I'm wondering if an AC motor can easily be converted to variable speed. I've got an old vacuum cleaner motor I was thinking of using to drive a home-made lathe. Problem is...I'm not sure if I can put something like a rheostat in the circuit like can be done with direct current. And if so, then is such an animal available from a hardware store, or can it be built? I'm assuming the heat and such would be plenty an issue for the ones for "normal" electrical applications.