The mystical ways of VFD’S

Variable Speed Drives can be found everywhere in manufacturing. In fact, Industrial output would be reduced by a considerable amount without them.

The intended audience for this article is people not necessarily technically inclined, or simply work in related areas, so I will try to refrain from getting too technical and provide a view for the curious.

The most common type of electric motor used in industry today is the three phase squirrel type induction motor. By three phase I mean a power source which is alternating current. It has three lines and an earth ground wire. Each line or phase waveform is 120 degrees from the other two. This type of power was defined along with three-phase motors by Mr. Tesla, back in the days when men were men and sheep were scared.

However, I digress…

Moving right along…

These motors were designed to be hooked directly to three-phase power, and their speed of rotation is tied directly to the frequency of incoming power, so if you change that frequency you automatically change the speed of rotation of the motor.

VFDs are placed between the incoming power and the motor so they can alter the waveform fed to the motor. By doing so they can accomplish many seemingly magical tricks.

Following is an illustration of a VFD’S relationship to incoming power and the motor:


VFDs, until the eighties were only used on dc motors principally to change speeds. Those motors require a change in voltage level to change speed, unlike AC motors which require a change in frequency to change speeds.

This could be accomplished with Direct Current without the need for a semiconductor which could handle the large current loads, which didn’t exist until the eighties, so up until that time if you had an application that required variable speed you had to use a DC motor.

However, AC motors are more desirable because they do not require a large permanent magnet, and there is no need for brushes which cause a carbon mess and need to be replaced often.

So because of the advent of semiconductors which could handle the load, the eighties saw the first VFDs for AC motors.


Alternating current VFDs output pulses instead of sine waves, using a scheme called PWM. (Pulse width Modulation) By varying the width and amplitude of the pulse, great things can be accomplished. Things like varying speed and torque of the motor.

This would be a good place to say that standard motors are “tuned” to 60 hz, they offer more resistance to current flow at 60 hz than any other frequency. So if you fed a 30hz Sine wave to one of these motors current flow would be excessive and the motor would burn up. But this can be compensated for with PWM modulation. The processor within the VFD knows the current draw and adjusts the pulses to bring that current draw in line.

The following illustration shows the differences between input and output of the drive:


These fun little critters are run by a microprocessor and offer hundreds of different parameters which can be set by the user, such as torque, speed, direction, max current draw, ramp up and down, ( how long it takes to get to target speed) and many, many other things. They have inputs which can define behavior by whether the input lines are high or low.

They usually have small displays and push buttons which allow for programming.

They can also be networked on an industrial Ethernet line which allows a PLC to control them according to the program running in the PLC.

(PLC’S were covered in an earlier article.)

I hope this article satisfied your curiosity.

Wayne Dover

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