Preventing VFD-induced bearing damage in electric motors

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eMotors Direct experts outline how VFDs damage bearings and discuss measures to prevent it.

Electric motors fitted with VFDs (variable frequency drives) are at risk of bearing damage due to a build-up of current in the shaft. The current discharges through the bearings. This damages the lubrication and bearing surfaces, and produces harmful vibrations that eventually results in bearing failure.

Bearing damage will most likely:

• Reduce the lifetime of the motor
• Significantly impacts productivity and efficiency
• Incur considerable costs in equipment repair and motor parts replacement

How VFDs damage bearings

Three phase electric induction motors operate on a power supply that consists of three alternating current (AC) inputs that are each 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 shaft of the motor.

Using VFDs is one of the most popular and useful methods of speed control for induction motors. This device allows the speed and torque to be finely controlled by modifying the frequency and voltage of the power source. To do this, a VFD typically converts the power from AC to DC and back to AC at the desired frequency. However, due to inherent limitations in the way VFDs work, the output is not a perfect sine wave, and the common mode voltage fluctuates between negative and positive values.

Due to this ‘capacitive coupling’ effect, the common mode voltage induces a current in the spinning shaft of the motor. Because the shaft of the motor is electrically insulated from the electrical ground by the bearing lubricant, 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 in turn produces a type of 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 total failure of the bearing occurs and costly repairs are required.

How to prevent bearing damage in electric motors

There are two principal ways to prevent VFD-induced bearing damage.

1. 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.
2. By fitting the motor with specially insulated bearings that prevent harmful amounts of current from passing through.

AEGIS shaft grounding rings

AEGIS Shaft Grounding Rings from eMotors Direct fit around the shaft of an electric motor and extend its lifetime by diverting shaft currents safely to the electrical ground. Utilizing advanced electron transport technology, it features a set of highly conductive, wear resistant, microfibres 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

FAG Insulated Bearings are a range of bearings distributed by eMotors Direct that are specially designed to prevent shaft currents from passing through, preventing bearing damage and extending the life of the motor. The dimensions of these bearings are in accordance with 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.

1. FAG’s ceramic-coated bearings feature a hard, wear resistant oxide-ceramic coating that provides excellent insulating properties while retaining good thermal conductance. This is important for preventing heat build-up in the bearings. A range of different coatings are available with varying properties to suit different applications.
2. Hybrid bearings feature ceramic rolling elements and steel races, which makes them able to run at higher speeds with less friction than ceramic-coated bearings. They retain very good insulating properties and are more cost effective than ceramic-coated bearings for smaller applications.

Conclusion

VFDs create currents in the shaft of electric motors that can build up and damage bearings, reducing the life of the motor, impacting productivity and incurring increased operating costs. By fitting AEGIS Shaft Grounding Rings or FAG Insulated Bearings from eMotors Direct to electric motors, operators benefit from the full-service lifetime of the motors and maintain peak performance of their equipment.

Contact us

eMotors Direct is a Canadian company focused on supporting customers in manufacturing, maintenance, and industrial settings with all their motor questions and supply needs. eMotors Direct inventories over 20 trusted brands across their 30 warehouses. Brands include Baldor, Leeson, TECO Westinghouse, KB Electronics, WEG, AEGIS Rings, FAG Bearings, and more.