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Determining the Polarization Index of Machine Windings

Knowing the polarization Index of a motor or generator can be useful in appraising the fitness of the machine for service. The index is calculated from measurements of the winding insulation resistance.

Before measuring the insulation resistance, remove all external connections to the machine and completely discharge the windings to the grounded machine frame.

Proceed by applying either 500 or 1000 volts dc between the winding and ground using a direct-indicating, power-driven megohmmeter. For machines rated 500 volts and over, the higher value is used. The voltage is applied for 10 minutes and kept constant for the duration of the test.

The polarization index is calculated as the ratio of the 10-minute to the 1-minute value of the insulation resistance, measured consecutively.

Polarization Index =

Resistance after 10 minutes
Resistance after 1 minute

The recommended minimum value of polarization index for ac and dc motors and generators is 2.0. Machines having windings with a lower index are less likely to be suited for operation.

The polarization index is useful in evaluating windings for:

  • Buildup of dirt or moisture.
  • Gradual deterioration of the insulation (by comparing results of tests made earlier on the same machine).
  • Fitness for overpotential tests.
  • Suitability for operation. 

The procedure for determining the polarization index is covered in detail by IEEE Standard No. 43.

CAUTION: Before proceeding with this test, the winding must be discharged against the frame.

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