Power and energy producers are turning to energy efficient devices to save money. One such device is the variable frequency drive (VFD), an electric motor accessory that can improve a motor’s life and reduce maintenance.
VFDs have given manufacturers the energy-efficient approach they desire. Yet using them without taking precautions could mean premature bearing failure and reduced motor life.
VFDs and bearing failure
VFDs are motor controllers that drive an electric motor by fluctuating the supplied frequency and voltage. This allows the motor a fast switching time and the ability to operate at less-than-full speed while maintaining the required torque. There are many benefits, including a reduction in power consumption and an up-to-30 percent cut in energy costs. Overall, users can expect their payback periods decreased to less than a year.
But VFD-run motors are more susceptible to premature bearing failure. This happens because damaging voltages are induced on the motor shafts. The extremely high on/off switching speeds of the pulse width modulation generated by the insulated gate bipolar transistors induce damaging voltages onto the motor shaft through parasitic capacitive coupling between the stator and rotor. These voltages discharge through the bearings and initiate damage in the bearing raceway, reducing the reliability of the bearings and causing motor failure.
discharge through the bearings and initiate pitting in the bearing raceway.
Why does bearing protection matter?
Bearings are essential to ensuring that a motor runs smoothly and consistently. Damaged bearings can lead to an increase in operation costs due to:
Reduced operating efficiency
Other issues have the potential to occur. Fluting, or concentrated pitting, can happen along the raceway surface. You can identify fluting by a distinctive washboard pattern. Fluting can cause excessive noise and vibration. These damages can lead to reduced motor lifetime and decreased system reliability. If you're operating a motor with a VFD, you also want bearing protection.
100hp Induction Motor
Repair cost: $3,500
Lost production cost: $$$$
Shaft grounding rings and bearing protection
Installing shaft grounding rings is the most reliable way to defend electric motors from electrical bearing damage. Shaft grounding rings are placed near either end of the shaft. They protect bearings with carbon fiber brushes attached to the ring. The ring offers an alternate path to discharge the capacitively induced voltages. The path moves the voltages away from the bearings and to the ground, preventing them from premature failure.
With the increased use of VFDs, we recognize the need to make shaft grounding rings standard equipment on motors. Marathon's Bearing Current Protection (BCP) usually consists of an AEGIS® Shaft Grounding Ring. This ring gives electric current a low-resistance path between the shaft and frame. Rather than arcing through the bearings, current flows harmlessly through the ring.