Epoxy losing its insulation?

[Skogsgurra]

A large synchronous motor had its rotor refurbished. The insulation was replaced with new material and epoxy resin was used to impregnate it. The insulation tested OK after curing. Around 100 Megohms.

When delivered and installed, an insulation test was made before energizing and the insulation was down to about 100 kohms or less. Hot air for a day and night didn't help. No extra circuitry present, only the open winding - still around 100 kohm, maybe less.

Don't know what epoxy was used, but these guys have done hundreds of rotors before and done so for at least fifty years. They know their thing. Anyone had this problem before?

Gunnar Englund

http://www.gke.org

 

[electricpete]

Shockwatch has a big line from simple go/no-go indicators to boxes that store time history in digital format.

Upon our request, our shop attached a similar product with the digital storage capability. Then we had a motor which vibrated badly upon installation after testing good at the shop. The immediate question was: "What did the recorder tell us?". It took a few days to get the recorder shipped to a place where it could finally be read and results reported. By then the troubleshooting had already progressed and the info was no longer needed. Moral of the story for us was to go with the simple go/no-go that you can read without any special equipment

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[Warpspeed]

This may be totally irrelevant, but I recently read a fairly old book (1985) on boat building.

In the evolution from marine ply construction to full moulded fiberglass hulls, one method tried was to impregnate a marine ply hull with epoxy, making a supposedly waterproof composite structure. It was quickly discovered that epoxy is naturally hygroscopic, it definitely does saturate and pass water, but at a very slow rate. So embedding something like wood or steel in epoxy is no guarantee of preventing rot or corrosion.

It may very well be that your windings need to be baked for a much longer time than you might have allowed to get a good result. All this is pure speculation on my part, but it may be worth some further investigation.

 

[itsmoked]

I would agree with that Warp. On fiberglass boats they often have a thin outer layer of special stuff which is more impervious to the water(though not perfect) which a motor wouldn't have.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

If they would have split the windings into sections they could have narrowed it down

If the insulation resistance remains uniformly low on all sections of the winding, we suspect moisture or some type of curing / off-gassing problem.

If the insulation resistance is much lower in one part of the winding but not others, then we suspect localized mechanical damage from handling or localized tracking damage caused by electrical test.

This is probably obvious to most of you guys but I just wanted to mention it.

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[motorspert]

Such a significant change suggests serious moisture ingress - or maybe something simple. Are there any clamps, v blocks etc that have been replaced - we have had instances where an insulation washer was not properly sealed - and it took in moisture just as you describe. In cases like this I always suspect the auxillaries, ie cable clamps, rectifier insulation, v-block supports etc.

 

[petronila]

Hi Skoggs,

The problem could come from : The Resin and the impregnation process or the slip rings.

Can you confirm the epoxy used? the required curing time & temperature? and the actual baking time & temperature? It might need to bake longer.
If the shop used one of the epoxies especially for wet-winding rotating poles (Sterling U300-20 or similar), some of those products require approximately 20 hours to fully cure. Some thixotrophic materials used for this purpose are semi-conductive until completely cured.

If so, you might be able to heat the coils using DC current.If the cure time and temperature seem correct, perhaps the low resistance is in the sliprings rather than the coils. If possible,disconnect the rotor leads from the sliprings, and determine whether the sliprings or rotor poles have the problem.

Best Regards,

Petronila

 

[oftenlost]

We are talking large synchronous here. That means either removable or starpunched poles. These are apt to be either edge wound or rectangular wire wound on the pole. If these are removable salient poles, they probably were not VPI processed but wet wound with something like sterling u300. if rectangular wire or B stage mica turn insulation with mica or nomex for pole insulation. Either way, the insulation appears to be defective in some fashion. I would isolate to make sure it is not in the brushrigging or rings, split the poles in the middle and remegger to see if it is a general problem or one pole, reisolate until the discrepant pole is found and then send it back for warranty repair

 

[esee135]

I'm with OFTENLOST, if it's a large salient pole, make sure your testing without the slip rings, brush gear etc. and start isolating poles. The dog house (slip ring housing) is notorious for field grounds. You should check the field bus to the slip rings, with new pole insulation the field bus should have been reinsulated as well. At worst start removing the horse-shoes, and doing individual Meggars.

Was the meggar done before after installation of the fan blades? I've noticed on quite a few of the machines I work with, the rotor fan blades sometimes get too close to the inner pole collars, sometimes sitting ontop off the pole collar. As another note, last I talked to a generator manufacturer, the outer pole collar isn't always necessary. Also check the rotor for excessive grinding materials on the poles, often the pole keys are cut off using a grinder flush to the rotor rim. A combination of grinding debris and 'tracking surfaces' will get you.

If you're concerned with the pole insulation, be sure to pole drop the rotor AC and DC after finding your field ground and check for equalized voltages across each pole to ensure the turns haven't been comprimised.

 

[itsmoked]

You guys realize this thread is 1/2 a year old?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Some questions are eternal, Keith. I am still open for ideas.

Gunnar Englund

http://www.gke.org

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[petronila]

Hello Skogs,
If they used one of the epoxies especially for wet-winding rotating poles (Sterling U300-20 or similar), some of those products require approximately 18 hours to fully cure. Some thixotrophic materials used for this purpose are semi-conductive until completely cured.Can you confirm the epoxy used, the required curing time & temperature,
and the actual baking time & temperature? It might need to bake longer.
If so, you might be able to heat the coils using DC current.
If the cure time and temperature seem correct, perhaps the low resistance is in the sliprings rather than the coils. If possible,disconnect the rotor leads from the sliprings, and determine whether the sliprings or rotor poles have the problem.

Regards,
Petronila

 

[Skogsgurra]

Thanks Petronila,

I checked with the winder today. He had bought a new epoxy ( could be the one you say) that needed more curing time than they had given it. The rotor is back and they are changing to the old insulation system. Another case of bad communication between producer and user. Rather high cost. I consider the case solved.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oftenlost]

There are recorded cases of partially cured resin which exhibit good megger readings at room temperature and then get wet and flow at slightly elevated temperature. Before I would replace the insulation, I would heat the rotor in a baking oven at 350 deg F for a period of 12 hours or more after the part gets up to temperature. Once this has been accomplished, I would cool to room temperature and with leads through the oven door, I would monitor and chart megger readings as the machine was heated back to 350 degrees. The labor to reinsulate these rotors on warranty is a big pill to swallow and I would invest a few hours in additional curing. It is little known that some of the accelerators utilized in polyester resins are actually deactivated by bare copper and need three to four times the curing time. Good luck. I have had to eat a big one due to this problem.

 

[Skogsgurra]

Yes oftenlost, it WAS a big pill to swallow. I am glad I didn't have to swallow it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...