The only way to do this successfully is to add a VFD to a 3-phase grinder. A single phase motor typically found in a bench grinder can't be speed controlled. Induction motors, both single and three phase are synchronous, meaning they lock their speed to the frequency of the incoming AC power. A 3600rpm motor operating on 60Hz will run at 3000rpm at 50Hz, if it is in the capabilities of the motor to run on more than a single frequency. Single phase induction motors used in most machinery need to have some form of starting mechanism, usually a centrifugal switch, multiple windings and often a capacitor or two. This is because single phase AC has no "direction", it just see-saws back and forth in polarity. Three phase power has three independent phases separated by 120 degrees of "rotation." A motor is able to lock in on this directional aspect of the power and spin up instantly with no additional startup circuitry. If you try to control the speed of either type of motor by varying the voltage, the motors will continue to run at the same speed, only the available torque (and with it, horsepower) will change. Once the motor falls out of its synchronous speed, the rpms drop drastically, the motor begins to draw a huge increase in current, and, in the case of single phase motors, will retrigger the startup mechanism. If you vary the frequency, you might succeed in controlling the speed without losing torque. This is very limited with a single phase motor, but with a proper VFD on a three phase, it can be done very precisely, without a loss of torque. A VFD electrically monitors the operation of a three phase motor and is able to modulate the incoming voltage and current along with the frequency in order to keep the motor up to preset speed but not allow it to stall or draw excessive power.
What does this all mean? Three phase grinders can be found in lots of different sizes and HP ratings, but usually no smaller than 3/4hp. They can often be found used for quite a bit less money than the same single phase grinder, since there is very little demand for 3-phase gear outside of industry. The money saved on buying a used 3-phase grinder can go toward buying a VFD. Mix the two together and you will have a very capable machine. Most smaller VFD units can be operated on single phase 240volt AC. Many can be run on either single or three phase.
