6.3kV motor winding failure in less than 1 year after re-winding

[hlrl]

Hello to electrical man,
I have a 1.2MW (6.3kV) motor which was re-wound last year (Oct05).
The winding was shorted again this year (Aug06) in less than ONE year after re-wound.

The shorted location is just a little dot near the winding overhung.

This motor did not show sign of overheating and it was not operating above its full load ampere.

Anyone has similar experience... please share... Thank U.

Cheers...

William

 

[jraef]

If the "overhung" is in the first turn, you may have a reflected wave problem. Is this motor on a VFD by any chance?

Even if not you could still be getting voltage spikes on your motor leads from some other source. There have been several discussions in here about vacuum contactor re-strike transients that may be worth your reading. Do a Keyword Search. Here is one of the old threads, there were others if you care to look hard enough.
thread237-42676

JRaef.com
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[hlrl]

Hi jraef,
Thank U for your respond.

a. This motor is not operating on VFD. It starts on direct on-line starting.

b. We are using vacuum combination starter unit. The contactor rating is 400A whereas our FLA is 181A. Running ampere is approx. 97% (175A).

c. When you mentioned re-striking transient on CBS, do you mean during starting condition (contactor clsing)?

 

[Skogsgurra]

The vacuum contactor says it all. They are known to do exactly this. Especially if you have a long cable.

Gunnar Englund

http://www.gke.org

 

[jraef]

Yes. Even vacuum contactors can "bounce" when they close, and in doing so they can create high voltage spikes because they tend to act like a capacitor every time they open. Then when they re-close, the potential difference is magnified and if they bounce again and again, it gets very high. Read that thread I linked to, that is discussed starting somewhere in the middle. Most vacuum contactor manufacturers will provide white papers on this issue if you ask as well. I know Toshiba will.

JRaef.com
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[hlrl]

Hi,
I have checked with the vendor for my vacuum contactor.
Vacuum contactor since the 80s used low surge material on the contactor surface which basicially creates very low voltage surge that is very popular for motor starting/switching.

With such low surge, this type of vacuum contactor doesn't require any surge protection (as in C-R arrestor or surge suppressor) for motor application even with frequent switching operation.

 

[jraef]

That's what HE says! Remember, he has a vested interest in your believing this; he does not want you to perceive that his contactor may have damaged your new motor winding. Post the vacuum contactor mfr.'s name and model here and see what kind of opinions you get.

Always be skeptical of a vendor's engineering advice. Remember who signs their paychecks!

JRaef.com
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[dpc]

rlyeo,

Even if the vendor is correct, a surge arrester/surge capacitor combo **at the motor terminals** is pretty cheap insurance. IEEE recommends surge protection at the motor for all medium-voltage motors, regardless of vacuum/air/whatever contactor upstream.

 

[aolalde]

The quality of the coil manufacture, winding assembly and resin impregnation are another factor. Was the winding resin VPI?
After rewinding, what type of acceptance tests were performed?
Air voids in critical areas could develop Partial Discharges and accelerated aging of the insulation.

 

[hlrl]

Hi,
We have nearly one thousand 6.3kV motors. Nearly 95% of these motors use vacuum contactor. Most of the vacuum contactors come from the same vendor.
This is the first time, we have motor winding that fails in less than one year after re-winding.

In the past, we do have winding failure, but is largely due to bearing failure which rubbed the winding or due to overloading.

Anyone has the IEEE standard number for this requirement to provide surge protection for all MV motors??

The re-winding vendor has used this motor coil manufacturer for the past 5years without any problem.
The new coils have been subjected to high voltage test starting from factory to the re-winding process. However, all these tests are considered as "GO or NO GO" test. It may not be able to detect minor "injury" to the winding in the process.

 

[jraef]

This is not an IEEE paper, but it does offer a good explanation of the phenomenon, even though it also appears to favor one particular solution.

http://www.accesstd.com.au/report.htm

JRaef.com
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[dpc]

See IEEE Buff Book (Std 242). Motor Protection chapter recommends surge protection for all medium-voltage motors rated 500 hp or larger.