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Epoxy losing its insulation?

Epoxy losing its insulation?

Epoxy losing its insulation?

(OP)
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
www.gke.org

RE: Epoxy losing its insulation?

For the new, epoxy impregnated insulation I would expect much higher resistance value than 100 MOhms, as stated (if measured in cold state) - so it might suggest something wrong in insulation/impregantion process. Maybe the rotor's drying before impregnation was insufficient?

RE: Epoxy losing its insulation?

(OP)
Thanks for the answer.

I actually do not know what the resistance was. It was said to be more than 100 Mohms. It may have been much higher. These guys (one of the better known rewinders in this country) have done this kind of job over and over again and have never had a problem like this before.

Is it possible that insufficient initial drying causes a problem like this?

Gunnar Englund
www.gke.org

RE: Epoxy losing its insulation?

On my latest project (3 phase generator stators) we are seeing about 500 Megohms after VPI.  100 seems very low to me too for a brand new modern insulation system.

RE: Epoxy losing its insulation?

(OP)
The initial 100 Mohms are not the problem. The insulation was probably a lot better than that. I shouldn't have mentioned that level. Sorry.

The problem is the lower than 100 kohms measured on site and that it cannot be improved by drying the rotor out. Any experience from that kind of situation?

Gunnar Englund
www.gke.org

RE: Epoxy losing its insulation?

are you able to split the winding to see if one portion measures lower than the rest?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

That's quite an attrition rate. I think you need to consider a better system by which you encourage the Ohms to stay. Perhaps an increase in minimum wage, or better benefits?

Seriously though, you have apparently eliminated the possibility of hygroscopic intrusion, but what about temperature? Was the temperature of the material the same when both measurements were made? Maybe the epoxy has a negative coefficient of resistance and was cold when they first measured it.

Any chance of measurement error in the first instance, i.e. the probes weren't touching hard enough? I would never accuse you of such a thing, but it appears that you were not there for the initial measurement. winky smile

Other than that, what you are implying is the possibility that as it cured, the electrical properties of the compound changed. That would mean that it was the volatile components that provided the insulation values and as they evaporated in curing, the remainder is more conductive. Or possibly something crystallized in the process, I remember from (long ago) chem labs that some materials change their electrical properties upon crystallization.

Just guesses though. I only added this to lend legitimacy to my irresistible joke in the beginning!

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Epoxy losing its insulation?

Did the repair shop pull a sample of epoxy from the source tank?  

Some grades and types are hygroscopic while in the tank and required various heating and drying schemes to keep the tank uncontaminated.  There may also be some other source of contamination in the source resin tank for the VPI process; perhaps from a previous job

RE: Epoxy losing its insulation?

I certainly agree with all previous comments.

I would also keep an open mind to the possibilities of contamination or hidden mechanical damage that occurred after the first test (perhaps during shipment or testing).  

Some things to check if not already tried
- Separating the windings and recheck.
- double check selector switches and scales.
- repeat with different instrument.
- very proper test voltage was used.
- was a hi-pot applied at the field?  what voltage?  (tracking during hi-pot can create tracking path that will lower insulation resistance later check by megger)
- Were proper techniques for discharging winding after test used (slowly ramp down voltage and maintain at zero or discharge thru grounding resistor).  Shorting a charged winding directly to ground can create a traveling wave transient which can damage the insualtion.
- contact rotor rewinder for advice

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

I once saw a similar thing with a stator that had been rewound. After VPI the stator was placed in the curing oven WITH another stator that was in there to be dryed out after the winding had been steam-cleaned. It was assumed that the moisture from the dry-out stator found its way into the rewound stator and dramatically lowered the IR before it fully cured.

RE: Epoxy losing its insulation?

(OP)
Thanks all,

It is good to have you available on-line.

No, I wasn't there either first time or second time. A guy just phoned me and asked me if I had any idea. I had not.

The temperature was probably around +20 C in both cases. The rotor had not been started when the second measurement was taken.

No, no sample left.

Jeff, these ohms were bought together with the epoxy from some overseas manufacturer. Difficult to negotiate - they do not speak our language or any language we know. They resist every attempt to communicate.

