US-based Electro Static Technology (EST) has collaborated with Fluke to produce a kit that tests whether an inverter-driven motor is affected by stray shaft voltages that could damage its bearings.

VSD-induced voltages can build up on a motor shaft and then discharge in short bursts along the path of least resistance – often the bearings. The discharge rate tends to increase with the VSD carrier frequency.

Continued discharges can pit the bearing balls and race walls through electrical discharge machining. This pitting can cause washboard-like ridges called fluting along the race walls, resulting in vibration and noise. By the time such damage can be heard, bearing failure is often imminent.

The new kit (above) combines a Fluke Scopemeter portable oscilloscope with a probe containing high-density conductive microfibres that ensure continuous contact with a rotating motor shaft. The probe can be held by in place by hand, or used with an optional magnetic base. The oscilloscope displays the voltage waveform and saves the image for analysis. The kit is said to make it easy to measure and document the damaging VFD-induced voltages before the bearings are damaged.

EST also produces products that extend motor lives by diverting the shaft currents safely to ground. The maintenance-free Aegis bearing protection rings can incorporated in a motor when it is built, or added later.