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Understanding Overpotential Testing

Any rotating electric machine undergoes final testing at the factory to show that it is in good working order. Part of that involves brief application of a suitably high a-c voltage between machine winding and frame. The NEMA standard for that "overpotential" test is twice rated voltage plus 1000 volts, applied for one minute. Too high a test voltage risks insulation damage, whereas too low a voltage will not expose insulation weakness.

At successive stages of production, individual coils or groups of coils are given in-process tests. Some manufacturers test each coil; others use a sampling method. The purpose is to search out insulation defects when repair is far simpler and less costly than on a completed winding. Initially at a high level, each succeeding test is at a lower voltage, because repeated stress contributes to eventual fatigue failure. Such tests are usually "momentary," lasting from 1 to 15 seconds (literature describing in-process testing seldom defines the time involved).

Accessory circuits built into the machine will also undergo overpotential tests. Because voltage ratings are typically much lower, test voltage is correspondingly reduced, For temperature sensors or other devices lacking a specified operating voltage, NEMA standards prescribe a rating of 50 volts.

Authorities advise against repeating the one-minute final test once a machine is in service. Any overpotential testing then should use a voltage reduced by one-third.

Although standards mandate a-c for the final factory test, any overpotential test is possible using d-c voltage at 1.7 times the a-c value. Reduced leakage current permits a smaller, lighter test power supply.

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