The Polarization Index (PI) is used to determine the fitness of a motor or generator for use. The index is derived from calculating measurement of winding (electrical) insulation resistance. The polarization index gives an indication of the buildup of dirt or moisture, the deterioration of insulation and the suitability for operation of the motor or generator. Testing for the Polarization Index is a safety measure for electrical equipment.

The reading that is significant to Polarization Index is the insulation resistance. Before measuring the resistance, you should remove all connections to the machine and discharge the windings (of electric wire) to the grounded machine frame. Using a direct-indicating, power-driven megohmeter (an electrical test instrument producing a DC voltage), an electrical current of either 500 or 1,000 volts DC should be applied between the winding and ground. The amount of current you use depends on the rating of the machine.

The voltage that is applied should be kept constant for 10 minutes. An initial reading of insulation resistance is recorded at one minute and a second reading is taken at 10 minutes. The resistance is measured consecutively and it is not necessary to take further readings beyond 10 minutes. The ratio between the 10-minute and the one-minute measurements provides the Polarization Index.

The recommended minimum value for the Polarization Index applies for both AC and DC motors and generators. The index should be at least 2.0. Machines for which the index is lower are less likely to be fit for use and need to be cleaned, rebuilt or discarded. Because the index is a ratio, no units are indicated.

The principle on which the Polarization Index test works is based on the idea that impurities in a winding act as charge carriers and cause current leakage. When the insulation is tested, current leaks. These impurities can be polarized over time. The index provides indication of the amount of impurities in the winding and the cleanliness of it. There is no relationship between the index and temperature, although there are some limitations when attempting to test the index in high temperatures.