How to do a proper insulation test on an electric motor?

[antisa555]

I want to check the insulation of the 3-phase 400VAC asynchronous squirrel cage motor.

Motor's stator windings are connected in delta.

I've read in some textbooks it is necessary to open motor's junction box and disconnect terminals and remove jumper bars (which are used to establish delta or star connection) and only then perform insulation test between:
a) each winding and earth and
b) each of the windings.

I think this might not be necessary and that it is enough to test the insulation from the motor's contactor output between any of the phases and earth.

This will let us know if there is an insulation problem between windings and earth.

However, testing this way doesn't allow us to measure the insulation resistance between each of the windings.

Questions:

1. How likely is it that there is a low insulation reading between windings and that this reading will not manifest itself when performing insulation test on the motor's contactor output between any of the phases and earth?

2. I am using Megger MIT330 and testing 3-phase asynchronous squirrel cage motor which operates at 400VAC.
Which voltage should I apply when testing and for how long should the voltage be applied?

3. If I apply 1000V for 20s on motor's contactor output between any of the phases and earth, I notice an increase in resistance from 50MΩ up to 250MΩ during the first 20 seconds and then it remains steady around 250MΩ. Why does this happen?

4. What is the acceptable insulation resistance when testing resistance on motor's contactor output between any of the phases and earth for the 400VAC motor?

5. Why is this considered acceptable insulation resistance?

 

[don_resqcapt19]

The worst case would be testing all of the windings at the same time since all insulation has some leakage. I would test it all at once. If you have a low reading, it really doesn't matter what winding it is on, the motor needs to be repaired.
The field megger test is normally twice the operating voltage.
The increase in resistance over time is a polarization index test, but is normally done for 10 minutes.

 

[ptonsparky]

I have had motor test a shorted winding but not show a fault to ground. It was on a VFD so the motor was likely shut down before a fault to ground developed.

If you're doing pass/fail testing take them free of the starter and check all at once.

 

[Joethemechanic]

If like a lot of motors they only bring three leads out for single voltage, that is really all you can do anyway.

 

[antisa555]

Thank you, guys.

 

[garbo]

They recommend that the Megger voltage be double the motor voltage plus a hundred volts so you should use a Megger that puts out 1,000 bolts DC. For years we only had a hand crank 500 bolt Megger that had no problems picking up grounds on a 480 volt motor. Large campus that I retired from had over 500 VFD'S and while some drive techs will tell you minimum Megger readings should be higher than 5 megohm to ground found that to be a little low. Cannot remember having any ground fault trips if Megger readings was above 10 megohms. Danfoss called ground fault earth fault and most of the time they were false trips. Would reset an earth fault and checking the memory for last ten trips a lot of times they did trip out again in the next year. I have seen the reading change like you mentioned and as long as it's so high should not be a problem.