• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer
  • Advertise
  • Subscribe

Motion Control Tips

Automation • Motion Control • Power Transmission

  • News
    • Industry News
    • Editor Blogs
  • Controls
    • HMIs
    • PC-Based Controllers
    • PLCs + PACs
    • Stand-Alone Controllers
    • Software
  • Drives
    • Servo Drives
    • Stepper Drives
  • Encoders
    • Absolute Encoders
    • Incremental Encoders
    • Rotary Encoders
  • Mechanical
    • Bearings
    • Brakes + Clutches
    • Belt + chain
    • Couplings
    • Gears + Gearing
    • Lubrication
    • Shock + Vibration Mitigation
    • Springs + Rings + Seals
  • Linear
    • Actuators
    • Linear Motors
    • Linear Encoders
  • Motors
    • AC Motors
    • DC Motors
    • Brushless Motors
    • Gearmotors
    • Piezo Motors
    • Servo Motors
    • Stepper Motors
  • Systems
    • Conveyors + linear transport systems
    • Gantries + Stages
    • Rotary Tables
    • Grippers + End Effectors
    • Robotics
  • Networks
    • Connections + Sliprings
    • Fieldbuses
    • I/O
    • Sensors + Vision
  • Resources
    • FAQs
      • Motion Casebook
      • Motion Selection Guides
    • Suppliers
    • Video
You are here: Home / FAQs + basics / What are bearing currents and what causes them?

What are bearing currents and what causes them?

October 8, 2020 By Danielle Collins Leave a Comment

Pulse-width modulation is a fundamental operating principle of variable frequency drives (VFDs), but the high-frequency switching that delivers pulses of voltage from the drive to the motor can cause bearing currents — high-frequency currents that flow through the motor bearings, often leading to damage and premature failure.


To achieve pulse-width modulation, insulated gate bipolar transistors (IGBTs) in the variable frequency drive switch on and off rapidly to create a simulated sinusoidal AC waveform. Because the power is delivered in pulses, rather than as a true sine wave, the power is never balanced — that is, the sum of the three phases of power supplied to the motor is never equal to zero. This imbalanced voltage is referred to as common mode voltage (CMV), and it is one of the primary causes of bearing currents in motors driven by VFDs.


Yaskawa provides the following definition of common-mode voltage in AC motors driven by VFDs:

The voltage difference between the power source ground (the DC bus midpoint) and the neutral point of a three-phase load (the stator neutral of the AC motor).

common mode voltage
Common mode voltage, also referred to as DC-offset, is unblanced power delivered to the motor by a VFD.
Image credit: Eaton Corporation

Capacitive discharge current

The fast switching time of the IGBTs creates parasitic capacitance, which causes capacitive coupling between the stator and rotor. This capacitive coupling induces a voltage (due to the common mode voltage caused by the drive) across the air gap between the stator and rotor. As the voltage searches for a path to ground, it travels through the motor shaft (which is connected to the rotor) and across the bearings.

Lubrication in the bearing normally acts as an insulator, but because the common mode voltage has a very high rate of change, known as dV/dt (due to the switching of the IGBTs), it can easily exceed the breakdown voltage of the lubrication — that is, the voltage at which the lubrication no longer acts as an insulator. When this happens, a burst of current is released, known as capacitive discharge current.

This capacitive discharge current can cause fretting of the bearing and raceways and produces temperatures high enough to melt the bearing steel and cause pitting through electrical discharge machining (EDM). This discharge can happen with every switching cycle, or at least frequently enough that over time, washboard-like ridges, known as fluting, will occur on the raceways, causing vibration and audible noise.

bearing currents
Capacitive discharge currents (yellow) and high frequency circulating currents (red) in an AC induction motor driven by a VFD.
Image credit: Aegis

High frequency circulating current

bearing fluting
Bearing currents can cause washboard-like ridges, known as fluting, on the raceways of the motor bearings.
Image credit: ABB

Common mode voltage induces common mode currents that travel through parasitic capacitances in the motor. This common mode current produces high frequency magnetic flux and currents with a frequency in the kHz or even MHz range. These currents circulate from the shaft, through one motor bearing, through the motor frame, to the other motor bearing, and back to the shaft, causing a potential difference across the motor shaft that can become high enough to exceed the bearing lubrication’s insulating properties.

This high frequency circulating current produces the same types of damage — fretting, EDM, and fluting — as capacitive discharge current. High frequency circulating currents are primarily an issue in motors over 100 hp (75 kW), with their potential for damage increasing with larger motor sizes.

You Might Also Like

Filed Under: AC Motors, Drives + Supplies, FAQs + basics, Featured, Uncategorized

Reader Interactions

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

LEARNING CENTER

Design World Learning Center

Motion Control Handbook

“mct
EXPAND YOUR KNOWLEDGE AND STAY CONNECTED
Get the latest info on technologies, tools and strategies for Design Engineering Professionals.

RSS Featured White Papers

  • Robotic Automation is Indispensable for the Logistics Industry’s Continued Growth and Success
  • Reliable Linear Motion For Packaging Machines
  • Polymers Outperform Metals In Precision Gearing

Footer

Motion Control Tips

DESIGN WORLD NETWORK

Design World Online
The Robot Report
Coupling Tips
Linear Motion Tips
Bearing Tips
Fastener Engineering.
Wire and Cable Tips

MOTION CONTROL TIPS

Subscribe to our newsletter
Advertise with us
Contact us
About us

Copyright © 2025 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy | RSS