phase burnt on 3 phase motor

[superdumpy]

Hey there,
i'm new to this forum but having found it through google it really seems that the people here know there stuff.
i'm a maint. tech and we had a problem with a motor. the 's' phase burnt out and caused the vfd to trip.this is the second time to happen in the space of a year.the first time we just changed the motor and all seemed fine till problem occured again.

this time we cut back the cable and re-connected just to get it running while we investigate further the cause.

has anyone heard anything similar to this? if so let me know it would be appreciated.

 

[DaveScott]

Could you be a little clearer?
You mentioned that, the first time, the motor winding 'Phase S' burnt out.
Then the second time, you mentioned that the problem was repeated, and to cure the problem you cut back the cable a little, and re-connected.

If the S Phase winding was burnt out a second time, I cannot see how cutting the cable back, and re-connecting, could solve the problem, unless the motor was changed as well? Did you cut back the cable in case the connection was at fault?

Was it the same phase winding that burnt?

Can you see my problem in understanding?

 

[edison123]

If the S phase cable was burnt, possibly loose (high resistance) termination.

* Algebra - The weapon of math destruction *

 

[powerjunx]

superdumpy,
Mostly, i encounter burning off was due to poor crimping of terminal lugs and loose tapping. This has a mean-time-failure that would led to long time interval of such failure. As heat build up at the tapping point, it later spreads up to the extent of considerable lenght, less than 5 inches, leading to burn out or even on worse case such single phasing.

If this not happen at the terminal point, as above, maybe you'll post some details.

 

[superdumpy]

sorry dave. the first time it happened we changed the motor out. this time we were under pressure to get the motor started so we cut back the cable and a downtime was planned to investigate further. the s phase had burnt and come away from its connection. all the windings were meggered so we are thinking it could be a lug/crimping problem. the motor is running as per spec for two weeks nowand i'm trying to get further info before i shut-down.
regards
tommy

 

[edison123]

Use a non-contact thermometer to measure the temperature of the terminations while the motor runs on load. High temps indicate improper terminations.

* Algebra - The weapon of math destruction *

 

[superdumpy]

thanks for that edison. the problem is the motor is in a h-vac unit and to be in there while its running,lets just say it would very windy!!!

 

[jraef]

So to get this clear, the first time you just changed the motor. The cable again less than a year later and the 2nd time, you changed the motor AND cut the motor lead back before reconnecting. The current installation appears to be running within specs for the past 2 weeks. Is that right?

If so, I would say that you had a bad termination the first time, which damaged the motor connection and the cable. Once a cable has been overheated to the point of melting, the damage extends far up from the termination point as fbcybil mentioned. The conductor material changes it's properties and can become more resistive, which accelerates future failure, especially in VFD output circuits because of the additional heating from harmonics. You changed just the motor, which left the cable vulnerable to the second failure. Hopefully you cut that cable back far enough to avoid this again. You may also want to look at putting a filter or even just a reactor on the VFD output to lower the thermal stress on those cables (and the motor).

http:/

http://www.jraef.com

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[superdumpy]

hey thanks for that. it sounds like it could be the reason. i wasn't around the first time it happened but i can find out if the cable was changed or not. i'll post it here.

thanks again.
tommy