Motor bearing current

[teke]

I am looking for a typical calculation for shaft voltage/ bearing current for VFD fed morors. Can you please tell me about any similar resources or papers - how to do those calculations? I understand that less than 300 mV shaft voltage is safe.

 

[Skogsgurra]

Yes. 0,3 V is definitely safe. But it depends how you measure. Put an old moving iron AC voltmeter on a motor shaft and you will certainly measure very low levels. Measure with an oscilloscope and you will likely see 10 - 15 V peaks - and that is not at all safe.

Oil film breakdown voltage is dependent on grease oil viscosity, speed, temperature, radial and axial forces on bearing and then some.

A practical lower limit for bearing damages is around 3 volts. But that shall be measured with an oscilloscope or special instrumentation that, among other things, measure peak voltages across the bearing.

The reason you need a fast measuring device is that shaft voltage consists of short voltage spikes that no ordinary voltmeter can catch - not even the faster ones that claim 100 kHz bandwidth.

The 300 mV are applicable when you have ordinary 50 or 60 Hz induced shaft voltage and could be measured with any voltmeter. It is no longer so.

Google "bearing currents" EDM PPS and such things.

Gunnar Englund

http://www.gke.org

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

 

[jraef]

Gunnar is the Guru of shocking shafts...

 

[desertfox]

Hi teke

Look at this site:-

http://www.ab.com/drives/techpapers/ieee/APEC95 bearing.pdf

desertfox