As the adoption of variable-frequency drives (VFDs) increases, so too will the need for the reliable mitigation of destructive shaft voltages, which will lead to increased demand for bearing protection rings, says bearing protection solutions provider Aegis sales and marketing manager Adam Willwerth.

He explains that bearing protection rings solve the problem of VFD-induced voltages in motors, which destroy bearings and can cause premature downtime.

“Bearings, which normally have a mirror-smooth bearing surface, can be damaged by electrical discharges; without protection, shaft voltages pass through the bearings and cause unwanted electrical discharges.”

However, in most cases, Willwerth explains that a reliable and cost-effective way of minimising electrical bearing damage and increasing the reliability of VFD-controlled motors and systems is to use an Aegis bearing protection ring, which is combined with an insulated bearing for large motors greater than 75 kW.

In contrast to the previously used shaft grounding technology of spring energised carbon brush, he further adds that Aegis bearing protection rings – marketed globally by static control solutions provider Electro Static Technology – offer far greater efficiency, especially in avoiding high-frequency bearing currents.

The bearing protection rings consist of highly conductive microfibres that are secured in the patented FiberLock channel. Scalable to any shaft diameter, the fibres surround the motor’s shaft and offer a low-impedance path from the shaft to frame, he explains. The fibres also offer a low-impedence path to the ground, which enables the channel to reliably divert damaging currents.

Since the microfibres work with little or no contact with the shaft, they are not subject to wear like conventional carbon brushes, Willwerth notes.

He further explains that the conductive microfibers discharge at any voltage peak, and are effective at very high frequencies, which are in the megahertz range in the switching frequencies of the VFD’s insulated-gate bipolar transistor (IGBT).

He adds that microfibres developed by Electro Static Technology are specifically designed to discharge VFD-induced voltages and high- frequency currents.

In addition, Willwerth notes that Aegis-protected motors do not require operational downtime, and last about three times longer because the bearings do not degrade from electrical discharges.

“Shaft voltage is a fact of life for VFD-driven motors; therefore, minimising electrical damage will extend the life of the motor and reduce the frequency of bearing replacements, as well as avoid premature and catastrophic motor failure,” he explains, subsequently leading to cost savings and increased productivity for the motor end-user.

However, Willwerth notes that shaft grounding in the form of high maintenance mechanical spring energised carbon brushes was the only solution available before Aegis bearing protection rings.

“Shaft voltages created by the pulse width modulation of the VFD are capacitively coupled from the stator in the motor to the shaft. “These voltages peak at a voltage higher than 10 V and will be high enough to overcome the dielectric of the oil film in the bearing. It will then arc through the bearing in a lightning-like discharge, which burns the lubrication and causes electrical erosion pits in the bearing race,” Willwerth explains.

This, he adds, happens millions of times an hour and can result in bearing failures from electrical damage in as little as weeks or months of operation.

The failed bearing will show a washboard pattern called “fluting” on the race, and black burnt grease, which is disruptive and expensive to the operation of the system, as it can cause premature bearing and motor failure.

Aegis bearing protection rings are, therefore, a maintenance-free circumferential ring of conductive microfibers that last longer than the design life of the bearings, which is equal to 200 000 hours, says Willwerth.

Aegis bearing protection rings have been installed in more than two-million applications globally, mainly in North America and Asia.