An article written by Adam Willwerth, Development Manager for Electro-Static Technology, describes the affect VFDs have on motors and how a shaft grounding ring can positively affect that relationship:
Already common in HVAC, pumping and industrial automation systems, the use of variable-frequency drives (VFDs) is growing. VFDs are smaller and more powerful, more reliable, easier to program and less expensive than ever. But, VFD/motor systems must be designed for reliability and trouble-free operation to keep the energy savings, which can reach 20% or more, from being wiped out by a system failure.
VFDs can induce electric currents on motor shafts that ruin bearings, shorten motor life and diminish system reliability. One way to mitigate the effects of these currents is to ground the motor shaft to protect bearings and eliminate expensive repairs.