Working Principle of Single-Phase Motor

A single-phase motor has only one winding and a squirrel cage rotor. When a single-phase sinusoidal current is passed through the stator winding, the motor will generate an alternating magnetic field. The strength and direction of this magnetic field change sinusoidally over time, but it is fixed in space, so it is also called an alternating magnetic field. This alternating magnetic field can be divided into two rotating magnetic fields with the same phase speed and opposite rotation direction. When the rotor is stationary, these two rotating magnetic fields will generate two equal and opposite torques in the rotor, making the resultant torque zero.

So the motor cannot rotate. When we use external force to make the motor rotate in a certain direction (such as clockwise), the movement of the cutting magnetic lines of force between the rotor and the clockwise rotating magnetic field becomes smaller, and the movement of the cutting magnetic lines of force between the rotor and the counterclockwise rotating magnetic field becomes larger. In this way, the rotor will rotate in a clockwise direction, because the total electromagnetic torque generated by the rotor is no longer 0.

If the motor can rotate automatically, we need to add a starting winding in the stator. The starting winding is 90 degrees away from the main winding in space. A starting capacitor should be connected in series with the starting winding so that the current in the main winding is 90 degrees away from the main winding in phase, which is the so-called phase-splitting principle. In this way, two currents with a time difference of 90 degrees pass through two windings with a spatial difference of 90 degrees, which will generate a (two-phase) rotating magnetic field in space. Under the action of this magnetic field, the rotor can start automatically.