Many industries use variable frequency drives (VFDs) with induction motors. Introduced in the 1960s, VFD technology has advanced considerably during the past 50 years. The engineering advancements have encouraged operators to consider other factors during particular applications, such as harmonics, energy efficiency, temperature, rise time, cable length and switching length.
Facilities that want to include VFDs often face problems during their installation. One of the most common issues associated with VFD installations is stray current, or shaft current, caused by common-mode voltage. Approximately 41 percent of the total failures in motors are the result of bearing currents. The primary cause is unbalanced current distribution with VFDs.
What Is Common-Mode Voltage?
The VFD inverter supplies unbalanced three-phase voltage to the motor. Common-mode voltages are coupled to the rotor through parasitic capacitances between the stator winding and rotor. This high-frequency voltage may result in excessive common-mode currents. Existing stray capacitances between the motor and ground may allow current to flow to the ground, passing through the rotor, shaft and bearings and reaching the grounded end shield.
The higher switching frequencies in the latest drives tend to increase common-mode voltages and currents.
In a typical greased-bearing motor, the bearings support the rotor with a thin film of grease between the balls and outer race. The rotor and grounded outer bearing race theoretically have no contact at operating speed. This is not actually the case. Residual imbalance and off-design operation result in elevated vibration that breaks down the grease. Discharge current then flows through the bearings.
The capacitive discharge component is the current that circulates whenever the grease film is momentarily broken down. The conduction component is another current induced by a ring flux in the stator yoke. This current permanently circulates through the characteristic conducting loop comprising the shaft, end shields and housing/frame. Both these electric discharges wear the raceways and erode the rolling elements of the bearings. Long-term flowing discharge currents result in furrows or fluting, reducing bearing life.
Stray Currents and Their Paths
Stray currents are typically seen in larger motors—above 444T—and poorly grounded systems, though they have been observed in all sizes of motors and applications. The current has several options when traveling to the ground, either through the motor bearings or other equipment bearings. This means that the motor does not always fail.
Some examples of paths that currents may take are: