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HV Motor Root Cause Analysis

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electricpete

Electrical
May 4, 2001
16,774
I'm developing a plan to do a root cause analysis for a 13.8kv 8000hp motor which has failed a dc step hi-pot test at approx 24kv. Before the hi-pot test insulation resistance (IR) was approx 8000megaohms to ground at 10kv. After the test IR is only 100Meg at 1kv and 30Meg at 10kv. It is assumed that the insulation to ground has been damaged somewhere.

Now assume that we can narrow the faulted coil to one specific coil (by breaking coil connections and doing IR test on individual coils). But there is no visible evidence of failure point on that coil.

What options are available to try to precisely identify the failure point and failure mechanism of that coil?

Should we try to raise dc voltage as high as possible and look for smoke? Or should we use ac hi-pot for same purpose? Will either of these likely destroy evidence? Is it reasonable to try to expect extract the coil for further examination without causing significant damage (global vpi)?

Would coronoscope be useful? Use it while coil remains in the slots? TVA probe? Other methods/suggestions?

We plan to test some other representative coils to failure (line-end coils, and coils electrically and physicaaly adjacent to the failed coils). Then possibly section those open for examination (along with the failed coil)... at locations possibly including midpoint of slot, first bend outside slot, knuckle.

Also we've checked entire winding terminal resistance with no anomalies. Will check again on the failed coil and compare to other coils.

Any other suggestions for items we should consider for a complete and thorough root cause analysis?
 
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Hi Pete :
Since the IR is between the Cu and Fe -- i.e. at a hidden place --I don't think there is a non-destructive way to determine the causation of the damage.

If you can afford a destructive test, you may cut the Cu e.g.1 inch from the Fe on both sides, separate and test the induvidual strands -- or groups of them . Now you can remove the undamaged strands leaving only the low-IR strands in the groove, which now can be removed without any further damage.
<nbucska@pcperipherals.com>
 
Good morning guys,

I would make sure there is no tracking from moisture, carbon, contaminates etc.. by first washing and baking the stator. Then perform testing again. If winding ground wall insulation breakdown is still present, I would put the stator in a dark room and perform a surge test by slowly bringing up the voltage, this can be a destructive test! The formula is double the name plate voltage plus a 1000. But to find where the insulation breakdown is, you might have to keep raising the voltage until it starts to go to ground.

If the breakdown is deep in the stator core iron area, you will only hear it going to ground and it will show the phase to ground on the pattern, but will eventually surface and appear near the top of the iron by raising the voltage. (very destructive) but at this point I don't see any other choices. If the area is out on the knuckle or out of the iron core area it is possible for patch insulation to be done but (risky)....

kind regards
Hammerhead1
 
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