In my post, “The Dark Side of AC VFDs,” we explored several options on troubleshooting drives based on many symptoms. One of the maladies variable frequency drives create is voltage traveling down the shaft damaging the bearings in the AC motor.
In this article, Adam Willwerth, Sales and Marketing Manager, Electro Static Technology, presents a kit that measures motor shaft voltage to determine vulnerability of VFD-controlled motors.
“ VFDs hold the promise of sizable energy savings, but without effective, long-term bearing protection such as the maintenance-free AEGIS™ SGR Bearing Protection Ring (manufactured by EST), those savings could be wiped out by high maintenance costs.
Whether they drive the fans and pumps of commercial HVAC systems or the conveyors of industrial production lines, motors controlled by variable frequency drives (VFDs) are vulnerable to electrical bearing damage. A savvy specifier will choose new motors that are already equipped with shaft grounding rings, but for retrofits the new AEGIS™ Shaft Voltage Test Kit makes it easier than ever to measure and document damaging VFD-induced voltages while there is still time to head off bearing damage and equipment downtime.
Because the kit can test every VFD-controlled motor in a whole production plant, office building, mechanical room, or anywhere else VFD-driven motors are operating, it provides a powerful tool for maintenance personnel and testing contractors — anyone who needs to determine and convince others that a motor is or is not subject to stray shaft voltages great enough to harm motor bearings. The result of a collaboration between Electro Static Technology (EST) and Fluke Corporation and available from both companies, the kit includes a special replaceable probe tip for highly accurate voltage readings on rotating equipment. Designed by EST, this tip is the first of its kind, containing high-density conductive microfibers that ensure continuous contact with a rotating motor shaft. EST manufactures the tip and extension rod that may be held or used with an optional magnetic base, while Fluke makes the 10:1 probe itself and the Fluke 190 Series ScopeMeter portable oscilloscope that displays the voltage waveform and saves the image for reporting.
If harmful voltage levels are identified, the installation of an AEGIS™ SGR extends motor life by diverting shaft currents safely to ground, thus ensuring the reliable, long-term operation of the VFD-driven motor system and locking in energy savings to make the system sustainable and truly “green.”
Without mitigation, VFD-induced voltages can repeatedly build up on the motor shaft to a certain threshold, then discharge in short bursts along the path of least resistance, which all too often runs through the motor’s bearings. The discharge rate tends to increase with carrier frequency, but continued discharges result in the pitting of the ball bearings and race walls through electrical discharge machining (EDM). Concentrated pitting at regular intervals along the race wall can cause washboard-like ridges called fluting, a source of vibration and noise. By the time such damage can be heard, bearing failure is often imminent.