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I have some experience with discrete electronics, but electrics was never in my interest. Currently I started making our home smarter and I hit the wall - there are no HomeKit compatible fan speed controllers.

For lighting I use very good Leviton DH6HD-1BZ dimmers, which according to the manufacturer work with following types of loads:

  • Incandescent fixtures
  • LEDs
  • CFLs
  • Mark 10

Nothing mentions fans. I started researching and people are warning that using classical dimmers is a bad idea, however nobody seems to have a technical explanation why? Is it really that bad of any idea to use smart dimmer to control fan? What is the difference between the modes selectable on the dimmer?

Now the fans are controlled by 5 speed switches, which somehow has to control the speed of the motor...

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    Right off the bat, induction. Motors are not lamps. A funny thing happens when people come out of low-voltage DC electronics and enter the world of mains power. They expect everything to work the same (or at least analogous). This anti-knowledge ends up being their worst enemy. Everything is different here. AC is feisty stuff and follows different rules because it's AC and at a much higher voltage. Lamp dimmers are one example, they are not potentiometers nor variacs, they do weird wave-shaping tricks that cause incandescent bulbs (themselves weird) to do the right thing. Motors not so much. – Harper - Reinstate Monica May 16 '18 at 16:29
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To dim a light, you decrease the amount of power. That can be done by lowering voltage (works well with some things but not others) or limiting current or by only supplying power part of the time - e.g., by changing the waveform (as noted by Harper). Some of these same "tricks" work fine with motors, but generally speaking modern dimmers don't work well with motors - or rather, the motors don't work well with the dimmers. There actually have been issues even with some lights - many fluorescent and LED lights do not work with dimmers and it is always best to make sure that a light bulb (unless it is incandescent) is specifically designed/tested to work properly with dimmers.

How does the 5-speed fan switch do it? It may be variable voltage (e.g., different taps on a transformer) or current (some sort of limiting device) or some other method, but designed specifically to work with the fan motor. Dimmers for lights simply don't work the same way.

If a light dimmer does happen to work with a fan (or other motor), I would watch out for overheating and/or lower motor life.

  • Usually different speed motors are different taps on the motor (more windings / less windings) and how they are electricaly connected. Some motors can be done with electronics (variable frequency drives are used in industry extensively) but changing the speed creates heat in many cases and the fan motor is not designed for this and will overheat the motor it ends drawing more current because the inductive reactance has changed, now the motor fails or the electronics. – Ed Beal May 16 '18 at 23:58
  • @EdBeal - Which is a more accurate & detailed version of my "watch out for overheating and/or lower motor life" :-) – manassehkatz-Reinstate Monica May 17 '18 at 2:05
  • Just adding detail as a comment instead of a separate answer. I don't think I have seen a dimmer that is a VFD most are just using a portion of the waveform much worse for motor life. Agreed. – Ed Beal May 17 '18 at 15:04

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