Hi Don,
I am not an expert in motor control.
For starters I am guessing that your motor is single phase without brushes in other words brushless. In my opinion that sort of motor needs fairly sophisticated control to vary the speed without losing too much torque, if all you do is lower the voltage to vary the speed there is a risk of overheating the motor.
These days there are fairly cost effective controllers available, but the first thing is what sort of motor are you trying to control?
Cheers,
Stephen.