CHAPTER 9GENERATOR ASSEMBLY REPAIR INSTRUCTIONS9-1. General.a. The main generator is a brushless type con-sisting of a stationary three-phase armature and arotating salient pole wound field. The exciter,mounted integrally with the main generator, con-sists of a three-phase rotating armature mounted onthe shaft with the generator field and a stationarysalient pole field.The exciter has ten poles.b. A full wave bridge rectifier containing sixdiodes is mounted on the shaft. Access to test orreplace the rectifiers is through the access hole onthe end of the endbell assembly. Access to the ex-citer stator and rotor, and inspection of the maingenerator armature and field, require disassemblyof the generator.c. The generator shaft is directly driven by theengine shaft through flexible metal disk coupling.The flexible coupling compensates for misalignmentbetween the two shafts to eliminate injurious stresson the connecting components.d. The main generator stator is installed in thestator frame and is fastened to and spaced by longi-tudinal ribs which are part of the frame. An eyebolt on the stator frame provides a means of liftingthe unit. The exciter stator is mounted in an end-bell which positions on the stator frame rabbit.e. The rotor is supported at one end by a singlehealing in an endbell bolted to the stator frame. Theopposite end of the rotor is provided with a blowerassembly and coupling disk which bolts directly tothe engine flywheel.The rotor is skewed to improvegenerator output voltage waveform.f. The bearing is a double-seal type ball bearing,packed with grease conforming to Specification MIL-G-23827. It provides a minimum of 5000 hours ofservice at continuous loads.9-2. Generator Removal and Disassembly.a. InsulationResistance. To test insulation resis-tance, use a megohmmeter to measure the resistancebetween a winding and ground. The insulation resis-tance of each of the windings should be at least 1 meg-ohm at 75°F.If this value is not met, clean or dryout the winding and repeat the test. Replace if de-fective.NOTELow insulation resistance may be causedby dirt or excessive moisture. Insulationfailure may be caused by wrong voltages,induced voltages caused by opening fieldcircuits too quickly, oil and grease, hightemperatures or excessive vibration.(1) Generator Stator- Disconnect all leads tovoltage regulator and all other points to completeyisolate the winding before meggering.(2) Generator Rotor - Disconnect both fieldleads from the rotating diode assembly before meg-gering.(3) Exciter Stator - Disconnect both field leadsfrom terminals 15 and 16 of TB16.(4) Exciter Rotor - Check this only if otherwindings have low insulation resistance as the leadsmust be unsoldered from all diodes before meggering.b. ElectricalMalfunctionsandIsolation. A mal-function of the main generator is usually indicatedby low output voltage, or no voltage output. To iso-late a malfunction to the main generator, proceed asfollows:(1) Tag and disconnect the two top leads onterminals 15 and 16 of TB16 (located on the currenttransformer mounting plate). Connect one side ofa dc ammeter to one of the terminals (15 or 16) andan adjustable dc voltage source to the other terminaland the dc ammeter.The adjustment shouId be setto mint mum voltage.(2) If the set is a 50/60 Hz set, select 60 Hzand start the engine.Bring the speed up to 1800 rpmmanually and turn on the adjustable dc power supply.Adjust the dc voltage so that the output voltage (noload) reads 208 or 416 volts (depending on low or highvoltage connection).The dc ammeter should read be-tween 1.25 and 1.55 amperes to produce either 208or 416 volts.(3) If the set is a 400 Hz set, start the engineand bring it up to 2000 rpm. Turn on the adjustabledc power supply.Adjust the dc voltage so that theoutput voltage (no load) reads 208 or 416 volts, (de-pending on low or high voltage connection). The dcammeter should read between 3.0 and 3.65 amperesto produce either 208 or 416 volts.(4) If the dc ammeter indicates incorrect in-put exciter current to obtain rated voltage in steps(2) or (3) above, the generator has failed. Proceedwith the following steps. If the output is as speci-fied, the excitation system assembly has failed (re-fer to paragraph 8-1).The following steps are pre-sented to enable isolation of the fault within the gen-erator.Only steps (5), (6) and (7) would normallybe done without some disassembly of the generator.(5) Remove six bolts securing air intake grilleand remove griIle.Remove screws, cover plate,and gasket from generator endbell. See figure 9-1.Remove bolts from the six diodes on the exciterrotor frame, one at a time, replacing each diodeafter testing.Test diodes per paragraph 14-12b.9-1
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