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3 speed switch to dimmer switch?

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  • 3 speed switch to dimmer switch?

    I need to put a new switch on a 3 speed exhaust fan in a small market. The fan is on the roof and the switch is inside the store. The store owner told me that previously the fan utilized a dimmer switch, which went bad and was replaced with an on/off switch so that now it only runs at high speed. I looked in the switch box and there are 2 white wires capped off, and 2 black wires connected to the switch.
    I am leary of putting a dimmer switch on this set-up, as I have never seen a fan controlled this way, and I am suspicious that the previous dimmer switch went bad because it couldn't handle the load. Also I wonder what the white wires are doing in the switch box, and what they are for.
    I would like to know if it okay to connect a dimmer switch to the fan. I want to do what is correct and what will work.

  • #2
    Re: 3 speed switch to dimmer switch?

    Originally posted by GreasyGirl View Post
    I would like to know if it okay to connect a dimmer switch to the fan. I want to do what is correct and what will work.
    Depends entirely on the motor.

    Actually a motor cannot be used with a "dimmer". You would need a motor speed control...IF the motor is capable of it.

    A capacitor start motor CANNOT run off a speed control. Typically only a shaded-pole motor can be controlled by a switch box mounted speed control.
    Beyond that a VFD, variable frequency drive, is required.

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    • #3
      Re: 3 speed switch to dimmer switch?

      What he said and speed drives are expensive.

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      • #4
        Re: 3 speed switch to dimmer switch?

        I wonder if the motor may be a 2 or 3 speed type with tapped windings or extra windings. That might be what the capped white wires are for.

        I know this isn't going to be easy, but is there anyway to get a look at the name plate on the motor? Maybe a really good digital camera with a telephoto lens and some good lighting would work for this. That's if you have access to such a camera. If not, maybe a pair of binoculars and a good powerful flashlight would let you read the name plate.

        Please everyone think "Commercial" for this fan. I really doubt it was made to be a variable speed. It may be two speeds such as 1140 (1200) and 1725 (1800) RPM.

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        • #5
          Re: 3 speed switch to dimmer switch?

          Originally posted by Woussko View Post
          I wonder if the motor may be a 2 or 3 speed type with tapped windings or extra windings. That might be what the capped white wires are for.
          The way I read the OP's post I'm getting that the neutrals are twisted together and capped (rather than a pair of individually capped white wires)...

          I'm guessing this would be something pretty big, like a wharehouse exhaust fan? Maybe belt driven?

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