Low Polarization Index on a 4.16 kV Motor After Heaters Were Off
A 4.16 kV, 2000 hp form-wound motor is on a scheduled outage. The winding temperature is 40 °C and the ambient humidity is 78%. A 1 kV insulation tester is used for a polarization index test. The 1-minute insulation resistance is 600 MΩ; the 10-minute value is 750 MΩ. Last year, after the same cleaning procedure, the 1-minute value was 1.1 GΩ and the PI was 2.3. The motor has not run for three weeks, and the space heaters were off because the breaker feeding them was locked out for panel work. The motor is dirty near the bottom end-winding and there is visible moisture on the frame. The maintenance planner wants to close the breaker and run the motor because the absolute IR is well above the minimum. The relay protection is set correctly, and the motor starts across the line.
1. Calculate the polarization index and assess whether the planner’s conclusion is justified.
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The PI is the 10-minute resistance divided by the 1-minute resistance: 750 MΩ / 600 MΩ = 1.25. The absolute IR of 600 MΩ is high, but the PI is low. Commonly applied IEEE 43 guidance treats a PI of 2.0 or higher as acceptable, below 2.0 as questionable, and below about 1.5 as usually unacceptable for form-wound insulation. A value of 1.25 therefore cannot be signed off as normal.
The planner is relying on absolute IR alone. That is not enough. The low PI trend, combined with visible moisture, dirty end-winding, high humidity, and space heaters that were off for three weeks, points to moisture or contamination rather than immediate proof of a failed winding. The motor should not be started until it is cleaned and dried and the test is repeated.
The previous PI of 2.3 is a useful baseline. A drop from 2.3 to 1.25 on the same motor is a meaningful change even though the absolute IR remains above the minimum.
2. What follow-up tests and actions should be completed before returning the motor to service?
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First confirm the test setup. Check the insulation tester, leads, guard connection, and winding temperature. The winding should be at a stable temperature, and the test should be repeated with the same voltage and duration used previously. Verify that the space heaters are restored and that the motor is dry.
Clean the dirty end-winding and dry the motor with approved heaters or warm air. Then repeat the PI test. A dielectric absorption ratio can provide supporting information if the PI remains marginal. If the PI recovers above 2.0 after drying, the likely cause was moisture and the motor can be considered for return to service with a trend record.
If the PI remains below 1.5 after cleaning and drying, do not close the breaker based only on high absolute IR. Investigate further, including winding condition, contamination, and possible insulation degradation, and involve engineering. Document the initial and repeat values, temperatures, humidity, and corrective actions so the next outage has a valid baseline.