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Ceiling fan speed?

Discussion in 'General Electronics Discussion' started by Doc Brown, May 25, 2014.

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  1. Doc Brown

    Doc Brown

    May 12, 2012
    Hey all.

    I'll start out by saying I'm a little embarrassed by asking this question but I have yet to find someone who can help me.

    The reason I'm embarrassed is because I'm an electrical contractor and I don't know the answer to a seemingly simple question. Even worse is the fact that I am 99% commercial and industrial and deal with controls and motor drives and soft starts on a regular basis.

    My question is... How in the heck does a
    rotary ceiling fan speed control work?!
    It's a simple ac induction motor and it shouldn't work without a vfd as far as I'm concerned. I can't hook up a fan speed control to any other motor and expect it to reliably control motor speed. Decreased voltage equals decreased motor torque leads to no power. I could see if a ceiling fan had motor speed taps that correspond to the speed control but they don't. So how is this working?

    Thanks all. o_O
  2. duke37


    Jan 9, 2011
    My thought is that the voltage is reduced and the speed drops. This could overheat the motor with a constant load but in the case of a fan or central heating pump, the torque is proportional to the square of the speed so comes down rapidly as the speed drops.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    have you not been to an electrical supply store and obtained one of the wall mount fan setting switches ( often also have an incorporated light switch)

    opening one up would surely lead you to its operation :)

    My previous home had them, my current one doesn't, so I cant look at one for you

  4. Doc Brown

    Doc Brown

    May 12, 2012
    Thanks for the replies all. I have been doing a lot of reading and it's not that mysterious after all. Many horror stories out there of people mistakenly using dimmer switches as a fan speed control!

    Not good to say the least.
  5. BobK


    Jan 5, 2010
    Care to share what you found? Now I am curious.

  6. shrtrnd


    Jan 15, 2010
    There are a lot of different types of dimmer switches, those SCR types were causing a lot of problems with the new CFL lights.
    I imagine the designs have been revised, trying to compensate for the new U.S. CFL rules for lamps.
    There's probably somebody out there making ones that will operate ceiling fans, and the light fixtures in a lot of them.
    You just have to be careful of what you're buying for your application.
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