AC GENERATORS sEctioN 2.4DiaGNostic tEstsRESULTS:1. Repair, replace or reconnect wires as necessary.2. Replace any damaged slip rings or brush holder.3. Clean and polish slip rings as required.teSt 13 – teSt rotor aSSemBly(12-20 kW)DISCUSSION:A rotor having completely open windings will causeloss of excitation current flow and, as a result,generator AC output voltage will drop to “residual”voltage. A “shorted” rotor winding can result in alow voltage condition.PROCEDURE:I. Disconnect the brush wires or remove the brush holder,to prevent interaction.2. Set a VOM to measure resistance.3. Connect the positive (+) VOM test lead to the posi-tive (+) rotor slip ring (nearest the rotor bearing); andthe common (-) test lead to the negative (-) slip ring.The meter should read rotor resistance. Compare to“Specifications,” in the front of this manual.4. Connect the positive (+) VOM test lead to the positive (+)slip ring and the common (-) test lead to a clean frameground. The meter should indicate INFINITY.RESULTS:1. Replace rotor assembly if it is open or shorted.2. If rotor tests good, perform “Insulation Resistance Test”in Section 1.5.SLIP RINGSBEARINGFigure 9. The Rotor AssemblyNOTE: Be sure to read Section 1.5, “Testing, Cleaningand Drying”, carefully. If the rotor tests good, try per-forming an insulation resistance test. Clean and dry therotor if it fails that test. Then, repeat the test. If the rotorfails the second insulation resistance test, it should bereplaced.teSt 14 – check ac output FrequencyDISCUSSION:The generator AC frequency is proportional to theoperating speed of the rotor. The 2-pole rotor will sup-ply a 60 Hertz AC frequency at 3600 rpm. The unit’sAC output voltage is proportional to the AC frequency.For example, a unit rated 240 volts (line-to-line) willsupply that rated voltage (plus or minus 2 percent)at a frequency of 60 Hertz. If, for any reason, thefrequency should drop to 30 Hertz, the line-to-linevoltage will drop to a matching voltage of 120 voltsAC. Thus, if the AC voltage output is high or low andthe AC frequency is correspondingly high or low, theengine speed governor may require adjustment.PROCEDURE:1. Connect an accurate AC frequency meter across theWires 11 and 44 terminals of the generator main linecircuit breaker (see Figure 1, Section 2.4).2. Start the engine, let it stabilize and warm up at no-load.3. When engine has stabilized, read the frequency meter.The no-load frequency for single cylinder units shouldbe about 62-63 Hertz. For V-Twin units, the no-loadfrequency should be about 60 Hertz.RESULTS:1. If the AC frequency is high or low, go on to Test 15 forsingle cylinder units, or Test 16 for V-Twin units.2. If frequency is good, but voltage is high or low, go toTest 17.3. If frequency and voltage are both good, tests may bediscontinued.teSt 15 – check and adjuSt enGineGoVernor (SinGle cylinder unitS)DISCUSSION:The generator AC frequency output is directly pro-portional to the speed of the rotor. A two-pole rotor(having a single north and a single south magneticpole) will produce an AC frequency of 60 hertz at3600 RPM.The generator is equipped with a “voltage over fre-quency” type AC voltage regulator. The units ACoutput voltage is generally proportional to AC fre-quency. A low or high governor speed will result in acorrespondingly low or high AC frequency and voltageoutput. The governed speed must be adjusted beforeany attempt to adjust the voltage regulator is made.Part 2Page 490G9266revA.indd 49 10/15/2008 11:27:18 AM
