Potentiometer / rheostat ??

Discussion in 'General Electronics Chat' started by Sparks614, Nov 29, 2007.

  1. Sparks614

    Thread Starter New Member

    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.
  2. Ron H

    AAC Fanatic!

    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.