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
2
0
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
7,710
1,277
Last edited: Mar 13, 2018
3. TryingToFixit Thread Starter New Member

Mar 12, 2018
2
0
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
6,028
3,450
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.