Stop bearing currents at the source

Bearing damage stopped at the source

Preface

This article was written by our partner Mark Gurney from 3Phi Reliability [1]. Mark is sharing his vast experience related to reliability aspects of VFD fed motors. The examples are from low voltage motor drives but the principles may very well also apply for medium voltage. Manufacturers of medium voltage VFDs often provide recomendations and detailed guidelines how to minimize bearing currents. Users shall always consider those recommendations to ensure optimum performance and maximum lifetime.

== Beginning of original article ==

Bearing damage from a variable frequency drive (VFD) is a common issue.

Real world example:

AF500 Fan 55 kW showing signs of bearing damage had a 10 second count 26’533 EMF with SKF Tked1 (electrical discharge detector pen). That is an elevated reading.

The first step in the troubleshooting is to ensure a proper grounding. 85% reduction 3’944 counts was achieved by:

– Earthing back plane

– Earthing cable glands

– Earthing cable screens

The second step is to fit EMF (Electromagnetic frequency) cores. These EMF cores contain a nanocrystalline powder that filters high frequency current typically > 40 kHz not affecting normal 50 Hz supply.

Final result reaches 190 counts in 10 seconds. That is a 99.2% reduction in harmful drive emissions that attack motor insulation, lubrication and bearings. This is a permanent fix at the source that does not degrade or fail.

Note: Insulated bearings or grounding brushes do not protect the motor insulation.