There are three main modes of operation for synchronous motors, namely, operation as a generator, an electric motor and a compensator.?
Operation as a generator is the most important mode of operation for synchronous motors, and operation as an electric motor is another important mode of operation for synchronous motors. The power factor of synchronous motor can be adjusted, and the application of large synchronous motor can improve the operation efficiency in the occasions where speed regulation is not required.
Synchronous motor can also be connected to the grid as a synchronous compensation machine. In this case, the motor does not carry any mechanical load, and it relies on the regulation of the excitation current in the rotor to send out the required inductive or capacitive reactive power to the grid, so as to achieve the purpose of improving the power factor of the grid or regulating the voltage of the grid.
Principle of operation
When a synchronous motor is in operation, the three-phase stator windings are fed with three-phase symmetrical currents, and the rotor excitation windings are fed with DC currents. When the three-phase alternating current is passed into the three-phase symmetrical windings of the stator, a rotating magnetic field is generated in the air gap. When DC current is passed in the excitation winding of the rotor, a static magnetic field of constant polarity will be generated.
If the number of magnetic poles of the rotor magnetic field is equal to the number of magnetic poles of the stator magnetic field, the rotor magnetic field rotates synchronously with the stator rotating magnetic field due to the magnetic pulling force of the stator magnetic field, i.e., the rotor rotates at a speed and in a direction equivalent to the rotating magnetic field, which is the basic principle of operation of a synchronous motor.
Reference: Baidu Encyclopedia-Synchronous Motor