Originally Posted by c0rr0sive
Some controllers actually do adjust the voltage for a 2-wire fan and dont really turn power on and off to get them to sit at a certain speed.
They use DC motors in those pc fans and the speed is easily controlled by the voltage, more volts = faster fan. Only in AC motors would you need to cycle the speed, but they do not use that method to adjust AC motor speeds, they use different sets of winding's in the stator and armature for different speeds. It's why your house fan can go only "hi" "med" and "low" while a dc fan motor is variable, and can ramp up or ramp down it's speed
If you put a rheostat on a dc motor and gave 1/2 it's voltage, it goes 1/2 speed. Do that with an AC motor and it'll sit there and fry itself (less volts = more amps)
In 3 phase a/c motors, they have controllers like c0rr0sive is stating