I had some wedges changed at a local rewind shop on a 1750 hp 4160 volt motor before entering the shop I got a clear 30 gig ohm megger reading at 5kv .After wedge change service and baking I meggered again,within 15 seconds at 5 kv the insulation began to track.I then ramped the voltage with the megger and the motor held good up to 4.5 kv though I got a lower reading about 2 gig ohm.performed a hi pot test slowly raising the voltage it again began to track at 5kv .May I ask the forum how do I move forward from here?
4160 volt motor problem
- Thread starter a.bisnath
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Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
a.Bisnath
What do you mean by "began to track"? IR reading increased or decreased!
Regards, Phil Corso
What do you mean by "began to track"? IR reading increased or decreased!
Regards, Phil Corso
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
A.Bisnath
1) Although IR readings are quite high, intermittent sparking is quite indicative of insulation breakdown, hence failure!
2) How long was the dryout period of the stator, before repairs commenced?
3) Were normal winding-to-ground resistance measurements monitored during Wedge replacement?
Phil
1) Although IR readings are quite high, intermittent sparking is quite indicative of insulation breakdown, hence failure!
2) How long was the dryout period of the stator, before repairs commenced?
3) Were normal winding-to-ground resistance measurements monitored during Wedge replacement?
Phil
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reply
the shop used a 1000 volt megger which showed that it was clear ,I have to find out about the dryout time in the oven -good point!.Also check if insulation resisitance checks were done in the change out.but, what can i do to try to find the point of failure,before the motor was sent to the work shop I was getting a clear 30gig ohm on the motor,can ultrasound help?
reply
the shop used a 1000 volt megger which showed that it was clear ,I have to find out about the dryout time in the oven -good point!.Also check if insulation resisitance checks were done in the change out.but, what can i do to try to find the point of failure,before the motor was sent to the work shop I was getting a clear 30gig ohm on the motor,can ultrasound help?
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
A.Bisnath....
Reur ultrasound question... NO! I'm certain you must conduct visual inspection! Several additional questions!
1) How many wedges were replaced?
2) Any evidence of rotor-bar extension?
3) Motor age?
4) How many starts had the motor experienced since initial commissioning?
Phil
Reur ultrasound question... NO! I'm certain you must conduct visual inspection! Several additional questions!
1) How many wedges were replaced?
2) Any evidence of rotor-bar extension?
3) Motor age?
4) How many starts had the motor experienced since initial commissioning?
Phil
- Occupation
- Licensed Electrician
I work on motors but not 4160V motors so I am curious.....why was the motor sent to in for new wedges? Is this some kind of preventative maintenance for these motors?
Bugman1400
Senior Member
- Location
- Charlotte, NC
If you got a spark and excess current from the hipot then, your insulation has been compromised and needs to be re-done. Perhaps while removing wedges, the insulation was cracked, scuffed or damaged somehow. I don't think a 1000V meg will work. I think it needs to be hi-potted at least twice the rated voltage. If its tracking from the wdg to the frame, the place in the wdg where the track starts is the pinhole. Many times it will have no visible soot or anything around the hole and the hole may be hard to see. There is a good chance that the motor will fail if put into service.
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