AC motor speed control

I have one of these

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941189) but the bugger blows way too much air and is very loud. So I bought one of these
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and that did slow things down a bit but not enough.

I have a 10A solid state relay and a microcontroller or two sitting around. I was curious if I could control the speed of the AC motor by cycling the availability of the juice like I could with a DC motor. IE: Hook the hot wire through the relay and cycle it on for 20ms, off for 80ms for a 20% duty cycle or some such (with the ability to adjust the duty cycle of course, 100% for 5 seconds at start, etc).

I don't know much about AC though and less about AC motors. Is this a valid approach? Will it burn up the motor, catch the relay on fire, destroy my karma, etc?


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You might end up with a mighty hot motor using it that way. It wont take kindly to fast on/off AC to control speed, all the higher harmonics in the applied AC are converted to heat.

Reply to
Sjouke Burry

Instead of doing asynchronous pwm to the ac, how about keeping the frequency at 60 hz. Turn your power on when close to the positive peak, and then off again just past the peak, and doing the same for the negative peak. So the main frequency is still 60 cycle, but you can control the total power delivered by making your pulse longer or shorter in time while keeping it centered on the peaks. A little microcontroller with an A/D converter should be able to do that, but you'd need a bit of analog circuitry to get the AC signal into the right range for the A/D. I've never tried this, just an idea.


Reply to
Kevin S.


So a transformer to bring the 120 down to 3 V peak-to-peak then boost the baseline to 2v DC put that into the ADC input? Then when it gets close to 4.5 or .5 then kill the motor and re-enable at the same voltage on the other side of the peak/trough?

Or could you accomplish similar with op-amps?

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If you just want to detect the zero-crossings of the AC supply, there's an even easier way. Take the 120V AC, feed it through a large resistor ( > 1 MOhm ) directly into a microcontroller pin. The input protection diodes will clamp the mains voltage to a nice square wave, and the approx. 100 microamps that flows through the resistor is well within their ability to handle --- check the data sheet.

However, I'm not sure that Kevin S's phase-control-dimmer-style speed control is a good idea ... a motor is an inductive load, and treating it like a simple resistive load is likely to burn out your switches, your motor, or both...

   Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1
  "We learn from history that we do not learn from history." -Hegel
Reply to
Wim Lewis

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