Shaft Voltage and Bearing Currents

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, over 100HP/75kW, this end-to-end shaft voltage is large enough to drive high-frequency circulating current through the bearings.

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

Although high-frequency flux is present in all motors on drives, the flux is usually only strong enough to cause circulating current in larger motors, over 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.

Measuring the shaft voltage on VFD-driven motors provides you with valuable information to determine if there is a potential risk of bearing damage from electrical discharges. We recommend surveying and documenting shaft voltage readings using the AEGIS Shaft Voltage Tester Digital Oscilloscope.

Examples of shaft voltage readings:

• High Peak-to-Peak Common Mode Voltage
• High Amplitude EDM Discharge Pattern
• Low Amplitude Voltage Discharge Pattern

Learn more about identifying shaft voltage types and the right solution to mitigate them in our handbook.

The AEGIS Bearing Protection Handbook walks you through how bearing damage occurs and what steps are needed to prevent that damage. Download your copy and learn how to apply the best practices for bearing protection in new and repaired motors, test in-service motors, and inspect damaged motors. Our guide is available in several languages so you can access this valuable information no matter where you are.

Download Bearing Protection Handbook