The Working Principle Of A Single-phase Motor

When the single-phase sinusoidal current passes through the stator winding, the motor will produce an alternating magnetic field, the strength and direction of this magnetic field change sinusoidally with time, but it is fixed in spatial orientation, so this magnetic field is also called an alternating pulsating magnetic field. This alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with the same speed and opposite rotation direction, and when the rotor is stationary, the two rotating magnetic fields generate two torques of equal size and opposite directions in the rotor, so that the synthetic torque is zero, so the motor cannot rotate. When we use an external force to make the motor rotate in a certain direction (such as clockwise rotation), the cutting magnetic field line motion between the rotor and the rotating magnetic field in the clockwise direction becomes smaller; The motion of the cutting magnetic field lines between the rotor and the rotating magnetic field in the counterclockwise direction becomes larger. In this way, the balance is broken, the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of pushing.
To make the single-phase motor automatically rotate, we can add a starting winding to the stator, the starting winding and the main winding are 90 degrees different in space, and the starting winding should be connected in series with a suitable capacitor, so that the current with the main winding is approximately 90 degrees different in phase, the so-called phase separation principle. Such two currents that differ by 90 degrees in time into two windings that differ 90 degrees in space will generate a (two-phase) rotating magnetic field in space.