bearing currents vs. common mode current 3

[MAGTiger]

Is anyone aware of documented research which relates bearing life to the magnitude of rms or peak(pulsing) current? I can measure and reduce common mode current near the motor terminals and must assume that most of the common mode current in this location particularly the peaks (current pulses) are passing through the bearings. So If I cut that current in half, how does that translate into bearing life? I realize this is going to depend on a number of mechanical variables, but I need a starting point. In cas it isn't obvious, I'm talking about a motor driven by a VFD.

Neil

 

[Skogsgurra]

Yes, we are a bunch that are aware of that problem. You are on the right track. But there's so much more to it.
The definitive text on the subject is Annette Muetze's paper:

http://www.ew.tu-darmstadt.de/media/ew/dissertationen/dissannette.pdf

Then, there are many other less detailed papers and articles. Google Bearing Currents with different qualifiers.

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.

 

[electricpete]

I can measure and reduce common mode current near the motor terminals and must assume that most of the common mode current in this location particularly the peaks (current pulses) are passing through the bearings.

Stator core is in intimiate contact with the stator winding on three sides of every slot. Rotor core is nearby only at the top of slot and at that location is separated by at least a slot wedge plus an airgap distance. Capacitive coupling from winding to stator core has got to be an order of magnitude higher than to rotor. I'd think the majority of common mode current flows to the stator core, nothing to do with the bearings.


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(2B)+(2B)' ?

 

[electricpete]

Forgot to mention:
The sides of the slot are larger than the tops and bottom, so top is small fraction of total area.
The material between winding and stator core in slot is likely high-dielectric constant somewhere from 2-4 whereas the material in airgap is air (1.0).
All of which would seem to contribute to much higher coupling to stator than to rotor.

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(2B)+(2B)' ?

 

[ozmosis]

MAGtiger
You should also read some of Gunnar's work too:

http://www.gke.org/presentationer/files/EDM Physics and Reality 1.pdf

You will find here a combination of theory but also many years (sorry Gunnar..) of practical experience.

 

[MAGTiger]

Electricpete I would agree that the the majority of the common mode current entering the motor terms gets to ground in the stator. The larger question remains, how to identify or separate bearing current from the stator ground current. I would expect the signature of the bearing current to be significantly different that stator ground current, more discontinuous. Other than isolating the bearings some way then providing a discrete ground is there a known way to separate these currents? If so then the the effects of mitigation techniques could be measured. It may be that the reduction in bearing current is insignificant through filtering or that there is a very significant reduction in bearing current. We just can't tell which without a long term side by side life test.


Neil

 

[electricpete]

Simplest start - you can measure voltage from shaft to adjacent frame. NEMA MG-1 gives some low limits around 500 millivolts if I remember right. Lower than that and you shouldn't have to worry. Higher than that is no guarantee of a problem, especially if you have insulated bearings.

Next more complicated approach was described in Gunnar's attachment (”Lill-Beppe”) - look at rate of change of voltage. High rate of change indicates a discharge.

I don't think you'll have much luck trying to measure the current. I've never heard anyone claim to see it on stator input current and we talked about challenges there. The linked Disanette paper talked about an experiment to measure that current. Bearing was insulated from housing. Then intentional ground jumper was installed between bearing outer ring and ground and the current in that ground jumper was monitored. If you care to send your motor to a shop for modification to allow measurement it probably is not a big job, but on an installed motor I don't see any hope.



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(2B)+(2B)' ?

 

[electricpete]

I forgot to mention that SKF has an antenna gizmo intended to detect discharges. Haven't heard much about it since it was introduced two or three years ago.

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(2B)+(2B)' ?

 

[electricpete]

Here's the skf link (thread by Gunnar)
thread821-231381

About measuring voltage on shaft - obviously you'll be using a brush and thinking about safety working near the rotating shaft.

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(2B)+(2B)' ?

 

[electricpete]

Gunnar - Where does the name "Lill-Beppe" come from?

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(2B)+(2B)' ?

 

[Skogsgurra]

That's a good question.

The original device was used to predict life of bearings, based on measured discharge (EDM) activities in the bearing during commissioning. If activities were low, then nothing more needed to be don. If high, a common-mode filter, a du/dt filter, equipotential connection or a shaft grounding device needed to be added. We then checked to see that EDM was acceptable, which it often is when you have done the right thing.

There are a few rough limits that have been verified throughout the years: If there are more than 10 PPS (pulses per second) from 20 V, or more than 100 PPS from 10 V, then bearing life will usually be less than one year. If there are hundreds of PPS at 20 V, then life may be as short as one or two months.

Also, if peak voltages are above 5 V (peak voltages are measured as representative levels that occur at least 1000 times per second), then life is also potentially at risk, even if no EDM is detected. The reason is that the speed may be high or oil film cold and then, there will not be as many discharges as when speed decreases or temperature goes up.

When we sought for a good name, I first thought of Karin (she is good at finding certain types of faults). But at last we changed it to Bearing Predictor 1 or B.P.1 for short. And if you pronounce B.P. it sounds like Beppe. Beppe was a popular actor in Sweden and the name caught on.

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.

 

[MAGTiger]

Good info, thanks all.

Neil