I. Definitions of Synchronous Motors and Asynchronous Motors

1. Synchronous motor

A synchronous motor is an AC motor, and its core feature is that the rotor speed strictly equals the synchronous speed of the stator's rotating magnetic field (the formula is n=60f/p, where f is the power supply frequency and p is the number of motor pole pairs). Its working principle is based on the "interlocking"; effect between the stator magnetic field and the rotor magnetic field. The rotor generates a constant magnetic field through permanent magnets or external DC excitation. When the rotor magnetic field and the stator rotating magnetic field reach complete synchronization, the motor can operate stably without any speed difference, and thus it is called "synchronization";.

2. Asynchronous motor

Asynchronous motors, also known as induction motors, are an important type of AC motors. The rotor speed of an asynchronous motor is always lower than the synchronous speed of the stator's rotating magnetic field, and there is a certain speed difference between the two (i.e., the slip rate s=(n1 - n)/n1, (0<s)). Its operation relies on the principle of electromagnetic induction: when the stator winding is energized to generate a rotating magnetic field, it will cut the rotor conductors and induce a 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. Without the speed difference, the driving torque cannot be formed. "Asynchronous"; comes from the difference between the rotor speed and the synchronous speed.

Ii. The Differences between Synchronous Motors and Asynchronous Motors

From the perspective of working principle, the rotor magnetic field and the stator rotating magnetic field of a synchronous motor have an "interlocking synchronous"; relationship. The rotational speed is completely determined by the power supply frequency and the number of pole pairs, and is not affected by load changes. Asynchronous motors rely on the electromagnetic induction of the rotor by a rotating magnetic field to generate power. The speed difference is the core prerequisite for their operation. When the load increases, the speed will slightly decrease.

In terms of rotor structure and power supply, the rotor structure of synchronous motors is more complex. It requires excitation through an external DC power supply or the use of permanent magnets. Some models also have slip ring and brush structures. The rotors of asynchronous motors are mostly squirrel-cage or wound-rotor conductor structures, without the need for an external excitation power supply. The rotor current is naturally generated by electromagnetic induction, featuring a simple structure and higher reliability.

In terms of performance, the prominent advantage of synchronous motors is the adjustable power factor. By changing the excitation current, they can achieve overexcited (providing reactive power to the power grid to improve the system's power factor) or underexcited (absorbing reactive power from the power grid) operation, and they have high speed regulation accuracy and constant rotational speed. The power factor of an asynchronous motor is always in a lagging state. It needs to absorb reactive power from the power grid to establish a magnetic field. The lighter 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 essence.

In terms of starting methods, asynchronous motors can be directly connected to AC power for self-starting, and the operation is simple and convenient. Due to the inertia of the rotor, synchronous motors cannot start directly by themselves. They need to rely on auxiliary methods such as asynchronous starting and variable frequency starting to achieve synchronous operation.

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