VFD Causing Bearing Failure 5

[richanton]

I have searched on this and found some discussion, but I am curious as to what anyone has found to be a good solution to the problem. In my case, I had a new 200HP motor/pump setup(pumping chilled water) that the bearings failed after about 6 months. The motor is fed by a new IGBT VFD. A single ground wire initiating at the source is run straight through to the motor and grounded at the termination box. There are currently no chassis grounds.

I have found much discussion online, particularly a white paper by Reliance Motor, but I was looking for some real solutions that have worked.

Right now we are looking at a ring assembly sold by a company called Shaft Grounding Systems. I am also considering adding some chassis grounds, and/or refeeding the motor from the VFD with continuously corrugated cable.

Any input is welcome.

 

[ozmosis]

I agree with Keith. I'm sorry if mentioning Beppe precludes you from any discussion but most of us know your intentions are wholly honourable in these discussions.

 

[electricpete]

Someone mentioned insulating ODE bearing. For vfd's insulating one only does no good... would need to insulate both if that is part of solution.

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[LionelHutz]

The reason I asked is because we have customers who have found some Weg motors have higher rates of bearing failures due to shaft voltages even when running directly on line power. One site was a big fan and it kept failing the fan bearings at $20k a pop.

If you insulate the NDE and put a ground brush on the DE then the ground brush holds the shaft and the motor case at the potential on the DE and that bearing doesn't necessarily need to be insulated. If you just ground brush both bearings you can possibly end up with circulating shaft currents. If you just insulate both bearings then high frequency components can still transfer across the bearing insulation due the capacitance of the insulation.

 

[starkopete]

Hi,
This link is to a pretty good practical article on the problem and remedies.

http://www.plantservices.com/articles/2007/022.html?page=1

Thanks

 

[Skogsgurra]

A little crash course on EDM in bearings attached.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

Gunnar,

Thanks for sharing that.

Dave

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[lukin1977]

maybe this can help,

To reduce the audible motor noise and telephone interference factor associated with PWM drive switching rates between 1 kHz and 8 kHz, manufacturers developed VFDs with switching frequencies as high as 20 kHz. While the higher frequencies did reduce motor noise, they also aggravated PWM pulse contribution to excessive motor-shaft voltage and current that can discharge across motor bearings. VFDs that use insulated-gate bi-polar transistors (IGBTs) as high-frequency switches are most likely to cause bearing discharge current

You can install a high-frequency compensated reactor on the load side of the drive, either at the drive end or motor end

lukin1977

 

[Skokian]

I don't know if the motor shaft is grounded by the load (pump). E;g.: Is the shaft coupling direct or thru a coupler, and is the coupler, if used, insulating or not.

The grounding rings or brushes being sold are not to simply ground (earth) the shaft; but rather, to shunt (bypass) the harmful currents around the bearing, rather than allowing the currents to flow thru the bearing.

Inverter duty (rated) motors have this provision built in, or use insulated bearings to avoid the problem. Inverter duty motors also have higher voltage insulation to withstand the modulation frequency spikes from the VFD. They also have higher heat tolerance since a VFD causes up to 5% extra heating in the windings, again due to the modulation currents.

Running a 115% S.F. (Servide Factor) motor at 100% of FLA is enough derating for the thermal considerations; but, using a conventional motor on a VFD requires attention to both the bearing current issue as well as the voltage withstand issue.

Hope this helps.

Regards . . .

Jim S. Nasby

 

[waross]

Thank you Gunnar.

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

 

[richanton]

Just as a final addition to close the topic, I found that NEMA actually publishes a document called "Application Guide for AC Adjustable-Speed Drive Systems". It has a lot of what has been discussed contained in it. See the link.

http://www.nema.org/stds/acadjustable.cfm

 

[edison123]

richanton

A LPS for that useful link.

 

[Skogsgurra]

I am surprised that the NEMA paper is so inadequate. Is this really the latest version of the paper? It is not only lacking lots of important information, it is also downright disinformative.

Picture 5-12, for instance, does not at all show a typical shaft voltage for a VFD fed induction motor.

Picture 5-13 is a little better, but still wrong. Typical EDM voltages are between 5 and 15 V. Not around 500 mV as shown in the picture.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...