Industrial Engineer · phase 10 of 16 · noun

V/Hz Ratio

Volts-per-hertz relationship used below base speed to maintain suitable motor magnetic flux.

The Explain card

Plain English
The volts per hertz ratio is the rule that ties motor voltage to frequency below base speed. Keep the ratio constant and the magnetic flux in the motor stays where it was designed to be.
Example
A 460 volt 60 hertz motor needs just under eight volts per hertz. At 30 hertz the drive applies about 230 volts, at 15 hertz about 115, and the motor makes full torque at each.
Why it matters
Too much voltage for the frequency saturates the iron and overheats the motor, too little starves the flux and the torque collapses. Every drive control mode rests on this ratio, even the ones that hide it behind a motor model.
Hook
Keep the ratio, keep the magnet.

Word knowledge

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

During commissioning, the technician set the V/Hz ratio to 460 V divided by 60 Hz before running the induction motor.

Why these words
  • V symbol for volt, named after Alessandro Volta supplies the numerator and identifies the voltage unit
  • Hz symbol for hertz, named after Heinrich Hertz supplies the denominator and identifies the frequency unit
  • ratio Latin ratio, reckoning or calculation marks the expression as a comparison between two quantities
Where it came from
Origin
20th-century electrical-engineering notation combining the SI symbols V and Hz with ratio from Latin ratio
Entered the language
20th century
What changed
Ratio moved from reckoning to mathematical comparison, while Hz replaced cycles-per-second notation in modern engineering usage.
How it is spelled
Pattern
V and Hz retain their official SI capitalization
Pattern
the slash is read as per
Pattern
ratio remains a separate lowercase word
Breaks the pattern
Hz has a capital H and lowercase z because it comes from the proper name Hertz

Spelled like

  • V/f ratio
  • torque-speed ratio
  • power-to-weight ratio
Broken into chunks
  • V
    • VDC
    • VAC
  • Hz
    • kHz
    • MHz
  • ratio
    • turns ratio
    • gear ratio
What it sits beside

Same subject

  • base frequency
  • motor nameplate
  • frequency reference
  • voltage boost
  • variable frequency drive

Same shape

  • V/f ratio
  • volts per hertz

drive commissioning settings

electrical unit symbols

  • V
  • Hz
  • A

Where it sits in the deck

Phase 10: VFD Electrical and Motor Fundamentals

The physics under the parameters: slip, torque, flux, and the relationship between frequency and voltage that every drive is really controlling.

Neighbours in this phase