What are good insulation test proceedures for motors?

The rule of thumb was 1 Megohm plus 1 megohm per thousand volts...acceptable limit from this rule would be 1.48 Meg for a 480V motor.

Fred

Thanks for the Reply. Seems we have had many motors in service with much lower readings than that. Guess it is just a matter of time.

Remove dashes "-----------" to e-mail.

snipped-for-privacy@shaw.ca

We often use two different numbers/rules. One for new or newly refurbished equipment, and one for installed equipment. For 600V and under, 1Mohm for new/refurbished equipment. For higher voltages, we use the voltage rating in kV converted to Mohms (i.e. a 4.16 KV motor would require a minimum of

4.16 Mohms). This usually is not much of a problem since rebuilt mid-sized motors can easily be >200 Mohms. Very large machinery (up in the >1000 hp range), it gets a bit problematic, but still achievable.

For installed equipment, we use 1/2 the new/refurbished number. So in your case, we would *consider* taking corrective action below 500Kohms.

But be sure of your readings. Take the readings right at the motor terminals with the power leads disconnected to avoid cable resistance from interfering. It's a b*tch to have a motor rewound and reinstalled, only to find out the problem was in the supply leads all along.

While 50K isn't about to start a fire or anything, it's a bit low. But if you've been tracking this for several years and it shows no signs of degradation, I'd consider just leaving it be. A PM program that tracks the insulation resistance and monitors a slowly degrading trend would indicate action is needed before failure. Or a sudden drop from one test to the next could indicate a sudden change in condition. But a steady reading for years may be ok and normal. You don't mention how old the unit is, or what type of insulation it has, so that's about as much as I can say.

daestrom