Zürich University for Applied Sciences on Motor Bearing Currents

In 2017, the School of Engineering at Zurich University of Applied Sciences (ZHAW) began a long-term study of electrical bearing damage.  Whereas most prior research had focused on large motors on drives, this research focused on smaller motors.

This project was commissioned by the BFE, the Swiss Federal Bureau of Energy.  The research consisted, in part, in running 11 identical 2 hp (1.5 kW) motors on drives for 10 months, and then cutting and examining their bearings.  Each of the 11 motors was equipped with different types of protection (or lack thereof) against electrical bearing damage.  Among other options, these protective measures included specialized VFD cable, common mode chokes, insulated bearings, and AEGIS Shaft Grounding Rings.

Before going into the test results, we should review the three types of VFD-caused bearing current.  EDM current, aka shaft voltage discharge or rotor discharge current, is a problem in motors of all sizes.  High-frequency circulating currents are a problem in larger motors, over 100 hp/75 kW.  And rotor ground current is a problem in poorly-grounded motors of all sizes.

Because these were small motors, circulating currents were not a problem.  The researchers found that both shaft grounding rings and insulating both bearings prevented damage from EDM and rotor ground current.  (Grounding the motor shaft, alone, would not protect coupled equipment's bearings from rotor ground current.)  Common mode chokes reduce rotor ground current, but do not decrease EDM current - and can in fact make it worse.  Likewise improved grounding and special VFD cabling prevent rotor ground current but do not affect EDM current.


The EDM "risk number" for each of the 11 motors. (Low risk number is good.) The least at-risk motors are #1, with insulated bearings, & #2 with an AEGIS ring. The next two are at risk: #3 has a well grounded frame, and #4 and 5, a common mode choke. #6-11 have various levels of shaft and frame grounding.

Their final recommendation for preventing bearing currents in small machines?  EDM current can be mitigated by effective shaft grounding (such as AEGIS Shaft Grounding Rings) or two insulated/hybrid bearings.  Rotor ground current can be prevented by good high good high-frequency grounding of the motor frame.

In addition to being part of ZHAW's recommendation, AEGIS Rings also come with a 2-year extended warranty against bearing fluting damage. No other form of protection against VFD-caused bearing damage offers a warranty like this.

To learn more about AEGIS shaft grounding and best practices for electrical bearing protection, sign up for a training. We offer monthly live training webinars, and - pandemic restrictions permitting - we can also visit your facility to review your exact application.