Electric motors fitted with Variable Frequency Drives are at risk of bearing damage due to a build-up of current in the rotor that discharges through the shaft, damaging the lubrication and bearing surfaces and producing harmful vibration that eventually results in bearing failure. This reduces the motor's overall life, significantly impacting productivity and incurring considerable equipment repair and downtime costs.

Power Supply

Three-phase electric induction motors operate on a power supply that consists of three alternating current (AC) inputs separated by one-third of a phase. When these alternating currents are perfect sine waves (which is the case with power sourced directly from the grid), the sum of the voltages of these three inputs, known as the ‘common-mode voltage,’ remains at zero, and current does not build up in the rotor of the motor.

Using a VFD With Your AC Induction Motor

One of the most popular and useful speed control methods for induction three-phase motors is using a variable frequency drive (VFD). This device allows the motor's speed and torque to be finely controlled by modifying the power source's frequency and voltage. To do this, a VFD typically converts the power from AC to DC and then sends out a DC pulse to simulate the sine wave. However, due to inherent limitations in the way that VFDs work, the output is not perfect, and the common-mode voltage fluctuates between negative and positive values.

VFD Voltage Issues

The common-mode voltage induces a current in the motor's spinning rotor due to an effect known as capacitive coupling. Because the motor's shaft is electrically insulated from the electrical ground by the bearing lubricant, the current may build up in the shaft like a battery until it becomes powerful enough to arc through the lubricant in the bearings and conduct to the electrical ground. When this happens, it produces cratering, which is when the hot spark melts a tiny area on the bearing surface and deforms it. This leads to vibration, which produces damage to the bearing races known as fluting, reducing the insulating effectiveness of the lubrication and exacerbating the problem. Eventually, the damage becomes so severe that total failure of the bearing occurs, and costly repairs are required.

Preventing VFD-Induced Bearing Damage

There are two principal ways to prevent VFD-induced bearing damage. The first is by providing a way for the current in the shaft to reach the ground without going through the bearings, which is accomplished with a grounding brush or ring. The second involves fitting the motor with specially insulated bearings that prevent harmful amounts of current from passing through.

AEGIS Shaft Grounding Rings

AEGIS Shaft Grounding Rings are the perfect option on motors that are under 100 HP. They fit around the shaft of an electric motor and extend their lifetime by diverting shaft currents safely to the electrical ground. Utilizing advanced Electron Transport Technology, it features a set of highly conductive, wear-resistant microfibers that remain in continuous contact with the spinning shaft and allow the current to pass through, keeping the bearings safe and the motor running smoothly.

FAG Insulated Bearings

Another option, or in conjunction with grounding rings, FAG Insulated Bearings are specially designed to prevent shaft currents from passing through, preventing bearing damage and extending the motor's life. The dimensions of these bearings are per DIN 616 (ISO 15), which means they can be used interchangeably with standard bearings. They come in two types: ceramic-coated and hybrid ceramic/metal.

FAG’s ceramic-coated bearings feature a hard, wear-resistant oxide-ceramic coating that provides excellent insulating properties while retaining good thermal conductance, which is important for preventing a heat build-up in the bearings. A range of different coatings is available with varying properties to suit various applications.

Hybrid bearings feature ceramic rolling elements and steel races, making them able to run at higher speeds with less friction than ceramic-coated bearings. They retain excellent insulating properties and are more cost-effective than ceramic-coated bearings for smaller applications.


VFDs create shaft currents in electric motors that can damage the bearings, impacting productivity and incurring increased operating costs. By fitting AEGIS Shaft Grounding Rings or FAG Insulated Bearings from eMotors Direct on your next overhaul or new purchase, you can benefit from the full-service lifetime of your electric motors and maintain the peak performance of your equipment.