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EMA Inc.

An Overview of Variable Frequency Drives

overview vfd illustrationAt EMA, we often laugh about how the quickest way to lose friends at a party is to discuss what we do for a living; in fact, we made a hilarious commercial poking fun at this. All joking aside, variable frequency drives — aka “adjustable-speed drives,” “inverters,” “drives,” “VFDs,” and “frequency inverters” — are extremely common in all types of applications and perform a lot of the behind-the-scenes work that makes many industries go. Although most people cannot tell you what VFDs are, these drives are everywhere!

In the most basic terms, a variable frequency drive is a device used to vary the speed of a driven load in order to exactly match the process requirements and, depending on the load type, achieve energy savings. VFDs have built-in electronic overload protection for the motor and allow the motor to be operated at any speed range, unlike other mechanical means of speed reduction. VFDs also greatly reduce initial inrush current on induction motors, acting as a softstart for the motor as well.

Industrial Applications:

It would be impossible to name every use for VFDs, but some of the more interesting ones we’ve seen are mechanical bulls, electric cranes, roller coasters, and juicing machines. Because electrical motors are so common and used in thousands of applications, VFDs are commonly mated to them to achieve the process results needed for the application. Ninety percent of “industrial” VFD applications are constant torque applications, meaning they’re used for process control and not energy savings. A very common industrial VFD application we see often is an extruder. Bonnot has written a very good blog post on how extruders work, but, basically, to achieve the correct mixture and viscosity of what you’re extruding, the speed of the motor running the screw has to be varied constantly so that the extruded material meets standards.

There is no better way to do this than with a VFD. At EMA, we service and retrofit VFDs on large extruders regularly. If the speed of the application needs to change at all, a VFD is likely involved. VFDs running in closed loop with encoders and resolvers can do very precise movements on motors, allowing them to perform very precise tasks.

Commercial Applications:

In the early 2000s, VFDs were highly touted as “green technology” in the commercial HVAC industry because of the energy savings potential for variable torque applications. Although flow control is important on commercial air handlers and pumps, the main reason VFDs are so prevalent on them is the immense energy-saving potential offered by variable torque loads. In a nutshell, a small reduction in speed is an exponential reduction in energy consumed, which is an exponential reduction in the monthly power bill. A variety of commercial HVAC applications use VFDs for this very purpose.

Matching a VFD to an Application:

Although many modern VFDs will essentially work out of the box on a number of applications, it is still important to understand the basic principles of physics when applying VFDs to applications. The application of an adjustable-speed drive to a machine is a mechanical, rather than an electrical, problem. When applying the drive, the speed, torque, and horsepower characteristics developed at the motor shaft must be considered. These must meet or exceed the torque and horsepower requirements of the machine being driven.

Here are a few other common pitfalls to watch out for when applying a VFD:

  • Variable or Constant Torque
    • Drives are often rated differently depending on whether they’re used on a variable or constant torque application; understanding what type of load you have is important.
  • Special Starting Torque or Stopping Torque
  • Size by Amps, NOT HORSEPOWER
    • Sizing by horsepower rather than amps is a very common mistake; make sure the VFD’s output current rating meets or exceeds the motor’s full load amps.
  • Control Scheme
    • The VFD chosen must have enough I/O (inputs and outputs) for the control scheme you are using. For example: If your machine has six digital inputs, don’t choose a drive that allows for only five.

Does this seem overwhelming to you? We’re here to help. Contact us for help on applying variable frequency drives or any other drive-related question. No one ANYWHERE is better at drives than we are!

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