Shaft current in AC motors controlled by variable frequency drives (VFDs, also known as inverters) can cause premature motor bearing failures.

Cii Service helps its customers solve premature motor bearing failures.

VFDs save energy and money by adjusting the motor speed on equipment to actual load conditions.  However, VFD”s can produce harmful electrical voltages in the motor shaft.  Shaft currents induced by VFD”s can travel to the ground through the bearings causing pitting and fluting, which can lead to bearing failure and later motor failure.

In order to avoid down time and costly repairs associated with this problem, it is very important to inspect the motor bearings frequently for early signs of problems. Regular servicing of your equipment through Preventive Maintenance contracts allows Cii Service”s professionals to examine your equipment and look for signs of bearing current damage. Our technicians can also listen for an audible bearing noise, which is usually the first symptom of a problem. Our technicians will also check constant speed operation.

There are several solutions to this problem including shaft grounding devices, insulated bearings, short conductor lengths, and shielded cable. There are many factors to consider when evaluating solutions for this problem.

Please contact a Cii Service professional in your area for expert advice on how to stop this type of damage before it causes motor bearing failures.


With our corporate office located in Richmond, Virginia, Cii Services has a
widespread presence in Virginia and North Carolina. Discover a Cii branch near you.