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In the phenomenon known as fluting (Figure 2), the operational frequency of a VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting can cause excessive noise and vibration that forewarn of imminent bearing failure.

The Search for a Solution
Motor failures caused by VFD-induced shaft currents can result in significant unplanned downtime. Additionally, these failures affect the performance and mean time between failure of the original-equipment systems in which the motors are used. In some production applications, even a momentary stoppage caused by motor failure can cost more than $250,000, excluding the cost of repairing/replacing the motor. Clearly, there is a need for a device that mitigates bearing damage from VFD-induced shaft currents.

Section IV, Part 31.4.4.3 of NEMA Standard MG1, Motors and Generators, recommends bearing insulation at one end of a motor if the NEMA-designated motor-frame size is 500 or larger and the peak shaft voltage is greater than 300 mv. In these larger motors, bearing damage may be caused in part by magnetic dissymmetries that result in circulating end-to-end shaft currents.

For smaller motors, Standard MG1 recommends insulating both of a motor's bearings with high-impedance insulation or installing shaft-grounding brushes to divert damaging currents. For these motors, a VFD can generate high-frequency common-mode voltage, "which shifts three phase winding neutral potentials significantly from ground," according to Standard MG1. Because the damaging voltage oscillates at high frequency and is capacitively coupled to the rotor, the current path to ground can run through one or both bearings.

Standard MG1 is quick to point out that bearing insulation will not prevent damage to other connected equipment. When the path to the bearings is blocked, the damaging current seeks another path to ground. That other path can go through a pump, gearbox, tachometer, encoder, or break motor, which consequently can wind up with bearing damage of its own. An ideal solution is one that redirects shaft currents along a low-impedance path from shaft to ground, protecting connected equipment as well as bearings.

Conclusion
Regardless of the application, the success of an automated control system depends upon its design. If in-house engineers lack the special expertise required, they should enlist the services of a qualified systems integrator who understands the engineering specifications, operating conditions, and performance curves of the entire system. With informed decisions at every stage from specification to commissioning, potential problems can be anticipated and resolved.

Especially important is the selection of VFDs and motors for such systems. They should be rated for compatibility with not only each other, but other components of the system. A savvy specifier will choose a motor that is equipped for use with modern fast-switching VFDs—one with adequate protection against bearing damage as well as winding damage.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan White at megan.white@penton.com.

Experienced in industrial product development and commercialization, Adam Willwerth is the development manager for Electro Static Technology. He has a bachelor's degree in management from the University of Maryland and a master's degree in business administration from Southern New Hampshire University.