Constant Torque vs. Variable Torque:

Our next topic for the “Did You Know” article series is a question that is thrown out quite a bit for VFDs. 

Do I need a Constant Torque Rated Drive or a Variable Torque Rated Drive? 
Sometimes simply referred to as CT and VT.

This has to be the most ridiculous concept for a “drive”, because drives have no twisting ability, which is exactly what torque is.  But what they are really referring to is the loading profile of the motor.

Variable Torque is probably the easiest to describe first.  This means that the faster the motor spins, the more load the motor sees.  Two applications fit this one perfectly, fans and pumps.  The faster the fan spins the more amperage it takes to do so. These drives have very little overload built into them, typically 110 to 115%.  Why so little overload capability?  Well, that’s because it very rare that all of a sudden a fan gets overloaded.

Now onto Constant Torque drives.  These are drives that see near constant loading throughout the speed range, but might see overloads on occasion.  The drive has at least 150% overload rating for 1 minute and around 175% for 3 or more seconds.  This means the drive keeps going with these types of overloads.  Probably one of the best examples is a conveyor that is being loaded with dirt.  Each time a new load of dirt gets dropped on the conveyor the motor sees this as a shock load, but has enough overload to keep going.

Drives are rated in HP for convenience, but in reality they are rated in amps and that’s what you need to make sure is covered. See the HP vs. Amps “Did You Know” article for more info.

Special thanks to National Power Quality and
Consulting for providing these Did You Know articles.

For electrical consulting needs, NPQC can be contacted at www.natpowerquality.com