AC fan speed control by periodic on/off switching

Should be able to cook something up with a solid state zero crossing switching relay and a simple op amp based pulse width modulator. you could also use the ever-popular dual 555 to generate the PWM waveform and power can come from the mains via a dropper resistor/rectifier/zener regulator arrangement. Safety first; plastic enclosure, plastic shaft on control potentiometer, plastic mains switch housing and operating lever. Proper cord anchorages (get the proper clamping grommets for this). Build it in the fan housing if you can to save some trouble. Five seconds for the switching cycle seems a bit long; a couple of seconds would reduce hunting. Watch for overheating of the fan motor.

Integral cycle control. Triac drivers wwith this feature are available.

ep,

the idea for speed control is good. However, instead of crating a few cycles of 120 60 hz and then a few off and aFEW ON ETC,, CONSIDER THE WAY THAT LIGHT DIMING IS PERFORMED... in this case each 1/2 cyle of the

marc

I would like to get power from the mains, yes. What do you mean by 'rectifier', by the way?

To get a low voltage DC supply from the mains, more must be needed than just only a resistor + zener diode, right? Could you elaborate?

Thanks.

I found a triac driver, the MOC3041, a 6 pin DIP. It has a zero crossing circuit, LED optocoupling. I can use this one; I'll need to have an extra triac that does the main switching.

But you say there are triac drivers that have integral cycle control BUILT IN. Could you tell a model number? I couldn't find it in Google. Are there any online searchable electronics parts catalogs?

Thanks !

You'd do better with a cycle control system using full cycles.

There is something that I have wanted to try out for quite a while. It is a variation on dropping cycles that I think would lead to lower losses in the motor. I will explain it and others can shoot it down if I am wrong.

To decrease the operating speed of an induction motor, it is best to lower both its voltage and frequency. A device like a triac can be fired to allow an alternation through or not.

The triac it fired on the positive alternation. The negative alternation is skipped The positive alternation is skipped The triac is fired on the negative alternation The positive alternation is skipped The negative alternation is skipped ... repeat

This lowers the frequency to 1/3rd that of the mains frequency.

The RMS voltage on the motor would be 1/sqrt(3) of the normal line voltage. However, the frequency content will be a bit funny. If I did my math right, only 0.4 of that will be at F/3.

Use a light dimmer.

Nah. LVMarc also suggested it. Not so good to cut up the phase (HF, net pollution, might have bad effect on motor). See also Rich Grise's reply.

If you could provide one, sure, thanks.

Found this webpage titled "PWM Fan Controllers":

Then it talks about using the 555 chip:

Exactly what I need! I can use a circuit from this webpage (best for me would be the double 555 solution) to trigger the MOC3041 triac driver, which in turn would drive the main triac...

Don't they aren't good for fans. they make triac based speed controllers, use one of them.

Bye. Jasen

They are asymmetrical - output some DC.

OK. And if later I have a working solution I'll post it. Thanks for your offer for the schematic!

We used to have an electric cooker where the heating elements was controlled like that..... Bursts of full sinewaves separated by gaps.

Try the keywords "burst control heater" as a starting point. You might even be lucky and find a complete knob-controlled module that does it.

This will instantly destroy any fan based on a conventional inductin motor. Only motors with brushes can be controlled by a lamp dimmer. However, the lack of speed feedback will cause major problems.