Change the motor to a 1/2 hp, 208-230 vac, 3 phase motor, and purchase a 208-230vac variable frequency drive. You will need your household 208-240vac power to power the VFD. You can feed most small VFD's with single phase in to get 3 phase out. If the drive does not specifically say it is rated for single phase input, then you will need to purchase a 1 HP, 3 phase rated VFD to handle the higher single phase current input.
You would be surpried by how cheap the motor, and VFD are on ebay. Just be sure the motor is the same frame size, and RPM rating.
The way to efficently change the RPM of an induction motor is to change the frequency of the power feeding to it.