To control the speed of a typical AC induction motor (not a universal
AC/DC motor) you need an inverter driver.  You can find them at
reasonable cost on eBay by searching for "inverter motor".  Be sure
to get a single phase model although you can use a three phase model in
some cases if the inverter is not of the 'smart' type that checks to
see what the motor is doing and just gives an un-monitored output.

Be warned, however that not all motors are happy with inverter control
and can actually be damaged.  It is best to replace the motor with an
"inverter duty" model.

As much as I like all this high tech new stuff that was unaffordable
just a few years ago, I think that a better solution would be to
replace the AC motor with a DC model and use a Variac and square wave
bridge to control the speed.

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/