I too am toying with the idea of speed control for an induction motor.
I am looking at altering the frequency of the A.C. input.
For a 50Hz motor, an increase or decrease of 10 Hz should result in a +/- 20% change in motor speed.
This project is aimed at controlling a telescope and hopefully should overcome the tremors inherent in using a stepper motor. Also, as steppers are limited in speed, they require more complicated gearing.
I am looking at an oscillator, variable over the range of 30Hz to 70Hz, feeding a 100W amplifier which has a reversed power transformer as the load.
Anyone ever tried this?