Parameter for motor should be rewinding

[jonos]

Dear All,

Is there any parameter if motor should be rewinding?

Is insulation resistance only can judge motor should be rewinding?

Your prompt reply will be highly appreciated.

Regards,
jonos

 

[icrman]

If the windings are burned and shorted.
If there is a voltage change.
Insulation has a very high resistance.

 

[jonos]

Is there any reference for that, Sir?

 

[stardelta]

Jonos?
We will need more information to give you a specific and accurate answer. Is the motor in question AC or DC and what, if any faults is it demonstrating?

 

[jonos]

Stardelta,

That motor is AC induction motor with 380 V, 68 kW, 3phase, 2 pole. There was any fault but insulation resistance problem only. Insulation resistance between U phase and V phase was about 0.541 Megaohm. The others were good. Is that condition can judge motor should be rewinding?
Is there any test should be done beside insulation resistance measurement? Is there any refference for that?

 

[jonos]

Dear all, I am sorry, I mean motor was not any fault but insulation resistance was low only.

thank you,
Jonos

 

[Addy71]

Hi Jonos,

If the IR value is low, just dry out the motor & re-test. I doubt if you a rewind for this purpose.

 

[stardelta]

Hello Jonos.
The figure you quote for phase to phase insulation resistance is very low and makes me suspect that the motor has suffered, or is in immediate danger of suffering from a phase to phase short. Particularly as you say the other 2 readings are good. We specify a minimum figure of 10megs for IR, both to earth and between phases. Any lower and we will advise our customers on a full rewind. There are 3 other static electrical tests you can perform although you will require specalist equipment that usualy only well equiped repair shops have. Phase resistance, measured with a low value ohmeter or a ductor, should be balanced across the phases. A high voltage flash test to earth and between phases, although I would not reccomend this on account of the low IR as it will almost certainly breakdown what little resistance remains. Finaly surge testing which will identify any breakdown in the isulation even a single turn to turn short.

 

[stardelta]

Further to the comments made by Addy71...
It is true to say that Insulation resistance can be affected by moisture, oil and product contamination etc and that it can be restored to an acceptable level by careful cleaning and controlled stoving of the winding. However if moisture was the cause of Jonos problem I would expect a low and similar IR figure across all three of the phase to phase paths and also a low reading to earth as the whole winding would be affected. Jonos has stated that only one path is low (U to V) and that the other two are good which suggests the problem is localised, also the winding to earth IR is unaffected. Its far more probable that a failure has occured to one of the phase barriers in the winding resulting in a phase to phase short.
The EASA website features a very good series of pics showing typical winding failures of 3ph stators including the phase to phase short condition I am describing.

http://www.easa.com

 

[electricpete]

I agree with everything said but I wanted to emphasize the points made by Addy71 it is certainly very possible that the situation can be resolved by cleaning and drying. At 0.5 megaohms I would not call it a phase to phase short. If it is a non-critical machine, you might just run it and see what happens. There machine might run for 30 years without any repair of this condition.

Also, another quick fix to think about if you're not happy with the insulation resistance and drying didn't work is dip and bake. But that can also create overheating problems and leaves a lot to be desired for random wound machines.

There are some other checks that can be done. Winding resistance test (not insulation resistance). Check for balance. Also surge test. Possibly DC hi-pot. Last two are potentially destructive.

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[jonos]

Dear All,

Thank you very much for your opinion. I'll try to run that motor based on Electricpete explanation and try to drying it.

Any way, thank you for all

Regards,
Jonos