and current metering are accomplished by a CT assembly containing twelve coils. Nine of these coils furnish inputs to the generator control unit and three coils are connected to ammeters.

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G.  A test module is provided which contains selector switches, test jacks, and circuits that allow portable meters to be used to check performance of the airplane generation system.
2.  AC Generators
A.  The generators are each 0.95 leading to .75 lagging power factor, 120/208 volts, 380/420 cps alternating current at 5700 to 6300 rpm. The units are designed without slip-rings, commutator, or brushes on either the main generator or the exciter. 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 (Fig. 2). The two engine-driven generators are supported by the CSD transmission on the underside of the engine. Generator cooling is accomplished 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 (Ref Chapter 75, Generator Cooling.) 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 3-phase voltage to be developed in the exciter armature. The ac voltage is rectified and fed into the ac generator rotating field. This field generates the useful ac output voltage in the ac generator stator. 

(2)  
The exciter stator has two windings; a shunt field winding connected between terminals F and A- and a stabilizing winding connected between terminals S and A- (Fig. 2). The stabilizing winding is not used with a transistorized-type voltage regulator. 

(3)  
Current supplied by the voltage regulator to the shunt field winding provides excitation for the exciter generator. In so doing, 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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H20867
 AC Generator  500 
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