A buying guide to help you choose the right VFD

A buying guide to help you choose the right VFD

Posted by Jason Justice on Sep 18th 2019

VFD Buying Guide

So your thinking about buying a new Variable Frequency Drive (VFD)? Finding the perfect VFD or motor controller can be a fairly complicated task if you are not familiar with sizing a VFD for your motor application.  However, our engineers are available by phone to provide you with the information you need to give you peace of mind when it comes to getting the right product. We want you to be able to shop for your VFDs and Variable Frequency Drive accessories with confidence. We know that having a firm understanding of the products you're shopping for is crucial to feeling secure about your purchase. That's why we've provided our customers with a wide array of VFD resources—from basic information about VFDs to VFD buying information, video tutorials and other VFD resources. Beyond putting the information at your fingertips, we also offer professional technical support for the products we sell. We have on-staff electrical engineers that are just a quick phone call or email away.

Before you get too far into the process of looking at motor controllers (VFD's), it is a good idea to gather the basic motor nameplate information that will help you in selecting the proper VFD, as seen below:

  • Horsepower
  • Full Load Amps (FLA)
  • Voltage
  • RPM
  • Service Factor
  • It is also helpful to know if the motor is inverter duty rated

Other information:

  • Type of Load (Constant Torque or Variable Torque)
  • Speed Range and Control Method
  • Special Enclosure Needs

Now that you have the motor nameplate information and how you would like to install and control the VFD, it is time to look at some of the important specifications on each inverter that will assist you in deciding what will work best for you.

Horsepower (HP)

Although it is important to size the VFD with the FLA of the motor(s) being controlled, knowing the horsepower of the load is a great way to narrowed down your search results.

Full Load Amps (FLA)

Full Load Amps are what the inverter will be sized off of.  Some people use HP, but FLA is the proper way to size VFD's.  When sizing the Variable Frequency Drive to the FLA of the motor, it is best to be a little conservative with your selection.  It is especially important if you have a high inertia load, or on that is difficult to get started.  Over-sizing the inverters amp rating will allow the VFD to run cooler and be more reliable in the long run, because you are not running the inverter at it's maximum capability.  


For three-phase input this one is easy. It is crucial to match the voltage of the VFD and motor to your available voltage on site. For the US low voltage application, this is usually either 208 VAC, 230/240 VAC, or 460/480 VAC. 

For a single-phase input, it is important to take a few more things into consideration. If the load is 3 HP (roughly a 230 VAC motor with an FLA below 13.3 amps) or below it is likely that we have a inverter built for single phase input in stock for you to hook up and run with. If your needs are greater than 3 HP, then it is possible to use a drive built for three phase input, however the drive must be properly sized for your application.  Usually this means over-sizing your FLA by 2 times to compensate for the current not being carried by the third leg of a three phase circuit.  To get around this we have come up with a single to three phase solution, for bringing in single phase into the system and getting three phase out to the motor, without over-sizing the VFD by 2 times. 

Type of Load (Constant Torque or Variable Torque)

Does your variable frequency drive application require a variable torque or constant torque drive?

If the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive will be more appropriate. Energy savings are usually the primary motivation for installing variable frequency drives for centrifugal applications, and variable torque drives offer the greatest energy savings.

For example, a fan needs less torque when running at 50% speed than it does when running at full speed. Variable torque operation allows the motor to apply only the torque needed resulting in reduced energy consumption, which is one of many VFD Benefits. Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. In which case, constant torque variable frequency drives would be more appropriate for the job.  Thankfully all of our VFD's are sized for constant torque(Heavy Duty 150%) and Variable Torque(Normal Duty 120%) applications.  

Speed Range

Generally speaking, a motor should not be run at any less than 20% of its specified maximum speed allowed. If it is run at a speed less than this without auxiliary motor cooling, the motor will overheat. Auxiliary motor cooling should be used if the motor must be operated at slow speeds.

In addition you should take care when over speeding a motor. Generally it is not a good idea to run the motor more than 20% above its rated speed. You need to check with the manufacturer of your motor to ensure what speed range will still be allowed within their warranty. Keep in mind that you also lose torque as you go above the design speed. Additionally, you should make sure that you do not run your motor consistently above the FLA rating.

Control Method

  • 2 wire control or 3 wire control (2 wire control usually is a maintained switch with the off position stopping the drive and the on position starting the drive.  3 wire control allows the use of a start and a stop button.)
  • Speed Potentiometer – Allows the operator to set motor speed with pot.
  • Digital Programming / Display Unit – Allows the operator to program and troubleshoot the drive by inputting values through a keypad with an LED or LCD display unit. Drive operation can also be monitored through this display.
  • Analog Signal Follower – 4-20mA or 0-10VDC; must provide variable frequency drives with an isolated input; must use a twisted/shielded pair and keep wire away from 3-phase AC, especially PWM.
  • Selector switch speed selection – Allows the operator to select from several preset speeds. Can also be used if the speed is being set via a PLC, and an analog output is not available.
  • Serial Communications – Allow variable frequency drives to communicate on a network, such as MODBUS RTU, enabling drive operation to be coordinated and monitored from a PC.
  • Ethernet Communications – Allow variable frequency drives to communicate on a network, such as MODBUS TCP/IP, RTMoE (Real Time Motion over Ethernet), Ethernet/IP, etc.
  • System Integration module slots are available to fit additional fieldbus, Ethernet, position feedback, machine control, and expanded I/O options.

Custom VFD Buildups

At this time you may also consider other options and VFD Accessories such as:

  • Disconnect or Circuit Breaker
  • HOA (Hand/Off/Auto Switch)
  • Pilot Lights
  • Bypass
  • Line Reactor
  • Harmonic Mitigation
  • TVSS
  • dv/dt filter

Give us a call or browse our website to see the options and features of having a system custom designed and built for you by our engineers, application specialists, and certified UL 508a panel shop.

Final Recommendations

This guide is intended to be of use for general application sizing and is not intended to be a comprehensive guide. There are applications and loads that may require special sizing and consideration. When you are sizing or specifying a VFD for any application it pays to be conservative and leave some buffer room in the FLA and overload ratings. This is especially true if your load is hard to start or sees heavy loading during operation. If you have any questions about your application or in sizing a new VFD call and talk to one of our application specialists before purchasing.

CALL (317) 844 - 7328