13.2kv motor coil inspection û missing carbon

[electricpete]

Attached is boroscopic picture taken looking into the vent duct area of a 13.2kv motor during teardown inspection following 20 years of service. The motor passed a dc step voltage test to 32kvdc with no non-linearity.

The motor has Thermalastic insulation (wound by Westinghouse in 1980’s).
The corona suppression system in the slot is provided by tape (polyester glass tape impregnated with carbon).
A red surface coating is applied over all.

The photo shows apparent white residue from partial discharge.
The red outer coating is missing.
What is visible appears to be glass type tape, but there is no hint of carbon black material.

That is the real puzzle... how can there be no carbon showing in this photo?


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(2B)+(2B)' ?

 

[Motorwinder]

Seems the red insulating paint would hinder the semi-conducting paint. Never seen red insulating paint over the conductive material. Are you going to rewind this? Or try to apply more semi-conducting paint over it.

 

[electricpete]

I think the red paint/coating is standard for Westinghouse and possibly other vendors. I don’t think it intereferes with the semicon. For one thing, it is applied after the motor is wound, so it won’t get into the area between the coil and the slot wall (where the semi-con function is most critical). Even just outside the slot, the red coating sits “on top” (outide) of the stress coatings, so shouldn’t interfere with the semicon extending the ground potential out to the beginning of the grading. There is certainly no credit taken for the red coating as an insulation.

We decided to rewind this motor, even before it got to the shop based on a variety of factors unrelated to the as-found condition of this particular motor (criticality of the application, performance of similar vintage/style motors, very long time until next availability). I am still very interested to understand what the heck is going on with this indication.

Another observation from the same motor was: looking at where the coils come out of the core, you can see white powder at the interface between the coil and the core. It seems the semicon is not present or not effective at that location also. I’ll try to get a photo.

The possibilities would seem to be:
A – no semicon (carbon) was ever applied
B – there is a semi-con layer under the armour
C – the semicon came off somehow, possibly because it was not correctly applied to begin with.

They all seem far out to me. C seems least far out among them.
What do you guys think?


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(2B)+(2B)' ?

 

[Motorwinder]

The white powder certainly looks like ozone damage. If all it had was painted coils and iron with semi-conducting varnish, (instead of semi-conducting tape) it could have worn off from coil movement. Make sure your rewind shop uses good high voltage coils with good semi-con filler material and wedges.

Did your shop do a Doble test?

 

[edison123]

Directly painting over end winding stress grading (SG) paint kills it (expensive personal lesson). After the SG paint is applied, allow it to dry (even slight warming of the area is recommended) and use sealing tapes over that SG area. Then paint the end winding.

Due to hassles with the paints, I have switched over to tape based systems for both slot and end winding more than 10 years back. Not a single issue so far. The additional cost of tape based system is worth the money.

Muthu

http://www.edison.co.in