Synchronous motor field voltage drop test

[beedubyuh]

We are performing an AC voltage drop test across the field windings of a synchronous motor. The windings are two parallel circuits fed from the same external DC source (through collector rings and brushes). The rotor is composed of 14 salient poles (7 poles per circuit). During the test the rotor is installed in the motor. The expected result when applying 120 Vac, 60 Hz to an individual field circuit is 17.1 Vac drop across each coil (+/- 10%). There are no reports of excessive vibrations when the motor is operating which would further indicate shorted field windings.

Actual voltage drops vary greatly from average:
Circuit 1 (coils 1,2, 5, 6, 9, 10, 13): 14.3, 10.1, 6.4, 24.3, 8.4, 14.8, 24
Circuit 2 (coils 3, 4, 7, 8, 11, 12, 14): 19.6, 29.2, 6.1, 7.4, 27.5, 32.5, 6.5

Could the position of the rotor poles in relation to the stator iron affect individual pole voltage drops (maybe due to inconsistent reluctance paths)? Or is this actually an indication of a failing rotor/shorted field windings?

 

[Tony S]

The position of the rotor will affect continuity readings. I don’t know the theory but I found out about the rotor position through bitter experience.

Just a thought, are you testing the two circuits individually? Do you allow enough time? Adjacent coil magnetic historicise may mess things up if you just “blip” the CR test button.

 

[beedubyuh]

The position of the rotor will affect continuity readings. I don’t know the theory but I found out about the rotor position through bitter experience.

Just a thought, are you testing the two circuits individually? Do you allow enough time? Adjacent coil magnetic historicise may mess things up if you just “blip” the CR test button.

Yes we tested the two circuits individually. The first circuit tested included the coils marked 1, 2, 5, 6, 9, 10, and 13. Then the other circuit was tested (3, 4, 7, 8, 11, 12, 14).

We spent about 2 hours perfoming this test. 120 Vac was appleid for extended periods and was not momentarily blipped. The motor was deenergized for at least 24 hours prior to the test.