Get a VFD instead of a phase convertor. With that you can change speeds electronically. My VFD was only $150 for a 2hp motor. You can put a potentiometer on the machine to control the frequency/speed. Mine is hooked up through a computer for my CNC converted mill and it starts the motor, changes speeds, reverses and stops the motor.
Here are my new toys. I still need to get a good Bridgeport style mill for the projects where you need to have some sort of feel when machining.