The AC voltage on the stator winding series wound with it and the AC voltage on the other stator winding generate 90° phase difference, thus forming a complex rotating magnetic field and generating starting torque to enable the rotor to rotate. The rotor rotating direction depends on which stator winding that the split- phase is connected in series, therefore the function of split-phase capacitance Cd is to generate starting torque and to change the rotating direction.

If Cd is in open circuit, then the complex rotating magnetic field cannot occur. Without the starting torque, the motor will not be started.

If Cd is in short circuit, then the two windings of the motor pass the in-phase current, the motor is in electric braking status and will not rotate.