What you have is very good idea and I am sure that you aren't the first person to come up with it. What you are referring to is a "VARIAC" or variable AC transformer. Simply stated, it just won't work for a variable motor speed application. The motor will burn up in short order. With DC motors, varying the input voltage will work to some degree. Our problem is that we don't have a DC power supply normally available and if we did, low speed torque may present to be a problem.
There are devices on the market that are built for this purpose but are relatively expensive. Ever notice the price differential between a fixed speed and variable speed grinder? About six hundred US! The difference is the SCR controller and motor.
I really wish it would work, thats what we all would be doing.....
CLWilkins