Bearing currents have been around in one form or another since Tesla’s invention of the induction motor in 1887. A recent search of the Institute of Electrical and Electronics Engineers (IEEE) Xplore database revealed more than 4,200 papers written and archived discussing this subject. Of these, roughly half were authored in the last 10 years, indicating that the understanding of these currents continues to grow through experimentation and analysis. Understanding the mechanism by which bearing currents are generated provides insight into best practices for remediation.
Bearing currents occur when voltage is induced on the motor shaft that is high enough to overcome the breakdown voltage of the bearing lubricant. There are two typical paths for this current to flow. The first is from the shaft, through a bearing, and through the motor or load frame to ground. The second path is for the current to circulate from one end of the shaft, through a bearing, through the motor frame, into the opposite bearing and back into the shaft.
The source of the induced voltage on the shaft can vary, depending on several factors. For fixed frequency, line-powered motors, the bearing currents are internally sourced, which means the net flux encircling the shaft is caused by magnetic imbalances inherent to the machine. Electrical steel, for example, is not totally homogenous, resulting in flux paths that are not perfectly symmetrical. This asymmetrical flux results in time-varying flux lines that enclose the shaft. As Faraday’s law explains, this time varying net flux gives rise to current flow down the shaft and through the bearings. As C.T. Pearce stated in 1927, “if it were possible to design a perfectly balanced and symmetrical machine, both theory and practice indicate that no bearing current could exist.”1 Small shaft voltages [on the order of 500 millivolts (mV)] can lead to bearing currents above 20 amps.
Common Mode Voltage & Bearing Current
For induction motors operated by adjustable speed drives (ASDs), such as inverters or variable frequency drives (VFDs), a second, external source of shaft voltage exists. This external source is a result of the voltage wave shape provided by the inverter. Unlike balanced, three-phase sine wave operation, VFDs create switching patterns where the instantaneous average voltage to ground is not zero.
This instantaneous voltage is referred to as common mode voltage (CMV). The voltage changes rapidly in magnitude with respect to time (high dV/dt), so its frequency content can be in the MHz range. As the current through a capacitor is defined by the equation I=C*dV/dt, this rapidly changing voltage can result in capacitively coupled currents from the motor windings to ground through several paths.
The impedance of a capacitor varies inversely with frequency. High-frequency currents (such as those from a VFD) can flow through paths normally considered to be insulators such as stator slot liners, stator-to-rotor air gap or the grease film between the bearing race and balls.
The key to mitigating these currents is to provide low impedance ground connections or alternate conductive paths to ensure the current is channeled away from the bearings. (2)
Image 4 shows four potential paths for high-frequency currents caused by inverter usage. The path in red is a capacitively coupled current from the stator to the rotor through the air gap, with a return path through the motor bearings and ultimately to the drive ground. Current may flow through the motor bearing if the shaft is bonded to the frame (through bearing ball contact) at the instant the dV/dt transition occurs in the CMV.
Discharge current may flow if the bearing first acts as an insulator, then becomes a conductor (due to ball spacing or grease film thickness combinations). Discharge current may also flow if the voltage across the bearing grease film exceeds its breakdown voltage.
The green current path is also due to capacitive coupling between the stator and rotor (across the air gap). In this case, the current flows through a conductive coupling, through the load bearing and load ground, ultimately to the drive ground. Discharge current may flow just as with the previously described red path case. Damage to the load bearing and/or coupling may occur.