'smart' variac or other variable AC supply (10A)

[Purduephotog]

Ludicrous, I hear you saying- I fully understand.

I have a very large fan- pedestal- pulling nearly 7.5A @ 120V. It can move quite a bit of air and, as you imagine, can be quite loud. I've run it via a variac transformer for quite some time at as low as 60V and it spins nicely, moves air, and I don't have the blade/cage interference noise. Makes the room livable (I heat with wood burning stove and need to help move air around).

However I would like to be able to regulate the speed of the fan better without cranking the knob. I understand I could build an Arduino, some very high torque high range servos, and probably pull something off- but I'd rather find something OTS- because we all know how much 'free time' there is in the world.

The fan itself actually has 3 speeds, and I believe they're cap based- but there are also another pair of wires going in for (I assume) another set of windings. At one point I ohmed everything out and decided that the variac was the best choice.

I do not believe a chopper type circuit would work although a stepped AC Sine Wave might- it is a motor, after all.

I know it's an oddball question but I've come up blank with ideas to try, and I'll be grateful to the suggestions you have.

Thanks kindly

Fan in question:
https://www.harborfreight.com/30-inch-pedestal-shop-fan-47755.html
Autotransformer- not the exact model, but close: https://www.ebay.com/itm/173631031654

[CaptDon]

It won't be high enough amperage probably, but you can adapt my idea. I built a similar controller for the fan used in a wood burning fireplace insert. I detected the available outlet temperature and with some comparators with both hysteresis and a bit of dead band to prevent hunting created a two wire dual polarity output to drive a D.C. gearmotor. I used an adjustable pulse width that varied from as short as .1 second to as long as 1 second and a time between 'updates' from a short of once every 10 seconds to only once per minute. This allowed me to 'tune' the incremental and maybe the 'proportional' as it were. The 'variac' was a dimmer control specifically designed to control ceiling fans and the gear motor was a bit of genius used on Seeburg jukeboxes to remotely control the volume. The Seeburg volume control is a typical 270 degree potentiometer and the D.C. bi-directional gearmotor drives that pot with what resembles 'Velcro'. Regardless of how long the motor continues to drive the pot there is no damage because the velcro simply slips. I put a limit switch with a diode for the high limit of travel. At the high end the limit switch opens and 'inserts' a diode across the open contacts preventing the motor from turning 'up' but not inhibiting the 'down' direction. On the low end of things there was a comparator that simply opened the motor circuit completely and the whole system went dormant until the next time a good fire was stoked up. I had a tiny yellow indicator driven by the down command and a red indicator for up. They were wired to the gearmotor directly and diode steered to control which lamp would illuminate based on which way the motor was turning. Those lamps also indicated the duration of the motor pulse and how often. It was actually entertaining to watch this unit 'do its thing'. Stoke up a fire and watch it go to work. Fan speed varied directly by available outlet temperature, higher temperature....faster speed. It tried to regulate the outlet temperature at about 160f. Of course with a big fire the temperature would exceed 160f when the fan maxxed out at high speed. At low temp the unit went dormant and opened the gearmotor circuit as well as the fan motor circuit.