Although motor shaft grounding rings have been around for many years, I have finally had a chance to experience the use of them first hand.  One of my industrial clients had installed 2 new water pumps with 10 HP Baldor motors a couple of years ago.  One of the motors(#2) experienced an opposite drive end bearing failure about a year after startup, that I replaced under warranty by the supplier.  About 6 months later the drive end bearing failed, so I replaced both motor bearings in this motor for my customer, who paid for the service.

Six months later, I returned to replace both bearings in the other #1 motor. Then, 6 months later, the bearings in the #2 unit were noisy, and bad, again!?!  Something wasn't right.  The motors are controlled by VFD's, and they only operate at about ¾ speed(45 - 50 Hz).  Investigation by company electrical department personnel found the bearing failures were most likely caused by excessive shaft voltages.  They used a simple voltage meter to measure the volts between ground(the SS shim under the motor foot), and the shaft during normal operation. 

Normal voltages range from 1V - 5V.  They measured 15V - 20V.  Shaft grounding rings were ordered from Aegis, and I replaced the bearings and installed the rings on both motors.  Measured voltages were reduced to less than 1 volt!   It hasn't been six months yet, but I am confident these bearings will last-hopefully as long as 10 -15 years as designed!

Variable frequency drives control the speed of motors by switching DC voltages on & off very quickly.  The switching scheme, coupled with rotor/stator capacitance, can induce damaging voltages onto the motor shaft.  The voltages try to find their way to ground by going through the ball bearings, where the arcing between the bearing races and the steel balls causes surface defects which eventually leads to fluting and premature bearing failure.

The grounding ring is aluminum with thin carbon fibers that encircles the shaft.  The carbon gives the voltage an easier path to ground by dissipating the voltage back through the motor frame.  There are a few different types, depending on the customer preferences, but they all are either drilled and tapped onto the motor end bell, or epoxied in place.  Most motor manufacturer's are now installing the rings at the factory.  So if you order a motor to use with a VFD, be sure to check this option.  

The use of shaft grounding rings is a very good Preventative Maintenance procedure.    Invest in PM, it will save you money!