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Thread: Rheostat on capacitor start motor??

  1. #1

    Default Rheostat on capacitor start motor??

    I have an old Dayton 1/3 HP 1725 RPM capacitor start 120V motor that I am trying to put into use as a bench grinder/buffer.

    I would like to be able to control the speed of the motor through use of a rheostat without significant loss of power.

    I had thought about using a ceiling fan speed control, but I don't think those motors are usually capacitor start

    Can any of you motor experts out there tell me if this is possible; and if so, what kind of rheostat (or other device) I should use?

    BTW, thanks to those who helped me with the starting problem on this motor. It was a loose capacitor connection.
    William (Bill) Smith

  2. #2

    Default they make them for routers

    to control the speed on them,it's possible it might work with yours also.

  3. #3
    Join Date
    Dec 2003
    Welland Ontario


    Ceiling fancontrol is too small. It will probably burn out on a 1/3 hp motor.

  4. #4
    Unregistered Guest


    You can't use a rheostat on a CSIR motor. You'll burn it out in short order, especially at slower speeds. The only way to use this motor as you intend would be to use a small inverter drive (VFD). Very expensive. Could you just rig up several sizes of pully's for the various speeds that you might need? An adjustable sheave or adjustable speed gearbox may work well for this application as well.

  5. #5


    Like the previous poster said, it would be real hard on a CSIR motor to run it slower. To use a rhestat on this motor, you'd actually need a pretty hefty
    "variac", costing a couple hundred bucks. For that money, you might as well buy a VFD. The router control won't work, because routers use a "universal motor", that can run on ac or dc, much like a vacuum sweeper motor. The router control us basicly a solid state version of a variac, and wouldn't last for your application.

  6. #6


    Thanks for the replies. Looks like I'll have to resort to the pully method. I thought that would be the case. No problem though. That system has served me well on my drill press and lathe.
    William (Bill) Smith

  7. #7


    You have two types of motors normally used for speed control work. A multispeed motor with high/medium/low windings in the motor much like the fan motor in your car or what is called a DC servo motor that is rather salty in cost for the horse power you need using a grinder design.

    The series of pulleys is your better choice to attack this desire you have.

    Good Luck


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