Synchronous Motor Fault Finding

Q: We have a 2250HP Synchronous motor used for compressor application. It was observed that IR value in on phase was very low0.007 while the rest of the phases were about 50 Mohms. We cleaned the stator with solution, heated the windings to approx. 60deg. But even then faulty phase did not show an improvement in IR. Although rewinding seems to be an option, what needs to be done to pin point the fault?

A: When working with motors or xmfrs, especially MV, you need a megger/IR tester with enough amps to fully charge across the insulation. If it is a dead short, then nano-, milli-, micro-amps ... it does not matter. But usually it is necessary to fully charge the inherent capacitance of the insulation to get a good, stable reading of the insulation resistance. Unless you want to wait all day for a good PI, you need a tester with at least milliamps of capacity.

I really liked my old motor-driven megger. Never tested the amps, but it could charge the biggest transformer or longest cable in a few seconds. Our PIs were 1 minute, not 10.

In this specific case, I would look carefully at all connections. That is very low and a dead short. I assume you have a 6 or 12 lead machine and all connections are open? Maybe you have a Y connected PT on one phase? Or a testing problem? Remove all testing variables and agree with others...99% you are looking at a rewind.

  1. Be sure there is not an issue in power connector or where copper wires of the winding joint internal cables. This could be the lucky case where it is possible to repair without rewind the stator.
  2. Instead of a Megaohmmeter, if you have it, you can try with an insulation tester, where you can set the Test Voltage and Fault Current as low as possible (I talk about milliAmps for the current). This because you have without any doubt a big short. The scope of this test can be to observe the stator and check if you see some lightning somewhere in the stator when the test fails. This can help to localize where the short is.
    In this case, 99% you need to rewind. The short can be hidden or even if you see a light as probably the stator is packed in resin you have no many chance to escape from rewind (or change rotor and repack if we talk about resin rotor). If the stator is not resin and you see the light came from some skeins, maybe it is possible to enhance insulation with proper product.
  3. From my experience when an issue like that came out and is clear that the resin rotor is guilty, the cheaper option is ask for a repair or exchange service. The time that you invest can be useful for learning, not for solve the problem.

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