Ceiling Fan - converting resistors to capacitors

Discussion in 'General Electronics Chat' started by TryingToFixit, Mar 12, 2018.

  1. TryingToFixit

    Thread Starter New Member

    Mar 12, 2018
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    My ceiling fans date from the 1950s. They are controlled by a dial that progressively connects more (or less) resistance in series. At the lowest speed setting, the total resistance is 260 ohm as read by a DC multimeter.

    I want to convert to capacitive controllers. What uF rating should I use?

    I’m on 240v, 50Hz (Australia).

    If I convert using the formula C = 1/2/Pi/50/R I get about 12uF as equivalent to 260 ohm. I would need a C value of 24uF to replicate a mid setting on the resistor.

    But, fan controllers on the market have C values of 1uF up to 6 uF which is much less.

    So I am wondering if I am doing something wrong with the calculation? Or is it because the fans are so old?

    Thanks!
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Last edited: Mar 13, 2018
  3. TryingToFixit

    Thread Starter New Member

    Mar 12, 2018
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    Thanks Dodgydave, very helpful... it seems like the calcs were about right.

    So I tried a 3-speed fan controller I had lying around just to see what speeds I’d get. It had 1.6uF, 2.3uF and full voltage. Not surprisingly the fan is too slow at 1.6uF and 2.3uF. I estimated its RPM at 40 and 62 respectively. Modern fans seem to do about 100 RPM on low and 150 RPM on medium. So, as expected, I need higher C values, probably more like 10uF and 20uF.

    Given I have two data points for RPM at 1.6uF and 2.3uF, is there some way I can figure out what C values I need to get 100 RPM and 150 RPM?

    Out of interest, why does a fan from the 1950s need C values that are so much higher than modern fans?

    Thanks!
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Just a guess. The motor is more efficient? Many things from back in the day weren't built efficiency in mind, even the bearings in a modern fan are much better.
     
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