Industrial Engineer · phase 9 of 16 · noun
Variable Torque Rating
Drive duty rating intended mainly for centrifugal fan and pump loads whose torque demand falls rapidly with speed.
The Explain card
- Plain English
- The variable torque rating is the drive's rating for centrifugal fans and pumps, whose torque demand falls with the square of speed. Because those loads never need heavy overload, the drive is allowed to run at a higher continuous current.
- Example
- The same drive that carries 40 amps constant torque is rated 50 amps variable torque. On a cooling tower fan that only ever starts unloaded, the 50 amp rating is honest.
- Why it matters
- Variable torque is the cheaper way to buy a drive for a fan or pump, and the wrong way to buy one for anything that can stall or start loaded. Ask what the load does at low speed before you take the discount.
- Hook
- A lighter duty for loads that go easy on the motor.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
During drive selection, the engineer used the Variable Torque Rating column to match the VFD output current to the pump motor nameplate.
Why these words
- variable Late Latin variabilis, changeable modifies the type of duty being rated
- torque Latin torquere, to twist names the rotational quantity used to classify the duty
- rating English rate plus -ing marks the phrase as an assigned equipment capability
Where it came from
- Origin
- English engineering compound formed from variable, torque, and rating
- Entered the language
- 20th century
- What changed
- Variable moved from general changeability into technical classification, torque acquired its mechanical sense of rotational effort, and rating developed an equipment-capability sense.
How it is spelled
- Pattern
- The phrase is written as an open compound with spaces between its three words.
- Pattern
- Variable uses the productive suffix -able.
- Pattern
- Torque ends in -que, with the final letters representing a pronounced k sound.
- Pattern
- Rating drops the final e of rate before adding -ing.
Spelled like
- adjustable
- programmable
- technique
- opaque
- coding
- making
Broken into chunks
-
vari
- vary
- variation
-
able
- adjustable
- programmable
-
torque
- torsion
- contort
-
rating
- rated
- derating
What it sits beside
Same subject
- Constant Torque Rating
- overload rating
- output current
- motor nameplate
- VFD sizing
Same shape
- Constant Torque Rating
- current rating
- power rating
VFD duty ratings
- Variable Torque Rating
- Constant Torque Rating
drive selection data
- output current
- input voltage
- overload rating
Where it sits in the deck
Phase 9: VFD Ratings, Sizing, and Motor Data
Nameplate arithmetic, where the currents, voltages and duty ratings decide whether a drive can actually run the motor you bolted it to.
