How to Test

Routine winding field tests are valuable for identifying signs of deteriorating insulation, since timely repair or replacement of a troublesome motor helps avoid costly disruptions in productivity, regular field testing is invaluable. This is especially true for plants running applications within harsh operating environments or with a history of winding failures.

When maintenance records reveal a tendency toward periodic winding failures, the condition of system insulation can be efficiently probed by way of a resistance test. Such testing is especially important when a motor's surroundings are wet, potentially corrosive, or hot.

The most simply executed field test, the ground-insulation or megger test, helps prevent a large proportion of common winding failures. Applying 500 to 1,000 Volts of DC power to the motor windings, the megger calculates the resistance of the insulation. NEMA standards require minimum resistance to ground at 40 degrees C ambient of 1 Megaohm per rated kV, plus 1 Megaohm. Medium size motors in good condition generally have megohmmeter readings in excess of 50 Megaohms. Low readings may indicate seriously impaired insulation. This is most often due to moisture, oil or conductive dirt contamination, or deterioration due to age or excessive heat. The megger test indicates the condition of ground insulation. It does not, however, measure the turn-to-turn state of the insulation, and may not register localized weaknesses. Moreover, voltage peaks may stress the insulation more severely than megger voltage. For example, the DC output of a 500 Volt megger is below the normal 625 Volt peak each half cycle of an AC motor operating on a 440 Volt system. Experience and conditions may indicate the need for more intensive routine testing. The AC high potential ground test functions to verify the existence of a safety margin above operating voltage. The test involves application of a high AC voltage, typically 65%, times twice the operating voltage, plus 1,000 Volts, to the windings and motor frame. Although this test effectively detects the symptoms of poor insulation, the high voltage may arc to ground, actually burning the insulation and frame, causing failure during the test. DC high potential tests are becoming more popular than AC tests, because the equipment involved is smaller and the test current is less dangerous.

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