Synchronous motor is an AC motor that generates torque by the interaction of the excitation magnetic field supplied by a direct current and the rotating magnetic field of the armature, and rotates at a synchronous speed. It belongs to AC motors, and the stator windings are the same as synchronous motor. Because its rotor rotation speed is the same as the speed of the rotating magnetic field generated by the stator winding, it is called synchronous motor.
When a synchronous motor is working, three-phase symmetrical current flows into the three-phase windings of the stator, and direct current flows into the excitation winding of the rotor. As three-phase alternating current is passed through the three-phase symmetric winding of the stator, rotating magnetic field will be generated in the air gap. Static magnetic field with a constant polarity is generated when direct current is applied to the rotor field winding. If the number of magnetic pole pairs of the rotor magnetic field is equal to the number of magnetic pole pairs of the stator magnetic field, the rotor magnetic field will rotate synchronously with the stator rotating magnetic field due to the magnetic pulling force of the stator magnetic field.