I was thinking that, perhaps, something can happen with the epoxy after curing - getting bad like some wines do when getting older (do not really mean that, but something similar). They are taking the rotor back to the workshop and will probably have to do the whole thing over again.

Gunnar Englund
www.gke.org

RE: Epoxy losing its insulation?

Hi skogsgurra;
I would consider shipping damage or an unreported incident during reinstallation.
It reminds me of a 600 kw generator end we shipped to the US for a cleaning, redip and bake after severe hurricane damage. It was crated in a plywooud box by the repair shop.
It was repaired and returned to us, but when it was being unloaded from the ship, the lifting slings were placed around each end of the box. When the generator dropped out through the bottom of the box, they considered themselves lucky that it didn't hole the old freighter. It did shift the copper strip windings, and a complete rewind was required.
Some inquiries as to the shipping and handling methods of your machine may be in order.
Respectfully

RE: Epoxy losing its insulation?

I'd have liked to have seen that ..  I would agree too with your suggestion.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Epoxy losing its insulation?

(OP)
Bill and Keith,

That's a whole new idea. Might very well have happened. I will fwd the thinking to the winder.

Gunnar Englund
www.gke.org

RE: Epoxy losing its insulation?

Never assume that the shipper didn't drop or otherwise damage the shipment.

We shipped an electro-optical tester to a Navy shipyard for installation into an aircraft carrier.  When installed in the test bay, they couldn't get the system to perform calibration alignment.  Everyone swore that it hand not been mishandled.

Shipped the system back to the factory and the first thing we noticed after unpacking the crate was that the 2-inch aluminum base plate was bent up in one corner.

TTFN



RE: Epoxy losing its insulation?

You guys shouldn't have used such a big hammer when you packed it!!

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Epoxy losing its insulation?

Um... not quite a new idea if you look at my 6/23/06 13:31.  

"I would also keep an open mind to the possibilities of contamination or hidden mechanical damage that occurred after the first test (perhaps during shipment or testing)."  

But that's ok, whose keeping track winky smile

We have had two large motors that have failed (stator ground fault in one case and turn/ground in the other) within a very short time after shipping.  Neither one immediately after shipping.  Both tested very dry. One operated several hours before failure.

The cause was never completely determined (there was also work done on one of these motors prior to shiping) but the coincidence of timing leads us to keep an eye on shipping. We now require an impact recorder on all our large motors during shipping.  It's something that Westinghouse does for Reactor Coolant Pump motors and transformer customer/shops usually do... but other than that not a common practice for motors (invariably our shops tell us they don't know where to get an impact recorder because they have never had it requested before).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

Never said was a new idea, considering that Waross made a similar comment, not that anyone's keeping track winky smile


Just offering some evidence to back those assertions.  

When confronted with the evidence, the Navy said, "Oh yeah, it might have fallen off the forklift."

TTFN



RE: Epoxy losing its insulation?

I was responding lightheartedly to skogsurra who said "that's a whole new idea"

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

Tangential thought alert!

In a prior life we used to use a simple device called a "Tip-N-Tell" on the sides of MCC crates because Allen Bradley MCCs could get critical damage to the bus support system if the units were tilted more than a few degrees from vertical. These devices were simply a couple of angled plastic 1/2 tubes with a reservoir of blue glass beads and sticky glue on a cardboard placcard that was stuck to the crate on 2 sides. If the crate was tipped, the blue beads traveled up the tubes and got stuck in the glue, providing a permanent indicator that the crate was mishandled.


It made me wonder if there is something like that for indicating shock damage as well, and there is. Could be very useful for large electrical machines.

They also have a version that is reusable (keyed reset) and even one that records the time and date of the shock!

Shockwatch link

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Epoxy losing its insulation?

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

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.

RE: Epoxy losing its insulation?

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

RE: Epoxy losing its insulation?

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Epoxy losing its insulation?

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.

RE: Epoxy losing its insulation?

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

RE: Epoxy losing its insulation?

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

RE: Epoxy losing its insulation?

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.

RE: Epoxy losing its insulation?

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

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Epoxy losing its insulation?

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Epoxy losing its insulation?

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

RE: Epoxy losing its insulation?

(OP)
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Epoxy losing its insulation?

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.

RE: Epoxy losing its insulation?

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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