Maintenance Technology, May 2014.
http://www.maintenancetechnology.com/2014/05/grounding-rings-rescue-paper-mills-bearings/

"At a paper mill in the Midwest, every new motor that is to be controlled by a variable frequency drive (VFD) must be tested. If shaft voltages are found, the motor must be equipped with an AEGIS® Bearing Protection Ring to divert damaging currents to ground. The plant’s electrical reliability engineer established this money-saving specification after many years of frustration from recurring bearing failures."