When a single-phase sinusoidal current passes through the stator winding, the motor generates an alternating magnetic field
The intensity and direction of a magnetic field varies with time, but it is fixed in the spatial direction, so it is also called alternating pulsating magnetic field. The alternating pulsating magnetic field can be divided into two with the same speed, the rotation direction of the rotating magnetic field are on the contrary, when the rotor is at rest, the two rotating magnetic field generating torque of two equal and opposite in the rotor, the synthetic torque is zero, so the motor can not rotate. When we use a force of the motor rotation to one direction (e.g., clockwise), the rotor and the clockwise rotation direction of the rotating magnetic field between the motion of cutting magnetic lines becomes smaller; the rotor and the counterclockwise rotation direction of the rotating magnetic field between the motion of cutting magnetic lines becomes large. Thus, the balance is broken, and the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of the push.
To make the single-phase motor can automatically rotate together, we can add a starting winding in the stator, the starting winding and the main winding is 90 degrees in space, is connected in series with a suitable capacitor start winding, the main winding current and in phase difference approximation
90 degrees, that is, the so-called phase separation principle. In this way, the two currents, which differ by 90 degrees in time, pass through two windings in space with a difference of 90 degrees, which will produce a (two-phase) rotating magnetic field in space,
In the rotating magnetic field, the rotor will start automatically after starting to rise to a certain speed, with the aid of a mounted on the rotor of the centrifugal switch or other control device will automatically disconnect the starting winding, when working normally only the main winding. Therefore, the starting winding can be made to work in a short-time manner. But in many cases, the starting winding continues to open, we call this motor as a single-phase motor, to change the steering of the motor, as long as the auxiliary winding end of the switch can be.
In single-phase motors, another method of generating rotating magnetic fields is called the shaded pole method, also known as the one phase shaded pole motor. The stator of this kind of motor is made of convex pole type with two poles and four poles. Each pole in the 1/3--1/4 side at a small groove is arranged, the magnetic pole is divided into two parts, in the small part is sheathed on a short-circuit copper ring, if this part of the pole cover, so called shaded pole motor. The single phase winding is sheathed on the whole magnetic pole, each pole coil is connected in series, and the polarity of the pole must be arranged according to N, S, N and S in turn. When the stator winding is energized, the main magnetic flux in the magnetic pole, according to Lenz's law, in which the main flux through the circuit copper ring in copper ring generates a lag induced current 90 degrees in phase, the current flux generated in phase also lags behind the main magnetic flux, and it starting capacitor motor the winding, resulting in a rotating magnetic field of the motor to rotate.