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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.
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Polarization
Index =
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Resistance
after 10 minutes
Resistance after 1 minute
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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.
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The procedure for determining the
polarization index is covered in detail by IEEE Standard No. 43.
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CAUTION:
Before proceeding with this
test, the winding must be discharged against the frame.
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