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.
Calculating the Polarization Index
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
