By Scott Wolfenden, Updated March 24, 2022
The Polarization Index (PI) evaluates the suitability of motors and generators by measuring the insulation resistance of their windings. A higher PI indicates cleaner, drier insulation and lower risk of electrical failure, making it a vital safety check for electrical equipment.
Before measuring, disconnect all machine connections and discharge the windings to the grounded frame. Use a direct‑current megohmmeter and apply either 500 V or 1,000 V DC between the winding and ground, depending on the machine’s rating.
Maintain the test voltage for 10 minutes. Record insulation resistance after 1 minute and again at 10 minutes. The PI is the ratio of the 10‑minute reading to the 1‑minute reading. No additional readings are required.
Both AC and DC motors and generators should achieve a PI of at least 2.0. Values below this threshold suggest the machine may need cleaning, refurbishment, or replacement.
The test exploits the fact that impurities in a winding act as charge carriers, causing current leakage. Over time, these impurities become polarized, reducing leakage and raising the PI. The index reflects the amount of contamination in the insulation and does not vary with temperature, although high‑temperature conditions can affect test accuracy.
