What's you're intended control method? Phase angle control?
According to torque versus slip characteristic of an asynchronous motor, speed control by voltage (respective phase angle) variation has very limited capabilities. But it can work with fans or rotary pumps that tend to stabilize the speed despite of the motor's torque characteristics. They are basic dimmer circuits, but providing a more stable and symettrical phase angle than a lamp dimmer and suited for inductive load.