Electric motors fitted with variable frequency drives (VFD) are at risk of bearing damage due to a build-up of current in the rotor that discharges through the shaft. This damages the lubrication and bearing surfaces, producing harmful vibration that eventually results in bearing failure. The overall life of the motor is reduced and significantly impacts productivity while incurring considerable costs in equipment repair and downtime.
Three-phase electric induction motors operate on a power supply that consists of three alternating current inputs that are each separated by one third of a phase. Importantly, 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 (common mode voltage) remains at zero, and current does not build up in the motor’s rotor.
One of the most popular and useful methods of speed control for induction three phase motors is by using a VFD. This device allows the speed and torque of the motor 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 then sends out a DC pulse to simulate the sinewave. 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.
Due to an effect known as capacitive coupling, the common mode voltage induces a current in the spinning rotor of the motor. Because the shaft of the motor 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 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.
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 are the perfect option on motors that are under 100 hp. They 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.
Another option, or in conjunction with grounding rings, is FAG Insulated Bearings. They’re 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’re interchangeable 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. This is important for preventing heat build-up in the bearings. A range of different coatings is available with varying properties to suit different applications.
Hybrid bearings feature ceramic rolling elements and steel races for running 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.
VFDs create shaft currents in many electric motors that can damage the bearings, impacting productivity and incurring increased operating costs. By fitting AEGIS shaft grounding rings or FAG insulated bearings on your next overhaul or new purchase, you will benefit from the full service lifetime of your electric motors and maintain the peak performance of your equipment.
William Schrottky is a technical writer at eMotors Direct Inc., an online supplier of electric motors, parts and accessories based in Edmonton. For information about preventing VFD-induced bearing damage, visit https://www.emotorsdirect.ca.