Single Phase Motor Control

I would imagine that I get asked several dozen times a year about a VFC for
controlling a single-phase motor. Well, here it is in writing as far as what is
available at this time:

There is not one manufacturer building a single-phase input and single-phase output variable frequency controller, Not One. The only way to control the speed of a single-phase motor is via voltage control and this only works on certain motors. Voltage control as I am describing it in this document can be best identified as a “Dimmer Switch”. The only ones that work well are the Solid State type and are limited in use to just a couple of motor applications. They will not work on the following:

     - Capacitor Start type Motors

     - Dual Winding Motors (Start and Run) with speed engaged contacts

The above-mentioned are the most common type single-phase motors and if you try to control these with variable voltage, they will definitely burn up. These are the motors mostly found on Clothes Driers, Washing Machines, Drill Presses, Band saws, Table saws, Small Lathes, Air Compressors and so on.

Capacitor Start- Capacitor Run type motors have been used on variable voltage but I still don’t believe that these are reliable on variable voltage control as eventually, the capacitors overheat and wear out due to the lack of proper voltage applied to them.

Shaded Pole motors work well on variable voltage control but understand that these motors have very little starting torque to begin with; so using them with a variable voltage source will make them very, very weak when starting. These motors are commonly found in ceiling fans and will run little else. Next time you get the chance, pay close attention to a ceiling fan starting. You will notice that it takes it some time to get up to speed. That is because of its low torque, high slip characteristics.

I wish I had something to offer but I don’t think it will ever happen in my lifetime.


Scott