The Polarization Index (PI) measures the health of a motor’s winding insulation. PI is the ratio of resistance measured one minute into a Megger test to the resistance measured 10 minutes into the test.
Insulation resistance is affected by the type of insulation, the age of the material, its surface area, ambient temperature, humidity, and the presence of moisture (condensation) or contamination on and on the motor windings. While individual insulation resistance measurements will vary based on ambient temperature and humidity, the motor’s PI remains a consistent metric of winding health. Tracking PI over time helps to evaluate the buildup of dirt or moisture on a motor’s windings; the deterioration of the insulation; the motor’s fitness for over-voltage operation (including its tolerance for surges and transients); and a motor’s suitability for operation.
For most motors constructed after 1970, a PI between 2 and 5 indicates good health. While higher or lower PI values may be acceptable for certain specialty motors (e.g., high-voltage [> 1kV] motors with vacuum pressure impregnated insulation), A PI below 2 indicates very dirty windings and/or moisture penetration in the windings.