The vast majority of grinder motors are single phase motors of the capacitor start, or capacitor run, or capacitor start and run type. They will promptly burn up on low voltage OR frequency.

How about using a variable frequency drive?

http://www.precision-elec.com/shop/vari ... ip-rating/

If the motor is a capacitor run motor then the rated frequency is as low as you can go with RPM. Higher usually works though. IMHO, this will end in an expensive exercise in futility. What is usually called a brushless DC motor which is actually a stepper motor or synchronous motor depending on the actual construction is the easy and low cost way to do this. Keep in mind that many of these grinders have 1 1/2HP rated motors. Finding a brushless DC motor with adequate starting torque will be another interesting issue to solve.

Lok at treadmill motors and DC motor controllers. This can be made to work. http://www.surpluscenter.com/