There are some devices that look like ordinary light dimmers, but are sold expressly for ceiling fan speed control. For example: Lutron Diva Quiet 3-speed control. Unlike an ordinary dimmer, this has a three-position switch for low, medium, and high speeds, instead of a continuously variable knob. The manual shows typical wiring, which is like an ordinary dimmer:
(the unconnected red/white wire is for an optional 3-way circuit)
I've heard people say "don't run a fan on a dimmer; you will start a fire", but don't say why. Yet, that Lutron sells this device expressly for fans suggests it is something other than an ordinary triac light dimmer, and that it's totally safe, but they also don't say why.
I also know when I bought this house, a different, uglier fan controller of a similar type was installed. Could it be that this ceiling fan is designed for this type of control, where others are not?
Can someone tell me, as an electrical engineer, exactly what these fan speed controllers do, electrically? How are they different from ordinary triac light dimmers?