The KB Electronics Model KBGE-125C Chasis-Mounted Fan Speed Controller with external heat sink (rated for 20 Amps max) should control your 1/2 HP 1-phase 120 volt AC fans. It is a triac-based controller with wiring for a 2-wire potentiometer input.
Interesting. More details please. What model PowerFlex? How did you wire it up? What is the application? What type of motor? What's the low-end performance?