End users of inverter-driven motors have every right to expect uptime and reliability. After all, VFD-induced electrical bearing damage can be prevented, not just repaired. When bearings fail, proper repair practices can fix the problem for good, but value-added services such as inspection, testing, and analysis can prevent the need for repairs in the first place. Motor shaft grounding rings such as those offered by AEGIS® can be installed during motor repairs or on new motors before they are put into service.
Perhaps someday all motors will be built so well that there will be no more electrical bearing damage. Until that day comes, motor repair shops will continue to replace bearings eroded by voltages induced by variable frequency drives (VFDs, commonly known as inverters). And if the customer has to send the same motor back for new bearings again in six months, s/he is likely to develop serious doubts about the shop’s competence.
On the other hand, a repair shop that fixes a motor’s bearing problem properly only has to do it once, and is therefore more likely to earn customer loyalty. Better yet, a shop that offers the latest diagnostic services (vibration analysis, thermography, shaft-voltage testing, etc.) can show a customer how the right preventive measures can prevent electrical bearing damage or nip it in the bud. Working at the customer’s plant, on either a brand new motor prior to its installation or on a motor already in service, personnel who know what they are doing can now protect bearings for the life of the motor. This is what we mean by “best practices.”
By now it is widely understood that induced shaft voltages discharge through the bearings of many VFD-controlled, alternating-current (AC) motors (See Figure 1). The high switching frequencies of today’s VFDs produce parasitic capacitance between a motor’s stator and rotor. Once the resulting shaft voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance — typically through the bearings (See Figure 2). During virtually every VFD switching cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a tiny pit (usually 5-10 microns in diameter) in the bearing race.