Understanding Variable Frequency Drives

There are many reasons why a variable frequency drive makes sense to use with three phase electric motors, no matter what the size or power needed. There are many different factors that go into the type of variable frequency drive used and it is important to understand them. Here are a few of the common questions associated with variable frequency drives, or VFDs, to get you acquainted with them.

What is a Variable Frequency Drive?

A variable frequency drive controls alternating current torque and motor speed in electro-mechanical drive systems. In layman’s terms, it is an adjustable speed drive that regulates the frequency and voltage to the motor. You can find variable frequency drives, or VFDs, in everything from small appliances to giant compressors.

How Do Variable Frequency Drives Work?

VFDs are part of larger drive systems that consist of alternating current motor, main drive controller assembly, an operator interface and a drive operation. Let’s take a closer look at what each part contributes to the overall VFD system:

AC Motor

This is generally a three-phase induction motor because it is the most economical. There are times when a wound rotor, permanent magnet and synchronous motors can be used but usually you can expect to see a three-phase induction motor.


The controller in a VFD system has three sub-systems that make it run. Those are the converter or rectifier stage to convert AC voltage to DC voltage, a DC capacitor assembly and an inverter assembly to create the three phase voltage. Since the introduction of the insulated gate bipolar transistor in 1983, progress in power devices and computer processors has reduced VFD costs and increased amperage ratings.

Operator Interface

VFDs have an operator interface that starts and stops the motor, adjusting the speed of operation. An operator interface may also reverse, and switch between automatic and manual controls. The VFD may be controlled by a computer communications network, external wiring of analog and digital controls or by using the controls on the operator interface.

Drive Operation

There are three types of drive operations: single-quadrant, two-quadrant or four-quadrant. For the most part, applications use single-quadrant loads using variable- or constant-torque. A two-quadrant load is used with the speed is positive but the torque changes polarity. Still, there are some high-performance applications that need four quadrants to facilitate the power needed. With a four-quadrant load, torque and speed can go any direction.

What types of Variable Frequency Drives are there?

Though all VFDs serve the same essential functions, there are different kinds that address different needs.

CSI (Controlled Source Inverter) Design

The way a CSI VFD works is by converting the incoming power to DC voltage through a SCR converter section. CSI drives need a large internal inductor to function properly, along with a large motor load. Some of the advantages to a CSI design include:

  • Simple circuitry
  • High reliability, based on current limiting function
  • Capability to regenerate

Disadvantages of a CSI drive include:

  • Reliance on a big, expensive inductor
  • Cogging that is below 6Hz because of the square wave output
  • HV spikes because of motor windings
  • Poor compatibility with multi-motor applications

VSI (Voltage Source Inverter) Design

This type of VFD drive is similar a CSI in its main function: regulating DC voltage through a SCR converter section. VSI drives do not regulate current, though, like the CSI drive. Using SCRs, transistors or gate turn off thyristors (also known as GTOs), a VSI drive generates an adjustable frequency output for motors.

PWM (Pulse Width Modulation) Design

This is the most common drive controller, mainly because it will work with a large range of motors. Motors from ½ hp to 2000 hp can use a PWM drive and it is a very reliable choice. In fact, there are nearly 100 manufacturers that sell PWM drives.

A PWM drive works by converting AC voltage to a fixed DC bus voltage that then goes through an inverter section equipped with insulated gate bipolar transistors that produces a near-sine wave output. Before reaching the final output, the power supply passes through a convertor section that implements DC capacitors and a diode bridge converter.  Pulse width modulation is used to reach this sine-like output through a series of short pulses.

Advantages of PWM drives include:

  • Low upfront cost
  • Multi-motor application compatibility
  • Efficiencies as high as between 92 and 96 percent
  • Great input power factor because of fixed DC bus voltage
  • No motor cogging
  • Capability to operate through a 3 to 5 Hz power loss

Disadvantages of a PWM drive include:

  • Line-side power harmonics
  • Insulation and motor heating breakdowns in some instances because of the frequency of switching transistors
  • Operation that is non-regenerative unless a regenerative modification is specified.

What Are the Benefits of Variable Frequency Drives?

There are a lot of good reasons to go with VFDs – some more highly-touted than others. A few of those benefits include:

VFDs are Energy Efficient.

They work especially well saving energy when it comes to centrifugal load service. It is estimated that in the U.S., up to 65 percent of all electrical energy goes to power motors. Energy efficient technologies, like VFDs, could save an additional 18 percent of the energy used by those motors.

VFDs Control Performance.

When you go with fixed-speed options, motors may have to deliver more power than what is required due to the inability to vary the load speed. This results in poor process control.  VFDs, on the other hand, allow for infinite variability so that the energy consumed makes sense for what is needed.

What Kinds of VFDs Does Mitsubishi offer?

Mitsubishi has released a FREQROL FR-A800 series of variable speed drives that are compatible with existing models like the FR-A700, FR-A500 and FR-A200. These VFDs are made as small as 0.4 kW and as large as 500 kW. The upgraded VFD series takes advantage of power conversion and open network communications. All applications can use the FR-A800 model but it is especially effective for difficult applications like centrifuges, winding machines and high-bay warehousing systems. It can connect with a variety of networks that include CC-Link, Profibus, SSCNETIII and Modbus. There is an energy saving mode that allows for the reduction of motor loss.

The FR-F800 is designed for general purpose industrial and commercial applications where the performance features of the FR-A800 VFD is not needed.

By understanding exactly what function you hope to accomplish, you can select the right VFD for the job at hand.

Understanding Variable Frequency Drives was last modified: July 20th, 2017 by admin

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Jerry is the owner of Sloan Electromechanical and is active in all aspects of the company. He is passionate about doing the work RIGHT and proposing the best product solution, hence the Sloan team is focused on aligning company values with client values. Please post your questions or comments and Jerry will respond. For a faster or confidential response, please contact Jerry directly 619-515-9691 or LinkedIn www.linkedin.com/pub/jerry-gray/17/332/5a1
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