Insulated bearings

[thinker]

We are analyzing a bearing failure on VFD driven PM motor. Motor MFG uses DE and NDE insulated bearings (brand AKN, Germany) and DE ground brush. These bearings have the outer ring surface coating (medium grey color). This seems to be either ceramic or aluminum oxide insulating provision. Question for the community: for this kind of bearing, is there any quantification of its insulating properties (e.g. insulation resistance shall be not less then xxx )?

 

[Skogsgurra]

The insulation resistance is of very little importance. It is the capacitance between outer ring and housing that is the problem. I start to see frosting and fluting after a few months on bearings with Al2O3 insulation and bearing sizes 6210 and upwards.

IE4 motors are worse off than the less efficient types.

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.

 

[ScottyUK]

Thinker - an insulated bearing of this type typically has a very high insulation resistance. Most normal meggers just read over-range. If you get a reading then it is a result of coating damage or contamination on the surfaces. The very thin coating makes a great capacitor dielectric - the closer the capacitor 'plates' are to each other the greater the capacitance.

 

[thinker]

Yes, I understand that. The insulating coating is fractured in several spots and have missing pieces exposing bare metal. As we read in published white papers, the outer ring coating is not the best method of insulation (could be the cheapest). We will suggest the motor MFG to use ceramic ball bearings instead. They seem to have much better reputation/service life.