Common mode choke (CMC) manufacturers now go to great effort to market their products as a preventative measure against bearing failure. A recent paper presented by H. William Oh at the Motor and Drive Systems Conference in Orlando, FL explains why CMCs are not an effective solution. VFD-driven motors exhibit two types of bearing currents: inductively-coupled and capacitively-coupled bearing current. CMCs are indeed an effective tool at minimizing high frequency transients in the common mode current as demonstrated through numerous tests.  They do not, however, address or eliminate the capacitively-coupled bearing current, also known as EDM bearing current, which caused fluting damage in electric motors of all sizes.

Common mode voltage, which is capacitively coupled from stator to rotor, does not influence circulating bearing current, and it remains the primary source of bearing currents in VFD driven motors up to 100 HP and is present in all larger motors as well.

Shaft Voltage Discharge

Motors up to 100 HP









Inductively-coupled bearing current, or circulating bearing current, appears as motor frame size surpasses 100 HP.  Research has demonstrated that circulating bearing current is influenced by the inductance of the motor and common mode current variations.

Motor with circulating currents

Circulating bearing current will thus only present itself when end-to-end shaft voltage is present, a condition found only in large frame motors. CMCs can reduce the risk of damage through circulating bearing current by regulating the amplitude of high frequency common mode current. In a small frame motor, the lack of circulating bearing current renders CMCs unnecessary.

CMC’s, which are sometimes also referred to as “Inductive Absorbers,” only attenuate common mode current; they have no effect on common mode voltage. As a result, use of a CMC will not address the risk of damage from EDM bearing current. Indeed, quite the opposite may be the case: an improperly-selected CMC may result in resonance, amplifying EDM potential.

This fact can often be overlooked because of the way the industry defines shaft voltage.  That is, neglecting to separate the two types of shaft voltage, which separately drive EDM bearing and circulating bearing current.

Motors over 100 HP

The best solution to this problem in a large frame motor involves pairing the AEGIS Ring on the drive side with an insulated bearing on the non-drive side of the motor.  This insulation method interrupts and stops the high frequency circulating current and is recommended in our best practices handbook.

Common mode chokes on the other hand, when properly matched to the motor,  could reduce the high frequency circulating current, but an AEGIS shaft grounding ring is still needed to discharge the capacitive EDM currents.

For more information on shaft grounding, visit Electro Static Technology, designers of the AEGIS Bearing Protection Ring. EST presented Mr. Oh’s paper, “Common Mode Choke Cores (CMCs) Cannot Prevent Bearing Failure in All Motors,” at the 2017 Motor and Drive Systems Conference in Orlando, February 8-9.