Industrial Engineer · phase 13 of 16 · noun
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.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
During VFD commissioning, the technician measured common-mode voltage at the motor terminals before selecting the output filter.
Why these words
- common Latin communis, shared or public, through Old French marks a component considered jointly across multiple conductors
- mode Latin modus, measure or manner, through French classifies the manner in which the electrical quantity is arranged
- voltage volt, named for Alessandro Volta, plus the noun suffix -age serves as the head noun naming the measured electrical quantity
Where it came from
- Origin
- an English electrical-engineering compound formed from common, mode, and voltage
- Entered the language
- 20th century
- What changed
- Common developed from shared or public, mode acquired a technical sense for a signal arrangement, and voltage became the standard name for an electrical potential quantity.
How it is spelled
- Pattern
- Common-mode is hyphenated when it acts as a compound modifier before voltage.
- Pattern
- Voltage keeps the -age noun ending found in words formed from a base noun.
Spelled like
- common-mode current
- differential-mode voltage
- common-mode choke
- amperage
Broken into chunks
-
common
- communal
- community
- commune
-
mode
- modal
- modality
- modify
-
volt
- voltmeter
- voltampere
- kilovolt
-
-age
- amperage
- wattage
- percentage
What it sits beside
Same subject
- differential-mode voltage
- common-mode current
- ground-fault current
- motor cable
- output filter
Same shape
- common-mode current
- differential-mode noise
- common-mode choke
signal modes
- common mode
- differential mode
VFD output measurements
- phase voltage
- line voltage
- common-mode voltage
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.
