Where the voltage comes from
VFDs switch power at high frequency to build the output waveform, and that switching induces a common-mode voltage on the shaft as a byproduct. It builds until it finds a path to ground, and on most motors that path runs straight through the bearings.
What the discharge does
Each event is a tiny arc across the races, electrical discharge machining on a microscopic scale, thousands of times a day. Individually harmless, cumulatively destructive: frosting, pitting, and eventually fluting, a washboard pattern with audible noise. VFD motors with unprotected bearings routinely fail years early.
How a ring fixes it
A shaft grounding ring, AEGIS being the name most associated with it, is a ring of conductive microfibers encircling the shaft, usually on the drive end, giving voltage a lower-impedance path to ground than the bearing offers. Passive, maintenance-free, no wear parts.
One detail that gets missed
On motors 100 hp and up, pair the ring with an insulated bearing on the non-drive end to fully break the circulating current path. Below that, the ring alone is generally enough. It's also worth checking the ring periodically for contamination buildup, since fibers coated in oil mist or debris lose conductivity and quietly stop doing their job.
BOTTOM LINE Any VFD-driven motor is exposed to this, regardless of size or how new the drive is. A ring is cheap next to an early bearing failure.
