We are performing an RCA on a VFD failure, which also had a motor winding failure, and a bad bearing. It is a kind of Chicken and egg scenario. Did the VFD burn up the winding or did the winding burn up the VFD. But I want to poll a simple question for you guys, below

Question:
Have you ever seen a motor winding burn up attributed to the mechaincal resistance from a bearing failure?

Choices:
Yes, I've seen it happen
No, I've never seen it happen
Seems possible to me
Seems unlikely to me

 

I have voted "seems unlikely" for the following reason.

If the bearing has increased friction, it appears to the motor like increased mechanical load. The motor should have electrical protection to trip the motor in time to avoid motor winding damage when the current rises too high for too long ("overload protection"). If increased mechanical resistance from a bearing failure were the cause of your winding failure, it means you also had improper design or performance of your motor overload protection.

Martin,
Not the egg, not the chicken. IMO, it is the chicken soup: the high frequency (10 khz .. 1 Mhz) common mode current is the common denominator.
This current may detoriorate the di-electric intrigity of the stator winding insulation and at the same time this current induces circulating shaft currents killing the bearings. Double crime. Even insulated bearing does not always properly insulate against these hf currents.
Regards, Arie Mol, NL

I had once a case when a motor which runs across the line had developed a terribly pitted bearing fault all around the outer race but was allowed to run this way for a few months. The TWF amplitude went through the roof, clear harmonics of BPFO were present across the FFT all this time. They did not want to take it off service and ultimately the motor failed, but .... electrically. It tripped the main breaker but the winding has completely burned out.

It is only my speculations now that high amplitude high frequency vibration affected turn-to turn isolation. May be not...

David

You didn't mention if the rotor had impacted the stator following the bearing failure. This is a common cause of winding failure. On a VFD, however, there should have been a ground fault to prevent this,unless it wasn't set up.

IMHO, most bearing failures in motors on VFD's start as a result of EDM due to insufficient grounding for the higher frequency currents to return to the VFD through the power leads, therefore it goes to ground through the motor bearings and the driven equipment, if the couplings is conductive.

I have to agree with EPete. Most of the time there is protection in the electrical system to avoid any sort of over current situation. I have witnessed several good sized (4" or 6") shafts twist off due to a failed bearing before causing the motor to trip but never fail electrically until the rotor rubbed the stator.

My guess would be insufficient winding insulation for the VFD application and shaft currents on the bearings