What is the right pump VFD?

By Tyler Simmons on 13th Apr 2023

What VFD Should I use for my pump?

Variable frequency drives are a great way to control the speed of a pump. With many pumping applications, you encounter affinity laws. This means you save energy and cost with a VFD at a high rate while you decrease the speed. You also reduce mechanical stress when you control flow through a VFD rather than with valves. 

If you know that a VFD is right for your pumping application, the next question is which one to use? There are many brands, and each brand has multiple series. There are several factors to consider when choosing the VFD to run your pump. 

Load and Overload

Sizing VFDs is always based on the overload needs of the motor and driven equipment, in this case a pump. VFD manufacturer labeling of overload is not standardized. Many use terms like light duty, normal duty, and heavy duty.

To simplify your evaluation, look at the overload percentage over 60 seconds. A variable torque rating means the VFD will allow the motor to run up to 110% or 120% of its rated amps for one minute. A constant torque rating means the VFD will allow 150% for that same time. 

Centrifugal Pumps

A centrifugal pump is usually the easiest type of pump for a motor to run. The basic construction is to have an impeller that spins to output the fluid. 

These are often used in clean water applications or in chemicals. They have higher flow but work best with less viscous fluids. Centrifugal pumps usually only require 110-120% overload from the motor.

Reciprocating Pumps

Positive displacement pumps come in several forms. First is reciprocating pumps, including piston, plunger, and diaphragm styles. Your pumping process has two strokes. First is suction, which pulls in the fluid through the inlet. Second is compression, which forces the fluid out through the outlet. 

These pumps are often used in more intense applications, like slurries or wastewater. They have less flow than centrifugal pumps but work better on fluids with higher viscosity. These pumps typically need a VFD rated for 150% overload. 

Rotary Pumps

The other type of positive displacement pump is a rotary pump. These could be gear or screw pumps that work by turning and forcing fluid through the gaps of the gear or screw.

Rotary pumps are like reciprocating pumps. They have a lower flow rate and the ability to work better with more viscous fluids. These also need a VFD with a 150% overload rating.

Different rotary pumps


Where you place your inverter is also important. There are a few variables to watch for that tend to be more common in pumping situations than in general applications.


There are two main concerns with liquid. The first is direct contact, like splashing or washing down equipment. Some VFDs operate near the pump and may become wet. For this protection, you should enclose it in an appropriate panel, rated NEMA 4 or 4X.

If it is outside and rain or snow is the concern rather than splashing, then you would likely want to put the VFD in a NEMA 3R enclosure.

The other concern with liquid is humidity. Many pumping applications have high humidity, either naturally or because of the surrounding processes. Letting electrical equipment function in a humid environment can cause premature failures. Look into heaters, filters, and enclosures to solve this issue.


Pumps are often outdoors or in warm manufacturing facilities. Heat is one of the biggest killers of all electrical equipment, including VFDs. If the VFD is not kept in a climate-controlled area like an electrical room, it needs to have appropriate cooling fans or air conditioning units. 


Many pumps are moving chemicals or are near products that need to be cleaned with chemicals. Not only will the liquid from chemicals damage a VFD, but corrosive materials and gases can also cause damage. 

If you are concerned that chemicals may be an issue, you should investigate NEMA 4X enclosures. You also should reach out to our experts to talk through your situation as these ratings and protection methods can get complicated quickly. 

Questions? We can help!

Our indsustry experts can answer any questions you may have and find you the right VFD for your pump system.

Lead Length

Pump systems often have a long cable running from the VFD to the motor. This is especially prevalent in down well pump applications. Long lead lengths between the drive and motor can lead to reflective wave phenomenon, or dV/dt. This can cause your motor to prematurely fail. 

If you have a cable over a couple hundred feet, you should look into output filters. These may be load reactors, dV/dt filters, or sine wave filters based on your situation. 

Just because you have a submersible or down well pump does not mean your motor is far from the VFD. It’s possible that your motor is at the top of a well with your pump at the bottom. Make sure you’re confident in the lead length between the VFD and motor as that is what causes dV/dt. 

Which VFD to Choose?

Once you’ve evaluated the factors above, you should be ready to choose your VFD. For lighter applications like centrifugal pumps, you’ll often want to find a VFD labeled as “general purpose.” For tougher applications like positive displacement pumps, look for “industrial” or “heavy duty” VFDs. 

Make sure the NEMA rating matches. If you need a NEMA 3R, 4, or 4X enclosure, you will usually need to have your VFD chassis integrated into a panel. If you need an output filter, these will often be built into the same panel. 

Most brands will carry the VFD chassis you need, and many will know an integrator to work with. The brands we recommend most will vary, but we have seen success with Galt VFD chassis built into MDI panels when needed. 

The Galt G500 line is capable of derating for either variable or constant torque loads, is simple to interact with, and has a warranty longer than the industry standard. 

MDI control panels are custom-built in a UL508A shop. The brand has been dealing with VFDs, harmonics, and dV/dt for decades and has extensive experience in pumping applications. 

Let us help you choose

We’ve helped many customers put VFDs on their pumping applications. If you still have questions or want to talk through your specific situation, reach out to us and we can help you.