[914GT]

Induction motors are AC synchronous motors, which means their RPM is dependent on the power line frequency. There's no easy way to vary their RPM, and a dimmer switch won't work. You can chop the AC voltage somewhat and get a slight RPM reduction, but a little too much it will just stall. If it's belt driven you're best bet is to go to a larger pulley on the squirrel cage, or smaller on the motor shaft. If the cage is directly mounted on the motor shaft then you're SOL.