Monthly Archives: July 2016

Variable frequency drives (VFDs) enable precise speed control and significant energy savings, but they can also cause motor bearing failure. VFDs take smooth electric power and break it up into pulses. These pulses create a voltage imbalance in the motor that can build up on the shaft. This shaft voltage can discharge through the bearings, causing cumulative damage and premature motor failure.

These destructive discharges can be seen with a digital oscilloscope equipped with a conductive microfiber probe tip, like the AEGIS® Shaft Voltage Tester™. A typical bearing discharge is shown on the left below, where voltage gradually builds up to several volts (~20V here) and then suddenly collapses back to baseline.

meter readings

Shaft voltages on a motor before (left) and after (right) installation of an AEGIS® Shaft Grounding Ring. The voltage scale is the same in both images.

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