一、The definitions of synchronous motors and asynchronous motors

1. Synchronous motor

A synchronous motor is an AC motor, and its core characteristic is that the rotor speed is exactly equal to the synchronous speed of the stator rotating magnetic field (the formula is n = 60f/p, where f is the power supply frequency and p is the number of motor poles). Its working principle is based on the "locking" effect between the stator magnetic field and the rotor magnetic field. The rotor generates a constant magnetic field through permanent magnets or external direct current excitation. When the rotor magnetic field is completely synchronized with the stator rotating magnetic field, the motor can operate stably without any speed difference, and thus is called "synchronous".

2. Asynchronous motor

An asynchronous motor, also known as an induction motor, is another important type of AC motor. Its rotor speed is always lower than the synchronous speed of the stator's rotating magnetic field, and there is a certain speed difference between them (that is, the slip rate s = (n1 - n) / n1, where (0 < s)). Its operation relies on the principle of electromagnetic induction: after the stator windings are energized to generate a rotating magnetic field, they will cut through the rotor conductors and induce an electric current, thereby forming an electromagnetic torque to drive the rotor to rotate. This speed difference is a necessary condition for the generation of the induced current, and without the speed difference, no driving torque can be formed. The term "asynchronous" originates from the difference in speed between the rotor speed and the synchronous speed.

二、The differences between synchronous motors and asynchronous motors

From the perspective of working principle, the rotor magnetic field of a synchronous motor and the stator rotating magnetic field have a "mutually synchronized" relationship. The speed is completely determined by the power supply frequency and the number of poles and is not affected by changes in load; while an asynchronous motor relies on the electromagnetic induction of the rotating magnetic field on the rotor to generate power, the speed difference is the core prerequisite for its operation, and the speed will slightly decrease when the load increases. 

In terms of rotor structure and power supply, the rotor of a synchronous motor is more complex. It requires external DC power for excitation or uses permanent magnets. Some models also have slip rings and brush structures. The rotor of an asynchronous motor is mostly of squirrel-cage or wound conductor type, and does not require an external excitation power supply. The rotor current is naturally generated by electromagnetic induction, and the structure is simpler and more reliable. 

In terms of performance, the prominent advantage of synchronous motors is that their power factor can be adjusted. By changing the excitation current, they can operate in over-excitation (providing reactive power to the power grid and improving the system power factor) or under-excitation (absorbing reactive power from the power grid), with high speed regulation accuracy and constant speed. In contrast, the power factor of asynchronous motors is always lagging. They need to absorb reactive power from the power grid to establish the magnetic field. The lower the load, the lower the power factor. Although the speed regulation performance has been improved through frequency conversion technology, it is still not as precise as that of synchronous motors. 

In terms of startup methods, asynchronous motors can directly connect to AC power for self-starting, which is very convenient to operate; synchronous motors, due to the inertia of their rotors, cannot start directly on their own and require auxiliary methods such as asynchronous starting methods or frequency conversion starting methods to achieve synchronous operation. 

The application scenarios also vary due to differences in characteristics: synchronous motors are mostly used in situations where high requirements are placed on speed stability and power factor control, such as large industrial compressors, precision CNC machines, and power system generators; asynchronous motors, with their advantages of simple structure, low manufacturing cost, and convenient maintenance, are widely used in general machinery, household appliances, and ordinary industrial transmissions where the requirements for speed accuracy are not high.