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Is Connecting A Variable Frequency Drive (VFD) Onto A Single-Phase Motor Possible?

Mike Jeffries | February 8, 2021

The use of 3-phase motors is commonplace but sometimes using a 3-phase motor is unsuitable or one is just not available.  A single-phase motor will just have to do.  

The one thing that they both have in common is that, sometimes, the motors will require a speed adjustment for whatever application they are positioned for.  To do this, a Variable Frequency Drive (VFD) is needed.

Related Blog: Connecting Variable Frequency Drive (VFD) to a 3 Phase Motor

What Is A Variable Frequency Drive (VFD)?

Motors sometimes need different speed adjustments, but they can't do this on their own.  A VFD is connected to the motor.  By adjusting the frequency up or down, this adjusts the RPMs or speed of the motor up or down.  The faster the frequency is, the faster the RPMs and speed.  The slower the frequency, the slower the speed of the motor.

Let's say that a motor is running in an application that does not require it to run at full speed.   In this way, if the motor was running at full speed when it doesn't need to, it's consuming more power than it needs and is creating waste. 

It's more economical to connect a VFD that can, either manually or automatically, lessen the frequency and voltage to meet the motor's electrical load, the minimum amount of power it should be using.  

Another advantage that a VFD provides is being able to make a production line more efficient.  By operating the motors at an efficient speed for the production application, fewer mistakes happen and production will increase.

One more benefit a VFD adds is that it reduces the amount of maintenance and extends the life of the motors and associated equipment.

How Does A VFD Work?

There are three-phased VFDs that can be matched with three-phased motors and one-phased VFDs that can be matched with one-phased motors.

Because three-phase motors are commonly used, there are hardly any limitations on matching VFDs with the three-phased motors.  

Using 240VAC, 3-phase power as an example, this would be the input to the VFD.  The 240VAC, 3-phase power would then be put through an AC to DC converter, inside the VFD, to change the 240VAC into a VDC value.  This VDC value is pulsed to slow down or speed up the DC to AC converter switches on the output, to get the desired frequency and AC voltage output.

For example, if the input voltage is 240VAC and the frequency is 60HZ, but the desired frequency to slow down the motor is 30HZ.  The 240VAC would be changed in an AC to DC converter to a VDC value and pulsed like turning valves on and off at a high rate of speed. The pulsed voltage would slow down the switches and go through the DC to AC converter and come out at 120VAC at 30HZ to slow down the motor.

There are single-phase VFDs that have a single-phase input and a single-phase output.  The single-phase VFDs work in the same manner but have some technical limitations. 

With this VFD, the output amperage is limited and there are very specific compatibility options when trying to pair it with different types of single-phase motors.

If you want to learn more about VFDs and their application in your industry, contact us today.

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