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I have just bought a $15 table fan, AC, ie plugged into wall mains. The lowest speed setting is still too fast for my requirement, and hence also too noisy. I estimate it to be around 5 revolutions per second or 300rpm. How can I change the lowest speed to say around 150rpm?

Are there resistors I can simply change?

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    you can buy "electronic speed controls" for motors. you can try a dimmer switch, but stop using it if it gets warm, some kinds won't work with some kinds of motors. it's not likely a matter of swapping resistors these days; it's either micro-controlled and hard to mod, or is made cheap and relies on specific winding ratios and input voltages.
    – dandavis
    Sep 2 '17 at 9:51
  • AC fan? Buy a Variac: en.wikipedia.org/wiki/Autotransformer DC fan? hook it up to a wall wart that puts out lower voltage. Any resistor you use will have to be able to handle the wattage of the fan. They tend to be large, get hot, and suck power just l;ike the fan at full speed. If you don't want to buy a Variac a simple 24 volt transformer in a box might give you the right speed, on the high setting.
    – Wayfaring Stranger
    Sep 2 '17 at 15:02
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Try rigging up a fan control switch - pretty commonly available at most home centers and hardware stores. With a little minor electrical knowledge, you could rig one one up to a plug and put your fan on it. That'd give you a L/M/H setting, at an additional L/M/H. That's, what, 9 speeds? enter image description here

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  • Sorry, I omitted that it is an AC fan, ie it is plugged directly to the wall mains, 230VAC where I am now. Will it work with this device?
    – Old Geezer
    Sep 2 '17 at 15:49
  • Yikes. Sorry, no, not without really digging in there. There are lots of cheats for window units though - a friend of mine has a little cheater box that makes his window AC a chiller to keep vegetables cool. I'm sure somebody has developed a fan controller, just might take some innerwebs research...
    – NPM
    Sep 2 '17 at 18:19
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I don't know enough to reduce the voltage without dissipating as heat so I did it by brute force:

enter image description here

Bought a 50W resistor from Taobao for 50 cents and put it in series with the level 1 button. The speed at level 1 is now perfect. Heat loss is far from perfect.

I Googled and found many people with the same problem. Wonder why fan manufacturers don't do what people need. Most commented that the speed is controlled by routing the incoming mains voltage to different number of coils in the motor.

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  • This tends to waste power. the speed controllers are a more "normal" way to adjust speed without creating all the heat.
    – Ed Beal
    Jan 22 '18 at 18:38
  • I don't have the reactance of the fan motor but on 230v with 470 ohm resistor you would be 112w if my math is correct (less the AC resistance of the motor since in series) I hope it dosent melt down with extended use.
    – Ed Beal
    Jan 22 '18 at 18:49
  • The fan has a label that says 48W, whatever that means. Some current, I, is already flowing in that wire. I reckoned that, if simplified to an all resistive load, I should not be higher than 0.2A (48/230). Then adding resistance would only reduce the current and I²R won't be anywhere near 48W. Is there a flaw in this computation? I hope it doesn't blow up in the meantime.
    – Old Geezer
    Jan 24 '18 at 6:48

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