1. If I understand how these fan speed controllers work (I'm a fair tinkerer with DC, less knowledgeable about AC) - then I think this is fine.
I'll put out my explanation - which may in fact be quite wrong (hopefully someone more knowledgeable can clarify):
Without going over too much background (google or wikipedia if interested), AC voltage is a sine wave. It oscillates smoothly between full voltage one direction to the other.
The solid state speed controllers use the variable resistor (knob) to create a threshold voltage below which a device called an SCR will not turn on and not apply power to the circuit. Instead of limiting the maximum voltage, they keep the circuit off until the voltage reaches a particular threshold (say 20v instead of your fan getting juice the instant we pass 0v).
So instead of sweeping from 0-110v, powering the fan the whole time, you get a break of total offness followed by a sweep from 20v (or whatever, based on knob position) to 110v. This is conceptually sort of similar to pulse width modulation as a DC voltage fan speed control.
I believe this method of speed control is what limits these to single-phase AC motors.
So, picture your existing control limiting on time from 20v-110v. With the temp controller at 100% full power goes to the existing control and you then still only get 20v-110v. As the temp controller reduces from 100%, nothing really changes until it starts chopping things higher than 20v.
I can't claim to have a real solid grasp of how the variable resistor/SCR circuit works, but I think that they are sort of like a zener diode and its absolute voltage that matters, so the fact that the temp controller is already chopping the output would not matter to your existing control - it will still work exactly the same as if it was seeing a full signal (that is, the speed drops are not additive).
However, you almost certainly would NOT want the simple contoller ahead of the temp controller, since the temp controller is stepping down and rectifying AC to DC to run the "smarts" circuitry. You will want a normal full AC sine wave provided to that device.
I wouldn't mind testing this if I could think of a cheaper end device (not a $100+ S&P fan), since the only devices at risk would be the simple controller ($20) and whatever's at the end - maybe a cheap house fan or something? Not sure that an incandescent bulb would work - suppose an oscilloscope would show it clearly, but I unfortunately don't have one these days.
2. I'll bet the speed controller buffers itself a bit so its not constantly adjusting. It would be pretty poor design not to.
I'll put out my explanation - which may in fact be quite wrong (hopefully someone more knowledgeable can clarify):
Without going over too much background (google or wikipedia if interested), AC voltage is a sine wave. It oscillates smoothly between full voltage one direction to the other.
The solid state speed controllers use the variable resistor (knob) to create a threshold voltage below which a device called an SCR will not turn on and not apply power to the circuit. Instead of limiting the maximum voltage, they keep the circuit off until the voltage reaches a particular threshold (say 20v instead of your fan getting juice the instant we pass 0v).
So instead of sweeping from 0-110v, powering the fan the whole time, you get a break of total offness followed by a sweep from 20v (or whatever, based on knob position) to 110v. This is conceptually sort of similar to pulse width modulation as a DC voltage fan speed control.
I believe this method of speed control is what limits these to single-phase AC motors.
So, picture your existing control limiting on time from 20v-110v. With the temp controller at 100% full power goes to the existing control and you then still only get 20v-110v. As the temp controller reduces from 100%, nothing really changes until it starts chopping things higher than 20v.
I can't claim to have a real solid grasp of how the variable resistor/SCR circuit works, but I think that they are sort of like a zener diode and its absolute voltage that matters, so the fact that the temp controller is already chopping the output would not matter to your existing control - it will still work exactly the same as if it was seeing a full signal (that is, the speed drops are not additive).
However, you almost certainly would NOT want the simple contoller ahead of the temp controller, since the temp controller is stepping down and rectifying AC to DC to run the "smarts" circuitry. You will want a normal full AC sine wave provided to that device.
I wouldn't mind testing this if I could think of a cheaper end device (not a $100+ S&P fan), since the only devices at risk would be the simple controller ($20) and whatever's at the end - maybe a cheap house fan or something? Not sure that an incandescent bulb would work - suppose an oscilloscope would show it clearly, but I unfortunately don't have one these days.
2. I'll bet the speed controller buffers itself a bit so its not constantly adjusting. It would be pretty poor design not to.