Shaft Voltage, Common-Mode Voltage, and Bearing Currents
Last modified: Aug 02, 2026
Shaft voltage, common-mode voltage, and bearing currents describe a connected set of electrical effects that can occur when a fast-switching inverter drives an electric motor; they are related, but they are not the same quantity.
How the terms connect
- Common-mode voltage is the voltage component shared by the motor phases relative to the inverter or vehicle reference potential. Rapid switching creates high-frequency common-mode pulses.
- Shaft voltage is an electrical potential between the rotor shaft and motor frame or between the two ends of the shaft. Capacitive and inductive coupling can transfer part of the switching disturbance to the rotor.
- Bearing current flows when that voltage finds a path through a bearing or connected shaft system. If the lubricant film breaks down electrically, repeated discharges can pit bearing surfaces and contribute to fluting, noise, and premature wear.
Not every measurable shaft voltage causes damaging current. The dominant path depends on inverter edge rate, motor geometry, cabling, grounding, bearings, lubricant, and the complete drive-unit structure. Mitigation can include controlled switching, common-mode filtering, low-impedance bonding, shaft grounding, insulated or hybrid bearings, and careful cable and mechanical design.
For the wider construction and packaging context, see Electric Motor Construction and Design Choices.