With the increase in the use of variable frequency drives today, a maintenance concern has arisen and that is circulating currents in a motor. When the current exceeds the dielectric properties of the bearing grease it arcs over in the bearing and goes to ground leaving a small arc burn in the bearing race. As this continues to damage the race eventually the bearing will fail. Today 30% of premature motor failures are caused by bearing failures due to these currents.
With the increase in the use of variable frequency drives today, a maintenance concern has arisen which is circulating currents in a motor. When the circulating in the motor current exceeds the dielectric properties of the bearing grease it arcs over in the bearing and goes to ground leaving a small arc burn in the bearing race. As this continues to damage the race eventually the bearing will fail. Today 30% of premature motor failures are caused by bearing failures due to these currents.
Because of the high-speed switching frequency in Pulse Width Modulated (PMW) inverter, variable frequency drives induce shaft currents in AC motors. The switching frequencies of the switching transistors used in these drives produce voltages on the motor shafts during normal operation. These voltages which can register10 to 40 volts peak are easily measured by touching an oscilloscope probe to shaft while the motor is running.
Once these voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance. This is typically through the motor bearings to the motor housing. During virtually every switching cycle of the VFD, induced shaft voltage discharges from the motor shaft to the frame through the bearings, leaving a small fusion crater in the bearing race. When this happens temperatures are hot enough to melt bearing steel and damage the bearing grease.
These discharges are so frequent that before long the bearing race has numerous pits called frosting. Frosting may result in what is called fluting of the bearing race which is wash-board like ridges in the race. Fluting causes excessive noise and vibration and depending on what the motor is driving can be magnified and transmitted through the facility. The result is a motor failure that could be very expensive in repairs and lost production.