bearing currents vs. common mode current
bearing currents vs. common mode current
(OP)
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





RE: bearing currents vs. common mode current
The definitive text on the subject is Annette Muetze's paper: http://www.ew.tu-darmstadt.de/media/ew/dissertatio...
Then, there are many other less detailed papers and articles. Google Bearing Currents with different qualifiers.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: bearing currents vs. common mode current
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(2B)+(2B)' ?
RE: bearing currents vs. common mode current
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)' ?
RE: bearing currents vs. common mode current
You should also read some of Gunnar's work too: http://www.gke.org/presentationer/files/EDM%20Phys...
You will find here a combination of theory but also many years (sorry Gunnar..) of practical experience.
RE: bearing currents vs. common mode current
Neil
RE: bearing currents vs. common mode current
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)' ?
RE: bearing currents vs. common mode current
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(2B)+(2B)' ?
RE: bearing currents vs. common mode current
thread821-231381: SKF new device for bearing current detection?
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)' ?
RE: bearing currents vs. common mode current
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(2B)+(2B)' ?
RE: bearing currents vs. common mode current
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: bearing currents vs. common mode current
Neil