A triac voltage (speed) control for improved performance of split-phase fan motors

Fan speed control of permanent capacitor run motors using triacs creates unbalance resulting from both amplitude and phase. The use of a force-commutation triac has greatly reduced this unbalance by advancing the fundamental of the main winding current at reduced speeds. The extra harmonics generated only slightly reduce the benefits of less unbalanced operation, and by using two triac conduction periods per half cycle, the third harmonic has been eliminated yielding further reductions of current, amounting in all to 35%.