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. 

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. 

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.

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. 

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. 

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. 

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.