Electrical Bearing Damage

Bearing currents are high-frequency currents that are caused by voltage on the motor shaft, which drives a discharge current through the motor bearings. This is rarely a problem in electric motors operating on line voltage. But variable frequency drives (VFDs) put out pulse width modulated voltage, which can be a big problem for inverter-fed motors.

When operated by VFD, the power to the motor is a series of positive and negative pulses instead of a smooth wave. Therefore, the input voltage is never balanced because the voltage is either large and positive or large and negative, with rapid switching between plus and minus states in all three phases.

The average voltage between the three phases is called common-mode voltage. It usually has a “square wave” or “6 step” voltage waveform.

The two destructive types of bearing current in well-grounded VFD-fed motors are capacitive EDM current and high-frequency circulating current.

An electric motor works like a capacitor. The common mode voltage from the VFD creates a capacitively coupled voltage on the motor’s rotor and shaft. This shaft voltage discharges by arcing through the bearings to the frame. This arcing comprises capacitive EDM current.

The shaft voltages is measurable with an AEGIS^® Shaft Voltage Tester Digital Oscilloscope and AEGIS SVP Shaft Voltage Probe Tip. Discharges appear as very rapid (vertical line) changes of the voltage back to the baseline level.

These discharge arcs – which may occur tens of thousands of times per second – degrade the bearing grease and produce electrical discharge machining (EDM): thousands of microscopic pits in the bearing’s race. These pits, and the degraded grease, produce increased friction and noise coming from the motor. Eventually, the bearing may develop stripes called fluting, and even more noise. By the time fluting is audible, bearing failure is imminent.

High-frequency current in the VFD’s output moves capacitively to the motor frame and then travels through the frame seeking ground. As this current moves through the frame, it produces a high-frequency magnetic flux inside the motor. This flux, in turn, induces a high-frequency end-to-end voltage on the shaft. In large motors, with an output exceeding 100HP/75kW, the end-to-end shaft voltage is sufficiently high to drive a high-frequency circulating current through the bearings.

This circulating current moves from the shaft to the frame through one bearing, and from the frame to the shaft at the other. The net result is that current circulates between the shaft and the motor frame, damaging both bearings simultaneously.

Although high-frequency flux is present in all motors on drives, it is usually only strong enough to cause circulating current in larger motors, those exceeding 100HP/75kW. Capacitive shaft voltage, on the other hand, is present in all motors run by VFDs, so all motors on drives are susceptible to capacitive EDM current.

Over time, the microscopic pits caused by arcing through the bearing build up to a visible matte condition called frosting.  A frosted bearing is less smooth than a healthy bearing, and so has higher friction and produces more heat, which further degrades the bearing race.  Over time, a complex interplay of electrical damage and vibration produces a striped or picket-fence pattern known as fluting.

By the time fluting develops, bearing failure is imminent.  A fluted bearing causes excessive vibration and a characteristic shrieking sound.

The three stages of electrical bearing damage are shown at the left.  A comparison of a new, healthy bearing and a bearing with severe electrical damage is shown below.