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Variable frequency drives (VFDs) add an unprecedented level of controllability to motors, however they can sometimes lead to increased maintenance costs if care is not taken to ground the system properly. Specifically, improperly grounding the motor shaft can cause currents to damage the motor bearings, resulting in vibration, noise, downtime, and premature motor failure. This is especially true in large motor applications (those of over 600 volts or 60 amps); once these voltages exceed the resistance of the motor bearings’ lubricant, they discharge to ground – typically through the mounting equipment. This action typically causes fusion craters in the bearings.
To help combat this issue, Marathon Electric has introduced the Aegis Shaft Grounding Ring accessory. The Ring contains special conductive microfibers that allow these otherwise harmful currents to discharge safely to ground without going through the motor bearings. With a high shaft rating protection – up to 120 amps continuous and 3,000 peak volts – they ensure that your motor will continue to operate safely without any shaft arcing damage. Installation is a simple matter of attaching the Ring to the motor with four pre-drilled mounting holes, and the Ring can accommodate shafts up to 30” in diameter. The Ring has already been successfully proven in over 150,000 HVAC, pump, and power generation applications, among others. Best of all, the Ring can be added to most Marathon motors during assembly, and will fit most NEMA T or TS flanged and non-flanged motors from other manufacturers.
http://www.regal-beloit.com/
http://www.est-aegis.com/shaft-grounding-products.htm
http://www.prlog.org/10266896-current-diversion-with-aegis-shaft-grounding-ring-now-available-with-conductive-epoxy.html
http://www.morgancarbon.com.au/ipro.pdf