Polarisation Index of Electrical Motor 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
Therefore if you know the polarisation index of a motor or generator you can determine if the machine requires maintenance, or indeed, if it is fit for further use.
Polarisation Index Formula
To calculate PI after carrying out an insulation resistance test (IR test), use the following Polarisation Index formula;
PI = R10 / R1
where
R10 is the insulation resistance after 10 minutes, and
R1 is insulation resistance after 1 minute
Interpretation of Polarisation Index Results
Once polarisation testing is complete, and you have calculated a PI Index, you need to know what is a good polarisation index number, what is an acceptable polarisation index, and what the recommended minimum polarization index for an ac motor is?
The recommended minimum value of polarisation index for form wound or random wound ac and dc motors and generators is widely accepted as 2.0. So any PI number above 2 can be considered acceptable. If, on the other hand, your machine has an polarisation index of less than 2, then it is worth considering changing out.
A PI valuse of between 2.0 and 4.0 would be considered good.
A PI value greater than 4.0 would be considered excellent.
IEEE Std. 43
The original Institute of Electrical and Electronics Engineers (IEEE) Standard No. 43 covered the procedure for determining Polarisation Index (PI) in detail. This original internationally recognised standard has been superceded a few times, and at time of writing St No. 43-2013 (IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery) is the latest version. This standard 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. 43-2013 provides, in detail, a procedure for determining the polarization index of machine windings. However, the following provides a summarised guide to carrying out a polarization index test :
Caution: Before proceeding with a PI 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, i.e. perform an IR test
- Completely discharge the windings to the grounded machine frame
- Reapply the voltage for 1 minute
- Measure the insulation resistance
- Calculate the polarisation index using the polarisation index formula discussed above, i.e. divide the result of the 10 minute IR test by the result of the 1 minute insulation resistance test.
A full polarisation index testing procedure can be found in the latest edition of IEEE 43-2013. IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery.
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