Potentiometer / rheostat ??

Thread Starter


Joined Nov 29, 2007
Thank you for taking the time to read this post.
I have a small 40 R.P.M. motor that I am trying to use with a Potentiometer or Rheostat to control an adjustable R.P.M.
I am not very knowledgeable of electronics and I am hoping that I get some information from someone here who is.
I am not sure if I have the 120v 60Hz. 2.7 watt 40 R.P.M. motor hooked up to the Potentiometer/Rheostat (which I will refer to as the device) correctly. Looking at the device there are 3 legs. From left to right I number them as
legs 1-2-3 .
I hook up the 40 R.P.M. motor leads to legs 1 and 3.
I hook up the 120 volt power cord leads to legs 2 and 3.
Could you tell me is this a correct connection ? Again, I am totally in the dark with this stuff.

I get a flash and a spark:confused: because I feel I the rheostat is only rated at 25 ohms and I think I need higher.
If I have the correct connection do I need to work up to a higher ohm value or a differnet type of device ?
If so what higher value would you suggest or different device?

The rheostat that I have is a 3 watt 25 ohm. The 40 R.P.M. motor is rated as 2.7 watt.
All I am looking to do is wire up a Potentiometer/Rheostat that will let the 40 R.P.M. motor be adjustable R.P.M.
I am a complete rookie at this stuff, I do not know how to calculate the ohm value I need. Any information will be appreciated.
Thank you.

Ron H

Joined Apr 14, 2005
You are basically attempting to change the speed by changing the voltage across the motor. You can do this with some types of DC motors, but a universal motor is the only type of AC motor whose speed can be controlled this way. Otherwise, you have to use variable frequency drive, which is complex.
Furthermore, using a resistor (rheostat ) to control the speed of a motor is not effective unless it has almost no load, because it severely affects the available output torque.