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I'd like to control a couple ac fans running on mains voltage (US) using a potentiometer. I believe they're in parallel but I'm not certain. Each fan is rated at 15/13 watts @ 115 volts.

What's the best way to do this, preferably by dismantling broken appliances?

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If you mean that you want to control the speed of an AC fan with a potentiometer,
it is probably not possible.

Whether an AC "mains" fan can be speed controlled with a pot depends on the technology used.
Motor type is liable to be

  • "shaded pole" induction motor. Smaller fans used in appliances, fan heaters, pedestal fans etc. (All typical older type small fans and many new ones)

  • Single phase induction motor - capacitor start usually. Larger fans.

  • Brushless DC motor (newer but less common even now due to high voltages involved).

  • Series wound "universal" motor.

  • Possibly other. eg brush motor.

A brush motor may have permanent magnets and would then be a DC motor and would be unlikely to be found in an AC mains fan application.

A brush motor with a wound rotor may be
- Parallel wound - unusual - speed control possible but not without motor modification.
- Series wound - see below.

ONLY the universal / series wound motor is suitable for speed control with a series resistor. All other types will not be able to be properly speed controlled with a series resistor and may be damaged. Speed MAY be slightly affected by a resistor but the end result will not be predictable or consistent or safely useful.


Speed Control:

The speed of "Induction motors" is based on the AC mains frequency. They "slip" slightly relative to the 50 Hz waveform. Altering mains frequncy is the only way to control speed.

Brushless DC motors can be speed controlled using an appropriate electronic controller. A version of this will already be built into the motor BUT is liable to be a "go as fast as you can" type and is unlikely to be able to be accessed or modified.

Series wound universal motors can be operated on DC or AC and speed is related to applied voltage (not necessarily linearly. Unloaded they will spin up to very high speeds and may self-destruct. Vacuum cleaners often use series wound motors and their motors would be a suitable candidate for speed control. A series resistor for controlling speed would need to be able to handle power levels of the same level as the motor itself.

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You will need a dimmer circuit like on the page below, which will translate your potentiometer position to proper AC output for your fans.

www.epanorama.net/documents/lights/lightdimmer.html

(Note: The above link is now deliberately not "clickable". It contains three dimmer circuits (unsure which one the author intended to be the answer), but it was reported by a member as triggering their PC anti-virus for containing phishing. It seems likely that this was caused by an advert, and so may not always be a problem, as adverts change. Due to the usefulness of the circuits on that page, the URL has been left here for you to make your own decision. Visit the site at your own risk.)

Cheapest solution possible is to buy a 250W+ dimmer with embedded potentiometer that fits table light lamps in some electronic lights, bulbs and lamps store. It will be probably less then 10-15$. Then you connect fans instead of a lamp. If you don't know what you are doing, then it's better to have an electrician do the wiring instead of risking your life.

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    \$\begingroup\$ A dimmer has much the same effect as a series resistor. As in my answer, the ONLY motors that it will control the speed of are a series wound motor or a DC brush motor with a rectifier. Other sorts of motors will not be able to be speed controlled by this means. \$\endgroup\$
    – Russell McMahon
    Oct 4, 2011 at 9:59
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    \$\begingroup\$ I know this is an old answer but the site you have linked to was blocked by malwarebytes as having phishing on it. You should probably remove the link or find somewhere else that has the dimmer you were referring to. \$\endgroup\$ May 10, 2021 at 3:27
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    \$\begingroup\$ @DevinCarpenter - Thanks for the report. I'm running an ad-blocker and I get no reports from my AV. The page itself is minimal text without the adverts. So I expect the problem is with one of the adverts. || It's unclear which of the circuits on that page this answer intended to recommend, so we can't be sure to copy the right one into the answer. The page seems to be useful (so we don't want to lose that completely) and, as I said, no obvious issues without the adverts. || The best compromise I can see is to convert the clickable link into text and readers can visit at their discretion. \$\endgroup\$
    – SamGibson
    May 10, 2021 at 12:59

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