For those that don't understand induction motors. An induction motor is both a trasnformer and a motor. It's really a clever idea that recognizes that both motors and transformers are made of two coils. The speed of the rotor is important in an induction motor so speeds are controlled by the design of the motor. That's why multiple speed induction motors have mutliple sets of stator windings. Which winding you energize determines the speed. And also why messing aorund with the voltage, particularly with controls that simply clip the sine wave doesn't work very well. You can vary the speed by changing the frequency. But only to a point because low frequencies don't work so well with a transformer.