Alternator: Testing and Inspection

FIELD WINDING SHORT CIRCUIT-TO-GROUND TEST

Perform this test using a suitable ohmmeter set to the X1000 scale or by using a 110 volt test lamp.
1. Touch one test lead probe to rotor shaft and the other probe to one slip ring.
2. Repeat procedure for the other slip ring.
3. In each test, the ohmmeter should indicate infinite resistance (no pointer movement) or the test lamp should remain OFF.
4. If ohmmeter indicates other than infinite or the test lamp lights, a short to ground exists.
5. Check soldered connections at the slip rings to ensure they are secure and not shorted to the rotor shaft. Check for excess solder which might be shorting the rotor winding to the shaft.
6. Replace rotor, if necessary.

FIELD WINDING OPEN CIRCUIT TEST

Perform this test using a suitable ohmmeter set to the X1000 scale or by using a 110 volt test lamp.
1. Touch one test lead probe to one slip ring and the other lead to the other slip ring.
2. Ohmmeter should indicate 3.2 ohms or the test lamp should light.
3. If resistance obtained is less then 2.7 ohms (at approximately 80°F), the windings are internally shorted. If the resistance exceeds 3.5 ohms, the windings have excessive resistance.
4. If the ohmmeter indicates infinite or the test lamp remains OFF, the rotor (field) windings has an open circuit. Replace rotor, as required.

FIELD WINDING INTERNAL SHORT CIRCUIT TEST

This test is performed using a 12 volt battery and a suitable ammeter.
1. Connect battery and ammeter in series with the slips rings.
2. The field current with 12 volts applied, should be approximately 3.75 amps.
3. Current flow exceeding 4.5 amps indicates an internally shorted windings. The winding resistance and ammeter indication will vary slightly with winding temperature changes. A current flow that is less than the specified value indicates excessive winding resistance, which would indicate that the rotor requires replacement.

STATOR WINDINGS SHORTED-TO-GROUND TEST

This test is performed with an ohmmeter set to the X1000 scale or with a 110 volt test lamp.
1. Touch one test lead probe to the bare metal surface of the stator core and the other test lead probe to the end of one stator winding.
2. Ohmmeter should indicate infinite (no pointer movement) or the test lamp should remain OFF.
3. If the ohmmeter indicates other than infinite or the test lamp lights, the stator windings have a short circuit to the core (ground) and requires replacement.

STATOR WINDINGS CONTINUITY TEST

This test is performed using an ohmmeter set to the X1 scale.
1. Unsolder stator winding terminals from the bridge rectifier.
2. Touch ohmmeter test probes to any two stator winding terminals and note the resistance.
3. Test all stator windings in the same manner.
4. Equal indications should be obtained for each pair of windings tested.
5. An infinite resistance (no pointer movement) indicates open windings.
6. Inspect neutral junction splice for an inadequate solder connection.
7. Resolder connection even if it appears to be electrically and mechanically satisfactory.
8. Test for winding continuity.
9. If an open circuit still exists, replace stator windings.
10. An indication of more than one ohm indicates a possible cold solder joint. Inspect the neutral junction splice and resolder if necessary.

STATOR WINDINGS INTERNAL SHORT CIRCUIT TEST

An internal short circuit (between adjacent windings) is difficult to locate without the use of special test equipment. If all other alternator test results are satisfactory and the alternator fails to supply the rated output, shorted windings are probable.