Alternator Diagnosis

Parent topic: Alternator Operation and Diagnosis

Preliminary Checks

    Warning
  • Risk of Injury
  • Extreme care must be used when working with an operating generator set. Lethal voltage potentials exist inside the control box, at terminals on the DSR and at the power receptacle.

Before attempting the more complicated diagnosis procedures, check the following items to ensure a superficial problem is not overlooked.

Note Further diagnosis is a waste of time until these items are checked, since a problem in one of these areas will influence test results.

  1. If the generator malfunction is accompanied by excessive black exhaust smoke and engine lugging, double check all possible engine problems such as fuel supply, injection timing, engine speed, restricted air cleaner, etc.

  2. Disconnect the refrigeration unit from the generator and check the output voltage at the plug. Voltage between the three phases should be between 230 to 250 Vac or 400 to 500 Vac depending on engine speed and whether the alternator stator is wired for 230 or 460 Volt operation. All three phases should be within 3% of each other. If the voltages appear normal, make sure the refrigeration unit is not at fault. Reconnect refrigeration unit and run in Cool mode. Check the amperage draw with an induction type ammeter (amprobe), and compare it with the load plate on the refrigeration unit.

  3. Check all push-in plugs on control circuits for loose pins or sockets. Make sure all wire terminals are tight. Be sure J6 connector is plugged in to controller, if disconnected can cause a Message 122.

Test Instruments

If the preceding checks did not uncover the cause of the malfunction, more extensive diagnosis procedures will be required. The following tests will require various electrical test instruments, and the technician performing the tests should have a good working knowledge of their basic electrical principles.

The tests are intended to determine whether the source of difficulty lies in the generator itself or in the excitation control system. Following the procedures carefully will, in many cases, avoid unnecessary dismantling and reassembly of the generator when easily corrected problems may exist in the external circuitry.

The test instruments required:

  1. AC-DC voltmeter 2.5 Volts to 500 V ranges (± 2% max. error)

  2. AC induction ammeter (amprobe)

  3. DC ammeter (preferably induction type TK No. 204-947)

  4. Ohmmeter

  5. Megohmmeter (Megger®)

Alternator Troubleshooting

    Warning
  • Risk of Injury
  • WARNING: When servicing or repairing a generator set, the possibility of serious or even fatal injury from electrical shock exists. Extreme care must be used when working with an operating generator set. Lethal voltage potentials can exist at the unit power cord, inside the exciter control box, inside any high voltage junction box and within the wiring harnesses.

Normal alternator output voltage is 460 +/- 10 VAC with engine rpm 1800 +/- 25 rpm and no load applied. If the generator produces no or low voltage output at the plug, perform the tests listed below to identify the component that may be causing a generator malfunction.

  • Symptom: Low Output Voltage—0 to 100 Vac

Note The DSR has a glass fuse (5AF 250 Volt) on the board. Check fuse - if this fuse is blown, replace it and check output voltage.
Note Using a flashlight visually inspect exciter rotor for signs of being burnt, if burnt replace alternator.

Test 1 - Determine if problem is with the DSR or the Alternator

During the 2 minute delayed output, perform the following steps:

  1. Turn unit OFF.

  2. Open the junction box on the alternator and disconnect the Blue and Yellow wires from the DSR pins 1 and 2. Connect jumper wire from 12 VDC positive post of unit battery to the Yellow wire. Connect another jumper wire to the Blue wire.

    Note Do not connect blue wire to ground yet.

  3. Connect an AC volt meter to the output terminals U1 and V1.

    • Start genset - engine will be in low speed.

  4. Momentarily connect the jumper wire from the Blue wire to negative post of battery and monitor the output voltage. Output voltage should be >400 VAC.

    Note If output voltage is not >400 VAC go to Test 2

  5. If the output is >400 VAC,

    1. Stop genset and check the resistance on the Quad winding.

    2. Disconnect the Red wire on Pin 3 and Red wire to DSR wire,

    3. Check resistance between the Red wires, should be 1.6 ohms.

    4. If resistance is correct go to step 6.

  6. Check the Quad relay circuit. Connect the ohm meter to the DSR wire and Quad wire. Turn genset on and go to Commands/Manual Function Test, select Quad relay test. Observe if ohm meter changes when relay turns ON and OFF. If relay is working replace DSR.

Test 2 - Alternator Exciter Field Testing

  1. Disconnect the Blue and Yellow wires from pin 1 and 2 on the DSR field wire. Measure the resistance of the field circuit (Blue to Yellow). The standard value is 9.7 (±10%) ohms at 77 F (25C). If field is open or resistance value is low replace exciter field.

  2. Measure Blue or yellow wires to GND for possible shorted to ground coils. Megger blue to stator case to check for insulation break down, @ 500 Volts must be more than 1 M ohm. If exciter field is OK, go to test 3.

Test 3 - Alternator Stator Testing

Test 3 covers testing the main alternator stator, the rotating rectifier, the rotating exciter field, and the rotating field armature.

  1. Main alternator stator windings

    1. Disconnect the stator leads from the terminal board and neutral stud in the terminal box. Check for continuity between the following pairs. 1-2, 3-4, 5-6, 7-8, 9-10, 11-12. The resistance between any of the pairs should be 0.239 (±10%) Ohms at 25 C (77 F).

    2. Check resistance between the pairs of coils (example: 1-3, 1-5, 1-7, 1-9, 1-11, then 3-5, 3-7, etc...). If any continuity or resistance is found, there is a short between pairs (leg to leg) and the alternator needs to be replaced.

    3. Using a megger meter, check for insulation break down between each pair (1 to gnd, 3 to gnd, 5 to gnd, etc...)of leads to the stator case. @ 500 Volts must be more than 1 M Ohms. If less than 1 M Ohms, replace alternator.

      Alternator Stator

      1–12 Disconnect all 12 stator leads to test the stator.

Test 4- Exciter and Diode Test

Test 4 covers testing the exciter and diodes. Each plate has one positive and one negative diode mounted to it. Disconnect the main alternator field leads to test the diodes.

  1. Remove rear grille from the alternator.

  2. Disconnect wires from 1, 2, and 3.

  3. Measure the resistance between 1 and 1. The main field resistance should be 1.3 Ohms +/- 10%.

  4. Use a megger meter to check for a break in the insulation from 1 to rotor case.

  5. Measure the resistance between 2 to 2, 2 to 2, and 2 to 2. The rotating field and exciter armature resistance should be0.420 Ohms +/- 10%.

    Exciter Rotor Components

    1. Main Field
    2. DC Voltage
    3. Exciter Rotor
    4. AC Voltage

  6. Unbolt the main field armature leads. Along with the main field armature leads are leads to a MOV. The MOV is a high voltage suppression device. When measured it will show open, it closes with peak voltage more than 600V cannot test. Check for continuity between the armature leads. The resistance between the leads should be 1.33 (±10%) ohms at 25 C (77 F).

    1. Using a megger meter, check for insulation break down between leads to the rotor shaft. @ 500 Volts must be more than 1 M ohm.

  7. Test rectifier. Each plate as one positive and one negative diode mounted to it. Use a volt /ohm meter in the diode test setting to check each diode in the forward and reverse direction. A good diode will have a high resistance reading in one direction and no reading when ohmmeter leads are reversed. Replace if a short is found or the diode flows in both directions.