Electric motors are essential to modern mining. From conveyors and crushers to electric trucks and loaders, to pumps and the HVAC equipment that keeps the air breathable, all are run by motors. When a motor fails, production will be slowed or even shut down until it is repaired or replaced. So obviously, preventing motor failure is a good thing.
Electric motors are nowadays often controlled by variable-frequency drives for several reasons. But despite their advantages, drives have their drawbacks, too. They can cause premature motor failure by damaging both the motor windings and the motor bearings. Nowadays, most motors sold for use with VFDs are resistant to winding damage, but protection against electrical bearing damage from drives is still the exception, not the rule.
Case in point: A dredge mine in western Africa had been suffering repeated bearing failures on its 2 MW cutter motor. The bearings showed fluting damage (above), which is the hallmark of electrical bearing damage from VFD control.
This damage is caused by stray voltage produced by the drive on the motor shaft. A common solution to this problem is to ground the shaft to dissipate the stray voltage. So the mine installed carbon shaft grounding brushes... but the failures continued. They also found that the grounding brushes required frequent maintenance to keep the springs from jamming in the sandy environment.
Then they heard about the AEGIS® Shaft Grounding Ring. Rather than pressing a conductive block against the shaft, the AEGIS ring surrounds the motor shaft with hundreds of thousands, up to millions, of conductive microfibers. AEGIS microfibers were specifically chosen to bleed off stray voltage from VFD control because, unlike carbon brushes, they have a very high surface area per unit volume. This high surface area exploits the skin effect to provide low impedance to high-frequency current, such as the stray current produced by drives.
So in early 2025, the mine installed an AEGIS PRO Series ring on the cutter motor, as well as an AEGIS HFGS, a braided grounding strap that helps ensure stray current a path to ground.
A year later, the mine stakeholders remain happy with the AEGIS system's performance... So much so that they are installing additional PRO rings and grounding straps on other equipment.
The AEGIS PRO Series was designed to dissipate shaft voltage in large- and medium-voltage motors. For smaller motors, the AEGIS SGR provides excellent shaft grounding. Motors over 100 HP/75 kW are also susceptible to high-frequency circulating current, for which we recommend insulating one bearing or using a hybrid ceramic bearing at the opposite end of the motor from the AEGIS ring.
In hazardous locations, AEGIS rings may be installed inside explosion-proof (XP) and Ex d motors. If the manufacturer or another expert confirms that it would be safe, AEGIS can also be installed on Class I, Division 2, and Ex ec motors. For hazardous duty motor lines with AEGIS installed or optional, see this Application Note.
AEGIS rings are also the only electrical bearing protection to offer a two-year extended warranty against electrical bearing damage. The warranty applies to new and repaired motors, as well as to motors with AEGIS factory-installed.
To learn more about AEGIS shaft grounding and best practices for electrical bearing protection, sign up for a training. We offer monthly live training webinars, and - pandemic restrictions permitting - we can also visit your facility to review your exact application.
