what would happen if I used a rheostat in series with a motor to drop my 230v ac supply to 110v ac?
It is very unlikely to be satisfactory. The reason is that the motor, as an electrical load, is variable depending on speed and mechanical load conditions on the motor. You would have to adjust the rheostat faster than humanly possible to get this to work the way you probably want it to. Maybe it could work in some very limited conditions. Also, note that you will dissipate a lot of power in the rheostat. Depending on motor size, the rheostat may need a fan to prevent it from overheating, and it will also be energized at a hazardous voltage. Talk about double-trouble.