bearing damage like current machining in dol motor

[martinrelayer]

hello,

we have a hot water pump with a 690V 500kW 4 pole squirrel cage induction motor that had many bearing failure apparently as the ones caused by vfd's but strangely it is a DOL motor, without vfd.
The thing is that mechanical guys changed the non drive end bearing and installed insocoat skf bearing, and since then there was no vibration climbing and apparetly this was the final solution.
Im looking to the cause of this failure, and if anyone have any suggestion to measure it would be much appreciated.
in the main mcc we have 4% THD voltage.

regards,
martin

 

[Skogsgurra]

It is not at all unusual that a large VFD on the same transformer secondary as a DOL motor causes this kind of problem. But that is more often seen in DC machines, where capacitive coupling from grid to rotor is orders of magnitude greater than in an induction motor.

The other, and more probable, failure mechanism is that you have a so-called "frame voltage". The signification of that is that there is a voltage differential between motor stator (frame) and the driven machinery. That can be caused by voltage drop in long PE conductor - or by any other of a number of mechanisms. The remedy here is to bond motor frame to driven machinery so that voltage difference is eliminated.

Can you describe the installation a little fuller?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[martinrelayer]

we have local earth bonded to the motor frame and we also have with the mcc earth (they are connected between them also).
the coupling to the pump is made of hard rubber it should be insulated, anyway the pump bearing didn't have any problem.
this is an actual photo, we have also shaft earthing but is in bad shape, I don't think it's working.
but the insocoat made the job, later i'll attach the condition monitoring graphs.

 

[martinrelayer]

the motor is not vfd driven, is DOL without softstarter, just a big contactor and that's it.

 

[waross]

A question Gunnar:
Could a slightly asymmetrical rotor or stator or windings cause shaft currents?
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, Bill. That was supposed to be one of the reasons in earlier days. But manufacturing is now so well controlled that it isn't mentioned any more. Personally, I have never seen that problem.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

I assume the motor did not have insulated bearings, otherwise it would have Bergen mentioned. In this size range, some dol motors have insulated bearings some don't...

Going with Bill'd thought.
Do you know if stator laminations are segmented or one-piece? The latter should have insulated bearings. I've heard 2nd hand that rotor bar problems can cause currents also (long shot)

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(2B)+(2B)' ?

 

[electricpete]

Typing on my phone...had a few errors . It is of course the segmented lams that need an insulated bearing...one piece lams may not

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(2B)+(2B)' ?

 

[electricpete]

Prior to insulating the lower bearing, was it just one bearing that was getting damaged, or both?

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(2B)+(2B)' ?

 

[electricpete]

Answer to my previous question probably doesn't tell us much... most likely both for any scenario I can come up with.

On the other hand, the fact that we insulated only one bearing and made the problem go away (if true) does seem to tell us something.
I think it would rule out static electricity, frame voltage, common mode power supply influences. Most of what's left I think would be magnetic asymmetry I think. Please correct me if I'm wrong.

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(2B)+(2B)' ?