CAPACITANCE CALCULUS OF A BEARING

[logic5]

I would like to calculate theoretically the capacitance of a bearing with metallic rings and ceramic spheres (see

http://www.alibaba.com/catalog/10516334/Ceramic_Bearing_Balls.html).

The expression for a cylindrical capacitor is: (2*pi*eps0*l)/(lnb/a).

However, when adding some spherical balls inside the cylinder, how should I proceed?

Thank u,



logic

 

[Skogsgurra]

There's a very comprehensive thesis (243 pages, techn. dr) by Annette Muetze, Darmstadt. The title is "Bearing Currents in Inverter-Fed AC-Motors". It covers most aspects of bearing EDM and there are lots of calculations on bearing impedance with or without ceramic insulation, ceramic balls etcetera.

You should also look at the IEEE paper by Chen, Lipo, Fitzgerald "Modeling of bearing currents in PWM inverter drives" and also the classic papers by Erdman, Kerkman, Schlegel and Skibinski published by Allen-Bradley/Rockwell Automation.

I have never seen any calculation of ceramic (hybrid) ball bearing capacitance. I do not think that the ceramic balls contribute very much to the capacitance. Relative epsilon of the ceramic is usually in the 3 - 8 range and the Hertzian contact area is very small. The error in the capacitance calculation that you get from assuming a cylindrical capacitor is probably larger than ball contribution.

And, in the over-all picture, capacitance between rotor and stator is dominating over bearing capacitance.

Google can probably find something for you. I will have a look, too.



Gunnar Englund

http://www.gke.org

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