Do I Need Variable Torque or Constant Torque?
Posted by Beckie Sullivan on Dec 19th 2023
Constant torque, variable torque, heavy duty, normal duty… what does it all mean?
As a large, stocking distributor for variable frequency drives (VFD), this question comes up a lot.
This blog will help you determine what each means and steer you in the right direction for what you need.
Cheat Sheet
● Constant Torque = Heavy Duty
● Variable Torque = Normal Duty
Constant Torque Loads
Constant torque, also referred to as heavy duty, is used in applications where the torque requirements are steady and unchanging regardless of speed.
Constant torque loads (heavy duty) often require a minimum of 150% overload for 60 seconds.
Conveyors, mixers and presses are great examples of constant torque applications.
This load requires high break away torque which is why the minimum of 150% overload capacity for 60 seconds is important.
Variable Torque Loads
A variable torque load, often referred to as normal duty, has torque requirements that vary from low at lower speeds to high at higher speeds.
Variable torque loads (normal duty) often require a minimum of 110% overload for 60 seconds.
Most HVAC applications, including pumps and fans, are ideal for a variable torque drive.
These applications do not need high break away torque so a drive with at least 110% overload capacity for 60 seconds is enough.
Constant AND Variable Torque Ratings?
This just means that a manufacturer has tested, and offers, two different ratings for a drive; one for constant torque and one for variable torque. These drives are often referred to as ‘dual rated’.
You would not want to use a variable torque rating to choose a drive for a heavy duty application or a constant torque rating for a normal duty application.
Using the wrong torque rating could result in the motor stalling or not being able to use your motor at full operating capacity or the desired efficiency.
Dual ratings allow you to properly choose your drive based on the ratings your application needs.
Marshall Wolf Automation is the only source you need for variable frequency drives! We are an authorized stocking distributor for multiple lines.
This means we know our lines, if the drive you need is backordered we will help you find other options.
