Can I Control The Speed Of My Single-Phase Motor

  • By Ron Abma

The short answer to the above question is, no. You’d need to purchase a three-phase motor and use a variable frequency drive to control its speed.

If the single-phase motor is low torque, the speed can be controlled. For example, a ceiling fan would be a single-phase motor where the speed can be controlled by a switch or toggle. This drops the voltage using a rheostat - these can dim lights, and start or control the speed of motors.

If a single-phase motor utilizes a start winding and a start capacitor, the speed of the motor cannot be controlled. The start capacitor can only be in a circuit for a very short time; start capacitors draw a high current. The centrifugal switch will then open and when it reaches about 80 RPM it disconnects the start circuit and capacitor and your motor will run at full RPM.

Here is where it gets a little tricky

Trying to slow down the single-phase motor by removing voltage or attempting to adjust frequency can have poor results. If the centrifugal switch were to close again, for a period longer than one minute, the start capacitor (now full of oil) would explode. The high current creates heat on the inside of the start capacitor and they fail. This is called thermal overload.

Some electricians will install a thermal overload protection device so if the current increases more than the nameplate suggests, it will take the power away from the motor. Without this thermal protection device, you could potentially stall the single-phase motor as a result of something becoming jammed in the impeller, causing the start cap to blow out.

Now, when is a three-phase motor necessary?

For motors that have higher torque, or use a start capacitor to get the motor operational, the single-phase would have to be replaced with a three-phase.

Every motor has a frame size that dictates the bolt pattern and the diameter of the output shaft. We would then consult our catalog to determine the three-phase motor to replace the single-phase.

How do I know the three-phase motor will fit where the single-phase motor was?

There are no guarantees that the three-phase motor will fit, however, there are steps we take to try to mitigate any issues with compatibility. If the junction box is in a strange place, this may cause issues with replacement. What the frame size does guarantee is that the bolt-down pattern and shaft size will be the same size. Electram will send you a diagram of the dimensions for the three-phase motor to make sure it fits before we ship it to you.

For any additional information about this, or other electrical questions about motors and 3 phase power, please get in touch with us!