Interfacing to COTS AC Motor Controllers

This is never easy. Motor controllers evolve over time. The evolution is directed by engineers with years of experience who know way more about motor phases and minutia than you do. They have engineered every unnecessary part out of the circuit. Controls, such as speed, may not (usually ARE NOT) operating with one teminal at ground potential. All 3 terminals may be floating. The control likely does NOT operate with a simple DC signal.

Do This

1. Determine the voltage at all major terminals of the motor and control. Adjust the speed and measure them all again. Become VERY familiar with the basic operation.
2. Reverse engineer the circuit around the speed control. Take lots of data
3. Develop your control circuit to mimic the operation of the speed control.
4. Design an interface between the pot and the new controller that, in the best case, allows both to have some authority. Use the pot to set some minimum speed and the controller takes over above that point.
5. I'm tired and going to bed. Figure it out for yourself

Don't

Don't just pull out the pot and wire something else in its place. Don't put in a swith and try to select either the pot or the new controller. When you flip the switch, there is a brief period of time when there is NO control. Also, the abrupt change may not be well received by the motor and load.