OT: VFD induced damage to motor bearings.

TConnolly · Aug 29, 2012

Otis and Bubba just pulled a three year old 40 HP motor from service that has some apparent bearing problems at one particular part of its revolution. We haven't got into the motor yet and inspected the bearings so at this point I don't know why the bearings failed. Motor was running a hydraulic pump that seized and sheared the coupling so there are other potential causes. However, I have heard that VFDs can cause damage to motor bearings. Under what conditions does VFD induced damage to bearings happen and how is it prevented?

bce123 · Aug 29, 2012

http://www.control.com/thread/1026211551

The Plc Kid · Aug 29, 2012

This is the brand of shaft grounding rings we use and they have a lot of good info on the topic also http://www.est-aegis.com/

These are shaft voltage probes that will pulg into you fluke scope meter and you can see it http://www.est-aegis.com/shaft-voltage-probe.htm

It is less of a issue when you have conductive grease in the bearings and insulated bearings in the motor.

The Plc Kid · Aug 29, 2012

With most of the major motor mfg you can order aegis-est rings factory installed.

The Plc Kid · Aug 29, 2012

It is caused by capacitive coupling between stator and rotor and cause by the PWM high switching speed of today IGBT's.

The higher the carrier frequency the worse it is. Put a aegis probe on a motor shart and change the carrier frequency and you can see it very clear.

TConnolly · Aug 29, 2012

This motor has a noticeable clunk just at a single point in its revolution, same spot every turn. I'm thinking that VFD induced bearing damage would manifest itself through out a motor revolution, not in a single spot. Is that right?

bce123 · Aug 29, 2012

I don't have a clue but my thoughts are somewhere around arc welding where that would make sense? the damage being done at the point of least Resistance? interesting subject, I can't wait to here from some experts on this!

DickDV · Aug 29, 2012

The outer bearing races will display a distinct wavy pattern in the pitting if it is shaft currents.

Actually, the reduced width of the air gap as a consequence of higher and higher efficiencies contributes at least as much to the problem as IGBT switching speeds. This also means that you cannot run a premium efficient motor with rattling loose bearings like you used to be able to do in the old days or you will have the rotor rubbing against the stator laminations. If that happens, the motor is toast--not rewindable.

TConnolly · Aug 29, 2012

This is what I would expect VFD damage to look like.

TConnolly · Aug 29, 2012

DickDV said:
This also means that you cannot run a premium efficient motor with rattling loose bearings like you used to be able to do in the old days or you will have the rotor rubbing against the stator laminations. If that happens, the motor is toast--not rewindable.

Not what I wanted to hear...

I can't detect any side to side play or run out in the bearings but with the anomaly occurring at only one place in the revolution, every revolution, I've wondered if the rotor is hitting the stator at one location.

padees · Aug 29, 2012

TConnolly said:
This motor has a noticeable clunk just at a single point in its revolution, same spot every turn. I'm thinking that VFD induced bearing damage would manifest itself through out a motor revolution, not in a single spot. Is that right?

A bearing will not clunk in a single place, the same place, in one revolution. You must have had some other mechanical failure when the thing locked up. Possibly twisting the rotor shaft???

The Plc Kid · Aug 29, 2012

TConnolly I have found a lot of fluted bearings at my place and we started using the aegis rigs like religion and that has calmed that problem down a great degree.

I have never seen one damged in one spot from shaft currrents. Not to say it can't happen only that I have never ran accross it.

Does sound more like it could have happened from welding or twisted shaft or just normal wear.

I know for fact that welding will do it in one spot and in a fast fashion. We had a genius here do that to the bearing on one of the shop lathes because he was grouded to the lathe bed and not the workpiece he was welding so he was arcing accross the bearing in the lathe head. A few times at that and we had to replace the bearing and get the lathe setup and running true again. A real PITA that was.

The Plc Kid · Aug 29, 2012

Another thing I have found is that on identical systems with the exception of motor cable system that are wired with shielded vfd cable the problem is not nearly as pronounced. I have had a few people disagree witht hat but I have scoped twin systems and the only difference is the cable.

Tom Jenkins · Aug 29, 2012

It's also possible that you just have a bearing that is shot - a spalled race from over or under lubrication. Have you pulled the bearings? If it is just a bad bearing it can be replaced.

TConnolly · Aug 29, 2012

The more I've thought about this I'm leaning to some other kind of damage to the motor. A 40 HP motor doesn't like to stop suddenly. There are a couple of hundred pounds in motion and when the hydraulic pump seized the motor was almost instantly decelerated until the coupling sheared. That means a lot of flexing happens in the motor. I told management before I left that the motor might be toast. We'll have to see what we find when the repair shop gets the motor cracked open.

OT: VFD induced damage to motor bearings.

TConnolly

Lifetime Supporting Member

bce123

Lifetime Supporting Member

The Plc Kid

Member

The Plc Kid

Member

The Plc Kid

Member

TConnolly

Lifetime Supporting Member

bce123

Lifetime Supporting Member

DickDV

Member

TConnolly

Lifetime Supporting Member

TConnolly

Lifetime Supporting Member

padees

Member

The Plc Kid

Member

The Plc Kid

Member

Tom Jenkins

Lifetime Supporting Member

TConnolly

Lifetime Supporting Member

Similar Topics