The long axis of the rotor is in the same direction as the magnetic force line, and in this state the magnetic resistance on the magnetic force line is the smallest. If the rotor rotates at an angle as shown in the left figure, the path of the magnetic force line will become longer and the magnetic resistance will become larger. At this time, the magnetic field lines will generate torque on the rotor, pushing it back to the original position with the least magnetic resistance. The non-rotating part of the motor is called the stator, and the rotating part is called the rotor. If the rotor is inside and the stator is outside, it is called an inner rotor, and it can also be reversed with the rotor on the outside and the middle part as a stator. Both the magnetic field of the stator and the rotor can be generated by the current passing through the excitation coil, or one can be generated by the current and the other by a permanent magnet. Let’s talk about synchronous and asynchronous again. A motor with a long application history is the AC induction motor. The three-phase alternating current excites in the stator of this motor, and the synthesized magnetic field is a rotating magnetic field that changes with the frequency of alternating current. The squirrel cage rotor in the static state begins to cut the magnetic force lines to form a current, and the current in turn generates a magnetic field, which interacts with the stator magnetic field. function and start to rotate. But the speed of the rotor will always lag behind the rotation speed of the magnetic field in the stator, because once the two speeds are the same, the stator will no longer cut the magnetic field lines and lose power. There is always a certain speed difference between the rotor and the stator, and they do not rotate synchronously, so they are called asynchronous motors. If the rotor uses permanent magnets, or uses excitation current to generate a magnetic field, then it can rotate with the rotation of the stator magnetic field, which is called a synchronous motor.