The Need for Shaft Grounding on VFD-Driven Motors

In the quest for energy efficiency and machine automation, more engineers, facility managers, and building owners are turning to variable frequency drives (VFDs). As one of the most promising “green” technologies available, VFDs allow users to operate motors at less than full speed while maintaining the required torque. In fact, VFDs can reduce an electric motor’s power consumption, saving 30 percent or more in electric energy costs. VFDs, however, can require additional protection for the equipment. VFD-induced common mode shaft voltages can damage bearings through the process of electrical discharge machining (EDM), which can shorten electric motor bearing life and reduce the reliability of these systems. To mitigate these voltages, long-term shaft grounding is essential. An effective way to protect electric motors from electrical bearing damage is to install circumferential conductive microfiber shaft grounding rings. Most VFDs use pulse width modulation with insulated gate bipolar transistors to control switching. The output of these drives includes a fluctuating common mode voltage that electrostatically induces voltage on the motor shaft. This shaft voltage tends to discharge through the bearings, possibly causing EDM bearing damage. Shaft voltage can be measured using a digital oscilloscope—with a bandwidth of 100 megahertz (MHz) or higher—by touching a conductive microfiber probe tip to the spinning shaft of a motor. Without a circumferential conductive microfiber shaft grounding ring, shaft voltage will discharge through the motor’s bearings (or those of coupled equipment), then through the motor’s housing to achieve electrical earth ground. The shaft grounding ring protects motor bearings by offering an alternate path to ground for shaft voltage—directly from the motor shaft through the ring to the motor’s housing (ground), avoiding the bearings altogether.

Without shaft grounding, EDM erodes the rolling elements and the bearing raceway surface (Image 1), causing premature bearing failure. These frequent discharges create millions of micron-sized pits, called frosting, on the bearing raceway. As frosting damage increases over time, a phenomenon known as fluting occurs. Concentrated pitting at regular intervals along the raceway surface that form washboard-like ridges, fluting can cause excessive noise and vibration. By the time this noise can be heard, bearing failure is imminent. The most reliable way to minimize electrical bearing damage and ensure the reliability of VFD-driven motors and systems is a circumferential microfiber motor shaft grounding ring that protects the motor’s bearings as well as those of attached equipment. This technology is available from motor manufacturers that install it inside or outside the machine at the factory. Field retrofit products are also available. A caution: most electric motor manufacturers offer “inverter-rated, inverter-duty, or inverter-ready” machines, implying the motors are already protected. The VFD protection incorporated into these motors is the use of “inverter class” magnet wire for the motor windings required under National Electrical Manufacturers Association (NEMA) MG1 Part 31 guidelines. Inverter-class magnet wire protects the motor from spontaneous winding shorts and subsequent corona failures, both of which would typically occur within the motor’s warranty period. And while inverter-class magnet wire protects inverter-duty, inverter-rated, inverter-ready, and VFD-rated motors against spontaneous winding shorts and corona failures, it offers no EDM bearing fluting protection. If any motor fails due to EDM bearing fluting damage, the manufacturer’s warranty may not apply. Electric motors that are correctly designed to operate sustainably on a VFD must include circumferential microfiber shaft grounding rings. The addition of this technology to an electric motor provides a warranty against bearing fluting. Most electric motor OEMs offer product lines that are inverter-rated, inverter-duty, inverter-ready with circumferential conductive microfiber shaft grounding rings factory-installed. Ask a preferred electric motor OEM or electric motor distributor if they supply machines with this level of VFD protection. For trouble-free operation, always specify circumferential microfiber shaft grounding on any machines that will be controlled by pulse width modulated insulated gate bipolar transistor VFDs.