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I was looking at a project that controls the speed of a normal off-the-shelf AC-powered house fan by plugging it up through a consumer smart plug (specifically, the Kasa ones, if that matters) and effectively pulse-width modulating its speed by turning the smart plug on and off with periods on the order of a couple of seconds. In the intended use case, this would be running for potentially hours at a time.

My primary question is: Is this strategy safe? Is it actually ok to PWM control a normal consumer AC-powered fan by flipping a normal consumer smart plug/switch on and off every couple of seconds?

My secondary question is: How quickly is this frequent switching likely to wear out the relay(s) in the smart plug/switch? And does wearing it out potentially become a fire hazard?

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  • \$\begingroup\$ I wouldn’t call it pwm - more like bang-bang. If the motor is not running at its normal speed, then it will most likely draw more current. Running in this condition for a long time might cause overheating. You’ll most likely wear out the relay. The relay will most likely just stop working - no smoke or fire. \$\endgroup\$
    – Kartman
    Jun 19, 2022 at 4:54

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I assume you mean a box fan or oscillating fan or something like that, not the HVAC whole-house fan. Slightly different power level there.

Anyway, it'll probably have a single-phase induction motor, and the "correct" way to vary the speed of one is to adjust both the voltage and the frequency. For example, if 100% is 120Vrms @ 60Hz, then 50% would be 60Vrms @ 30Hz, etc. There are industrial VFD's (variable frequency drives) that do exactly that in "dumb mode", or can actually read what the motor is doing in "smart mode" and run some logic to control that. The concept is quite similar to an audio amplifier, except that it's hooked up to a motor instead of a speaker. Any industrial electrical supply place should have them, though you might have some difficulty finding one that can drive a single-phase motor. They're normally used with 3-phase motors.

You might get away with a triac dimmer switch, though the cheap ones of those can be fooled into latching 100% on or dropping out if they don't have their expected incandescent-lighting load or close enough. For this purpose, it's more-or-less reducing the voltage while keeping the same frequency, but what it's actually doing is chopping up the AC sinewave faster than the motor (or hot filament) can respond.
I just happened to have a variac (variable transformer) sitting around, so I'm using that to actually reduce the voltage to one of my fans, because even the low-speed setting was a bit much for what I wanted.

Reducing the voltage while keeping the frequency up like that, is effectively reducing the torque that the motor can produce, while keeping the same maximum speed. So it'll be more susceptible to wind gusts if it's outside, etc. But if that's enough for you, then it's okay. You're not going to blow it up by doing that.
Torque actually corresponds to the current that it draws, but less voltage means it can't draw as much, all else being equal. How much it actually draws for a given voltage depends on the difference in speed between itself and the electrical frequency, or "slip": running slower, with more slip, draws more current, which produces more torque.

An on/off period of a few seconds is probably slow enough that it can respond to that. Thus, every PWM cycle is a start/stop cycle, not a constant speed. And the most stress happens during a full-on start, because it's receiving the full voltage and frequency ("commanded" fast, with lots of juice available), but presently running much slower than that.


As kind of a "nutty" experiment, you might actually get a big audio amplifier, like a Behringer NX-6000 for example, wire its output to a normal 120V outlet, and feed it from your phone or computer's headphone out. Play around with your favorite free signal generator and see what works. (Audacity at minimum)
A little bit of math on the claimed power rating and impedance says that it should be capable of producing the required voltage, and I'm pretty sure a floor- or desk- fan would be a lighter load than the 4-ohm speaker that it's designed for...

(This can also be used to drill home the idea that amplified speaker leads should be treated with respect, which for some reason the "artsy" audio guys don't seem to understand...)

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  • \$\begingroup\$ "I assume you mean a box fan or oscillating fan or something like that, not the HVAC whole-house fan." Correct. \$\endgroup\$
    – reirab
    Jun 19, 2022 at 7:04

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