Polarization Index (PI) testing is performed to determine four things in any motor:
- Moisture in motor
- Gradual insulation deterioration
- Motor fitness
- Operation suitability.
This test is quite effective and is considered as one of the best for judging motor performance. Polarization Index is defined as the ratio of 10 minute to 1 minute insulation resistance values of windings, which is measured regularly for each reading. The machines whose windings have higher index are considered best for this test. The value of minimum polarizing index is 2.0 for induction motors.
Basically, a motor’s life generally depends upon the insulation conditions in the windings. There are two tests that are widely accepted to check insulation conditions: Insulation Resistance (IR) and Polar Index Tests.
The Polarization Index testing procedure, which PI testing is the extension of IR test facilities. Basically, IR test is performed to evaluate insulator resistance of conductors to that of the stator or rotor. The resultant resistance should have a higher value as a lower value shows the signs of motor deterioration. So, PI testing is done by varying the IR test. All readings for Polarization Index test are noted. If the values are very high or very low, then it indicates a problem in the machine’s insulator.
There are simple steps that are involved in the principle of polarization index testing:
- Remove all the connections (including external )
- Discharge windings to the grounded value
- Apply a very high DC voltage between the conductor and stator in the windings
- Measure the resistance by a Megohmmeter
- Readings should be taken at 1 minute and 10 minute to calculate the final value
- After getting the readings, turn off the power supply
- Calculate the final resistance by the formula of PI test
- Test duration should be bounded within 10 minutes
One important factor to remember is to always note winding temperature. This is essential because this test is done in single, as well as three-phase, induction motors. Thus, windings are to be kept grounded according to the testing procedure. The Polarization Index test is done to check the working conditions of insulators in any motor. The durability of motor is partially due to winding insulation and promises seamless motor functioning over their predefined lifespan.
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