insulation resistance of motor windings

[fg45]

In carrying out insulation resistance tests on three phase motors, is it possible for the resistance to earth to increase as well as decrease over time? Although over sufficient time it will decrease as the insulation breaks down.

 

[ScottyUK]

Yes. Moisture, ambient temperature, and other factors all have a contributory effect.

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[raok]

Insulation resistance to earth does not increase with time usually.
Any degradation, either by time, environment etc will cause the insulation properties to reduce.

 

[electricpete]

I'm with Scotty. Humidity can be a big factor. Temperature can be a factor (and temperature correction is not exact). You can expect readings to bounce up and down.

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[Zogzog]

Are you asking if the measured resistance can increase during the insulation resistance test?? If so yes, and that is expected as the excitation and absorbtion currents decay.

 

[electricpete]

Good point - a different spin on "over time". I certainly agree with zogzog if "over time" refers to the duration of a 10 minute polarization index test.

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