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what is PI value and its significance for insulation
testing?
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Answer Posted / biju9cr
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.
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