Thursday, August 15, 2019

How a VFD works and when to use it



Did you know you could save on energy consumption and costs by using a VFD? You can also tighten your processes, increase production, reduce maintenance, and extend the life of your equipment.

But what is a VFD, you ask? Let’s take a look.

Defining VFD

VFD stands for variable frequency drive. It’s a motor controller for electric motors. VFDs are also known as adjustable speed drives, adjustable frequency drives, AC drives, microdrives, inverters, and variable speed drives.

The word “frequency” in the name relates to the frequency of the power delivered to the motor, which is measured in hertz. Changing the frequency changes the speed of the motor shaft.  If your electric motor doesn’t need to run at full speed for the entire process, you can save some juice and some wear and tear by installing a VFD to vary the speed of the motor.

A variable frequency drive can also get a motor started and ramp it up to speed at a controlled acceleration rate. This makes the start-up smooth, while also saving on electricity and motor life.

A VFD allows one motor to be used for processes that may require or allow different speeds.
Variable Speed Drive in Operation
Variable Speed Drive


How Does a VFD Operate?

A VFD converts fixed frequency AC line voltage to DC, then makes new AC at whatever voltage and frequency are needed to run the motor at the desired speed. The VFD consists of a converter section, a filter section, an inverter section and control section. 
  • The Control section operates the entire VFD, monitors the VFD and motor for safe operation, and interact with the machine operator or automation control system. 
  • The Converter uses diodes and/or SCRs to change AC utility power to DC
  • The Filter ''Cleans'' the DC power with inductors and capacitors
  • The Inverter makes new AC power for the motor using transistors as switches

Those switches are what allow the VFD to function at different speeds. The transistorized switches let the VFD adjust the frequency and voltage of the power supplied to the motor. As the frequency changes, so does the motor speed.

What’s It For?

Anytime you have a system run by an AC electric motor, you may have a need for a VFD. For example, a common use is controlling the speed of a water pump. If the pump is part of a water treatment process, a low demand for water can mean that the water doesn’t exit the plant at the same speed it enters for treatment.

To slow down the supply-side, a VFD is used to slow the water pump.

As mentioned before, a VFD can be used to get a motor started and smoothly accelerate it to operating speed. Energy usage is reduced if operating speed is below full speed.  There is less strain on the motor, less wear and tear on the machinery, and it doesn't just start with a jolt. Using VFDs on conveyors and belts eliminates those jerky starts and increases throughput without damaging equipment.

VFDs can be regulated with a PLC instead of manual adjustment. It’s an easy way to automate a repetitive task and reduce labor cost.

A VFD is a handy little gadget that can help you tighten your process controls, increase production, and minimize mistakes. Your maintenance and repair needs go down, and so does your electricity bill. At the end of the shift, your company has made a little more money than it did before.
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About the Author:
With over 25 years of experience in the industrial automation repair industry, Jeff Conner is the Dallas Service Manager for Control Concepts and serves on the Advisory Committee for the Electronics Technologies Department at Texas State Technical College.
Control Concepts helps design, fabricate, install, test, and program control systems. They service almost any brand of control found in automated systems and can send an experienced technician anywhere, wherever one is needed 24 hours a day, 7 days a week.

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