New phase-sensitive technology for capacitor-start motor simplifies application

Single-phase capacitor-start induction motors drive the majority of residential room air conditioning products. Usually, a potential relay controls the start cycle. Proper selection of the correct relay from the hundreds of available models requires detailed knowledge of the application. We have been searching for a new motor-starting technology with the best characteristics of both solid-state and electromechanical relays. Our goal was to develop a universal device with only a few application choices. This hybrid device would electronically adjust to individual motor-starting requirements and would have the robust current-handling ability of a mechanical relay. First, this paper describes a new motor-starting technology that senses phase difference between the mains and start-winding voltage. This technique applies across a wide range of different motor types. The user need only specify line voltage for an application. Second, we describe the implementation of this control using a hard contact for switching the start capacitor and a microcontroller for implementing the control algorithm. Third, we demonstrate that these devices select the optimum point to disconnect the start capacitor through graphical display of performance. Finally, we report on the development and implementation of this new technology.