Adjustable ac capacitor for a single-phase induction motor

The most common practice for starting a single-phase induction machine (SPIM) is to install a starting capacitor in series with the auxiliary winding. In some applications, two capacitors are used. One is used during the starting period to help create the starting torque. The other one is used during the running condition to improve efficiency. This paper discusses the possibility of using an electronic switch in parallel with the running capacitor, thereby providing the equivalent of a starting capacitor. The capacitor is shorted during each cycle to vary the effective size of the ac capacitor. By using this method, only one capacitor is used for both the starting and running condition, and a similar starting performance can be obtained when compared with the conventional method using two capacitors.