In single-phase circuit conditions the motor will not start because it is difficult to obtain 90º out of phase currents from a single phase source and if the electric motor is activated out of the zero speed it will start in the direction of the initial speed. In order for the engine to start, it is necessary to have an auxiliary circuit arranged 90º in the space relative to the main and fed by a current also 90º out of time.
The phase difference of the electric motor can be achieved by means of a resistance, an inductance, or a capacitor connected in series with the starting winding. The most common is to use a capacitor in series with the starting winding and when the motor reaches the operating speed, a centrifugal switch shuts off the starting winding circuit. The centrifugal switch is required because most electric motors use an electrolytic capacitor that can be traversed by alternating currents for only a short time. The appropriate condenser produces close to a 90 ° lag and hence a large starting torque.