A single phase induction motor voltage controller with improved performance

Induction motors inherently operate with nearly constant air-gap flux and therefore almost constant iron losses. When the load does not require full flux, conventional voltage controllers utilize thyristors in series with the motor to reduce air-gap flux by decreasing the applied voltage. Thereby, iron losses decrease and the overall efficiency increases. However, thyristor voltage controllers tend to introduce harmonics into the current waveform which not only reduces motor efficiency but also causes harmonic pollution of the power lines. An improved voltage controller and control strategy for efficiency improvement of single phase induction motors is presented in this paper. In particular, thyristor voltage control by dynamic switching of the winding configuration is presented. Laboratory data for a voltage controller, thus enhanced, demonstrates a significant decrease in input motor current distortion and increase in efficiency below one-quarter load.