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?

Thanks in advance



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.


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

  • 2
    \$\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 '11 at 9:59

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