Motor Failure Problem-Need Some Advise 1

[nightfox1925]

What would be other posible cause of motor winding to be cut (lower winding part of the stator was cut, and the stator core lamination welded)

motor passed the following test
1. rotor bar crack
2. core/ring hot spot test
3. surge
4. ir
5. no load (current voltage vibration)

and at commissioning with load, the motor failed!
during commissioning, no over load and no overheating was detected


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[svceng66]

The description of the failure is not specific enough to say for certain, but this sounds like it could have started as a turn to turn failure within one of the stator coils and eventually deteriorated to the point of failure to ground within the slot. Why it would have passed the surge test is hard to say, but I have seen this occur before. During starting, when the machine is hit with high inrush current, failures can develope which may not be seen during a surge test which uses very little current and imposes no mechanical or thermal stress on the insulation.

 

[nightfox1925]

Thanks sveng66, I have advised to check the insulation resistance of the motor. If there was a turn-to-turn failure, there should be a breackdown on the motor insulation. If there is a high in-rush problem, what is the cause of this as motors are designed to have their own in-rush current withstandable right? Is there a possibility that the motor breakdown torque was exceeded by the mechanical load?

My guy informed me that the motor fails while under load? I will verify if they have conducted a no-load test prior to it?

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[waross]

From the information given I would suspect a mechanical fault.
Possibly a fan failure, possibly some foreign object found its way into the air gap and dragged some stator laminations. Bearing failure can cause this type of failure and I have seen bearings fail on new machines.
respectfully

 

[electricpete]

I agree with everyone, any more details would be helpful. Some ideas about things you mjight tell us:

Did it fail immediately upon start? Did it get up to speed? Did it fail sometime after start?

What do you mean by lower part of the winding? Are you talking about the bottom endwindings in vertical motor? Bottom coil in a slot?

What is the approx motor horsepower and speed and voltage? At least this will tell us if this is random or form wound machine.

Is there a vfd feeding the motor? Any strange reduced voltage starting means?

What do you mean by cut? Was copper exposed? Did it look like a mechanical cut?

What do you mean by core welded...is it at the same location where the coil is faulted? Is there any ground fault protection associated with this motor? Did the motor trip on ground fault or instantaneous overcurrent?

Have you traced the location of the failure in relation to the line end coil?

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[electricpete]

I tend to agree with the scenario by svceng66 with the details we have so far. Mechanical stresses causing turn to turn fault can occur during dol start and especially if this was the first dol start ever seen by this motor. And turn to turn often causes massive melting.

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[nightfox1925]

Thanks electricpete,

before testing PI and IR ok

We conducted at no load test (1-hour) the following
1. vibration ok
2. current logged ok
3. Temp ok (body using IR camera) every 15 min
it also passed core test (stator core lamination no hot spot detected)
4. Voltage acceptable (unbalance would just add 1 deg C temp rise as per my computation)

During load test:
1. pulley lazer aligned
2. phase current 90% FLA
3. Voltage acceptable (unbalance would just add 1 deg C temp rise as per my computation), about 6V highest voltage difference (our supply is 480v)
4. Vibration within standard
5. Temp ok
6. Motor failed after running 30 min

Motor failed after 7 months running, the above are conducted after the rewinding
By the way, 3 other identical motor were rewinded with same reading and data of this motor that failed, those other 3 passed.

motor is squirrel cage induction

Motor started by soft starter


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