Help: Velocity control of Fan AC motor

Hello everybody, I have been working with a friend on a project for a while, its about the construction of an evaporative cooler, we already made it but we are still missing the automatic part; we need to control the velocity of the fan motor using arduino by interpretating the signal of a temperature and humidity sensor, we tried a simple circuit that uses a potenciometer to regulate and it worked pretty well, its a circuit like this one:

The problem is now to add the part of the arduino to make it work as a potenciometer. I was wondering if I can change the potenciometer for a digital one like the AD5241 or AD5242 and make it work up to 1 MOhm? Do you think it can actually work as a normal potenciometer but interpretate the signal of arduino?

We already looked for other circuits, some of those use an optocoupler, but the problem is that motor is AC and inductive so it won´t work.

The motor we need to control is an AC fan of 1/8 hp 110 V 3.6 A.

Thanks for everything

I suspect you have an single-phase induction motor (judging by the 3.6A), which usually are not speed-controllable, they follow close to the mains frequency or else they stall (and overheat/burn out).

Some fan motors are able to be controlled somewhat, but they are specifically designed to take modest amounts of stall without burning out. They are very inefficient in this mode too.

Also triac circuits like the above don't like heavily inductive loads, normally you can control a universal motor with such a circuit, but not something that is mainly inductive, you just pop the triac.

For speed control you need either:

3-phase induction motor and V/f speed controller (inverter) DC-motor or BLDC motor and suitable supply and ESC

Thank you so much for the information, actually after reading your comment we went straight to an old car repair shop and we bought a new DC motor of a toyota, its the motor of the fan for the radiador. The motor works with approx 15 A, but the current for it to boot is almost of 40 A. It works with 12 V.

Now as far as I know the automatic circuit is easier to make since its a DC. But the problem is the source for that high amperage. Do you know of a way to control it without buying a source, they are way too expensive (sources for 30 A are almost $70).

Thanks again :)

BTS7960 module? If you ramp up the drive you won't see the 40A anyway, that's the stall current. You just want a driver than can handle the stall current briefly, not continuous (unless you expect to stall the motor a length of time - normally that quickly fries the windings).

Thank you so much for your answer, is there a way to ramp it up without buying the BTS7960? because I see its too expensive for me :S

And you are right I just want to control the stall current.

Big MOSFET? logic level MOSFET with 20V to 40V rating and 0.005 ohm on resistance or less (no heatsink) or 0.01--0.015 milliohm (heatsinking needed).