Industrial Engineer · phase 13 of 16
Common-Mode Voltage
Voltage shared by motor phases relative to ground that can drive leakage and bearing currents.
The Explain card
- Plain English
- Common mode voltage is the voltage that all three motor phases share relative to ground at any instant. On a sine supply it averages to zero, and on a PWM output it jumps in steps at the switching rate.
- Example
- A scope on a motor's neutral point shows a stair step waveform hundreds of volts high, switching thousands of times a second. That is the common mode voltage the cable and the motor frame are seeing.
- Why it matters
- It drives current through every stray capacitance, including the motor bearings, the cable shield and the ground system. Bearing damage, radio interference and nuisance ground fault trips all trace back to it.
- Hook
- The part of the output the motor was never meant to see.
Where it sits in the deck
Phase 13: VFD Power Quality, Wiring, and Motor Protection
Harmonics, reflected waves, shielded cable and bearing currents: the side effects a drive inflicts on everything around it.
