Soft-starting techniques for capacitor-run single-phase induction motors

The direct-online starting of induction motors produces troublesome torque and current transients. Soft-starting techniques, which are commonly used with three-phase induction motors, lead to more reliable operation and allow the reduction of the motor sizing: they limit the phase currents during the starting transient in order to obtain a smooth connection to the power supply. Unfortunately, little investigation has been made so far on the application of these techniques to single-phase motors, even if they are widely used, e.g. in households, and of course suffer of the same troubles as three-phase motors. The present work deals with different soft-starting techniques, implemented on a simulation tool. Some of them are based on the so-called point-on-wave switching, which is in fact a delayed insertion, either simultaneous or independent, of both the main and auxiliary phases to the main supply. In order to limit the total transient current within the allowed maximum for household appliances, the gradual insertion of the motor´s main phase, is also investigated and discussed