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Polarization Index of Machine Windings

For many years, the polarisation index of an electric motor or generator has been regarded as a useful tool in evaluating its windings for:
- Build up of dirt or moisture
- Deterioration of the insulation (this is done by comparing results of tests made earlier on the same machine, at the same temperature)
- Fitness for over potential tests
- Suitability for further operation
The recommended minimum value of polarisation index for form wound or random wound ac and dc motors and generators is 2.0. Therefore if you know the polarisation index of a motor or generator you can determine if the machine is fit for further use. If your machine has an index of less than 2, then it is worth considering changing out.

IEEE Std. 43

IEEE Standard No. 43 covered the procedure for determining Polarisation Index (PI) in detail. This was superceded by IEEE St No. 43-2000 which addresses some new issues regarding modern insulation systems. In particular, it identifies the complexities of performing PI tests and notes their limitations. Notwithstanding this, polarisation index is still regarded as a useful tool by many.

How to Measure the Polarisation Index

IEEE Standard No. 43-2000 covers the procedure for determining Polarisation Index in detail. However, the following provides a summarised guide to carrying out a polarization index test :
Caution: Before proceeding with this test, the windings MUST be discharged against the frame.
- Remove all external connections to the machine and completely discharge the windings to the grounded machine frame
- For machines rated 500 volts and over, apply 1000 volts dc between the winding and ground using a direct indicating, power driven megohmmeter. (If your machine is rated less than 500 volts then apply 500 volts)
- Continue to apply this voltage for 10 minutes
- Measure the insulation resistance
- Completely discharge the windings to the grounded machine frame
- Reapply the voltage for 1 minute
- Measure the insulation resistance
- Calculate the polarisation index by dividing the 10 minute insulation resistance by the 1 minute insulation resistance.

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