Pumping With VFDs - Don't Be Left Without Torque

Author: Tom Schroeder | December 11, 2014 | Category: Variable Frequency Drives

PumpSmartAs focus on energy efficiency grows, we see the use of variable frequency drives (VFDs) become more of the norm. Understanding how much horsepower your pump requires as you lower the speed can keep you from oversizing your motor and keeping you from running out of torque. This blog post will explain the terms related to, and effects of using, a VFD with your pump.


Pumping applications can be categorized as constant torque (CT) or variable torque (VT). This refers to the relationship of the torque required by the pump to a reduction in speed. Positive displacement pumps can be slowed to decrease the flow rate but the pressure may be constant. For example, if a gear pump is used to pump hydraulic oil to a pressurized cylinder, the pump can be slowed down to limit the flow rate but the cylinder is still at the same pressure.  

Conversely, when you slow down a centrifugal pump, the performance (pressure and flow) capability is reduced. This is a variable torque application since the pump will require torque proportional to the reduction in speed (see previous post, How To Use Affinity Laws To Calculate Best Efficiency Point).


A motor will carry a CT and VT rating (1000:1, 20:1, 10:1, etc) which designates the ratio you can turndown from the nominal speed without overheating the motor.  Variable torque motors have a torque output that is the square of the speed. As an example, a 10hp motor that has a nominal speed of 1800 will have 2.5hp rating at 900rpm. A CT rating is directly proportional to the reduction in speed. For instance, a 10hp motor with the nominal speed of 1800 will have a 5hp rating at 900rpm.


Specifications are loosely used to describe the requirement of an inverter rated motor. “Inverter duty” is a commonly used term which typically relates to a 1000:1 turndown. “Inverter capable” or “Inverter ready” commonly means a 10:1 or 20:1 turndown ration is required. Where these terms can convey a need for a motor capable of variable speed, the specific application should be reviewed to determine what speed reduction and the horsepower is required. Then a determination can be made as to what CT or VT rating is needed.

VFDs can help save a lot of energy, but knowing how to properly apply them is critical. If you’re considering adding a VFD, be sure to consult an experienced engineer. They can help you be sure you save as much energy as possible, while delivering the power needed for your process.

Got a question about applying VFDs in pumping systems? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin and Upper Michigan.

Tom Schroeder

Tom Schroeder

Tom Schroeder is an Application Engineer II at Crane Engineering. He has more than 7 years of experience in the general industry sector. He specializes in the proper selection of fluid processing equipment like pumps, filters, mechanical seals, and mixers.

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