Q: How to slow down an AC fan I have an AC fan rated at 220 V, 50 Hz, 12 W.
It's too noisy so I would like to install a capacitor in series to slow it down. I don't know how to calculate this.
What size capacitor is approximately correct to reduce speed as much as possible without stalling the ventilator?

A: The fan currently consumes 12W.  If you select a capacitor that would draw 12VA (it should consume minimal energy as it's a capacitor) if connected directly across the supply then if placed in series with the fan you might expect both to drop to somewhere between 3 and 6W, depending on the characteristics of the fan.  Every quarter-cycle the capacitor would charge or discharge to/from 311V if connected directly to 220V, and that would happen 200 times per second, so the capacitor needs to store 60mJ at 311V.  Using E=0.5CV^2 we get about 1.2uF so that would be a starting point.  As others have pointed out, the fan may not work at all, or may work with little power reduction depending on the type of motor it uses.  The capacitor needs to be bidirectional and rated at more than 311V (400 or 450 is advisable).

A: Assuming it's an async motor, which is very likely, you can use a triac based regulator like this
