The single-phase induction motor has a main winding connected directly to the mains supply and a lagging auxiliary winding, usually at 90 ° from the main winding and connected in series with a capacitor. Both the auxiliary winding and the capacitor are inserted into the supply circuit only during the starting period of the motor. The capacitor allows a larger angle of lag between the main and auxiliary winding currents, thus providing high starting torque. The main winding alone develops almost the same torque as the combined windings.
For higher velocities between 80% and 90% of the synchronous speed, the curve of the conjugate with the combined windings passes the curve of the conjugate of the main winding so that for speeds above this point, the motor has more conjugated with the auxiliary circuit switched off. Because the crossing of the curves does not always occur at the same point, and still, the centrifugal switch does not open at precisely the same speed, it is common practice to cause the aperture to occur on the average just before the curves intersect. After disconnection of the auxiliary circuit, its operation is the same as that of the split-phase electric motor.