While I agree in general with the cautionary comments above, SOME line powered induction motors can be controlled by a thyristor-type speed controller, as indicated by the common ceiling fan speed controller. However, this only works with purpose-designed motors in specific applications (ceiling fans, for example) with appropriate load speed/torque relationships and requires a controller designed specifically for motor control, not lighting control. Motors intended for such speed control are tolerant of high slip (difference between motor speed and synchronous speed).
So I think the appropriate question for kalt solutions is what specific motor and application are you trying to control?
awright