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Here at Marshall Wolf Automation, we receive calls on a daily basis asking which VFD is best suited for a particular pump or fan motor application.

While our tech support team is more than happy to help our customers find the right VFD, this simply isn’t a ‘right or wrong’ type of question. In order to best match your application needs, we generally start off by finding out what type of pump you have and what features are desired from the VFD.

First, let’s start with the absolute basics; what is torque exactly?

Torque is defined as a twisting force that tends to cause rotation. For those of you who may be wondering why we’re reviewing high school physics, stick with me here.

The first step we take to determine which VFD will work best is to find out what the application parameters are.

We do this by reviewing the current, heat, and voltage requirements of the motor and application. Even though we tend to think of motor power in HP, much of the specifications we need to take into account for sizing a VFD has very little to do with the actual motor HP.

Most of the specifications our tech support team will start out with can be found on the information plate on your motor.

Pumps can be divided into constant torque and variable torque applications. One of the more common applications we end up working with is the variable torque variety, mainly involving prototypical centrifugal types of pumps. The easiest way to distinguish variable torque and constant torque is to focus on the name.

VTL (variable torque load) drives are designed to deal with alternating torque demands and can range their speed/torque from low to high. Think about HVAC fans and sump pumps. Though the drive may need to run the motor at intermittent torque and speed, the application will not be at full capacity 100% of the time. Many times these drives are designed with less overload capability compared to their counterparts due to their variability.

Conversely, constant torque loads are not dependent on speed and maintain the same level of torque regardless of demands. Applications such as conveyors, compressors or hoists incorporate these types of drives due to their higher heat and overload ratings. Unlike VTLs, constant torque drives are generally used in torque-consistent applications and may only hit 150% overload current for 30-60 second intervals (usually at start-up or when another load of dirt is added to the conveyer).

Learn what a centrifugal pump is here!

So what types of pumps and fans are available?

Roto-dynamic (Centrifugal) – simply put…they move fluids. To quote an earlier blog, “Basically centrifugal pumps are the most common type of pump. The water/fluid comes in the side and the impeller throws it up out of the pipe.”
These pumps handle variable torque loads due to the nature of the pump affinity rules, whereby, head changes proportional to the square of the speed change.

The other type of pump we will discuss is the positive displacement pumps. These fall under the constant torque load category due to their ability to pump independently of speed and pressure. This is due to the construction of the internal compartments in which the fluid is moved.

During each revolution of the shaft a constant volume of fluid is delivered to the pump’s internal compartment and then exits through the discharge pipe. While this means a reduced speed will directly impact the flow delivered, it also means that there is no direct impact on the actual differential head or pressure that the pump will have to resist.  

When considering a centrifugal pump or fan, the Affinity Laws are our best friend.

By referencing these constants we can better understand how pressure (pump head) variation is a result of the square of the change in speed and that power is a function of the cube of speed.

Using a VFD on variable torque loads allows us to take advantage of these affinity laws. As the speed of a centrifugal load decreases, the horsepower requirement will decrease with the cube of the speed. Head pressure will decrease with the square of the speed, while flow is proportional to speed.

Brand Model Series Centrifugal Pump Type Positive Displacement Pump Type
AC Tech/LENZE SMV/I500 X X*
Delta Ind. Automation EL series, MS300 X X (MS300 only) 
Delta Ind. Automation C P series X
Delta Ind. Automation C Series X
FUJI VFD ACE, MEGA X X
FUJI VFD MINI, ECO X
GE LP, GP X X
GE FP X
Hitachi NES1 X
Hitachi WJ200, SJ700D X X
Leeson FHP X
LSIS C100, S100 X X
LSIS H100, H100 Bypass X
LSIS IS7 X X
Teco Westinghouse L510 X
Teco Westinghouse E510 X X
Teco Westinghouse A510 X X
Teco Westinghouse F510 X
Teco Westinghouse N3 X
Teco Westinghouse EQ7 X X*
WEG CFW300 X
WEG CFW500
WEG CFW700 X X
WEG CFW701 X
WEG CFW11 X X

*Oversized by at least one HP increment.

Still have questions? Feel free to contact our tech support directly or give us a call and we’d be happy to help you find the right VFD for your pump!

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