I have a 20" Box Fan, that has 3 speeds. It's a commerical model from Lasko, heavily built, puts out a lot of air... too much actually for my normal needs.
So I'd like to add speeds in addition way slower than what it has. As far as I can tell it's a straight power cord to the motor with a 3 way switch which I assume is some sort of resistor bank that limit the power to the motor.
In a perfect world I could have a similar switch with 6 or more settings, or alternatively another switch that works with the existing one, to multiply the 3 speed setting, which I think can be done by resistors in parallel or series.
I know a little about electronics, but not enough as to the best way to go from here. Part of me was thinking of a lamp dimmer but doubt those can handle the inductive load of a fan, or an AC fan speed control like this http://www.amazon.com/Speed-Control-Cord-AC-Cooling/dp/B00809BQZG
Here's a pic of the project if curious, it's DIY allergy filter using the box fan and a decent hepa furnace filter. https://instagram.com/p/z07BOOJ0pX/?modal=true