The reason why AC asynchronous motors are less efficient than permanent magnet synchronous motors

AC asynchronous motors and permanent magnet synchronous motors are two common types of motors in modern industrial applications. They have certain differences in structure and working principles. Although permanent magnet synchronous motors are relatively more efficient, asynchronous motors still have their unique advantages and applicable scenarios.

First of all, we need to understand the working principles and structural characteristics of AC asynchronous motors and permanent magnet synchronous motors.

AC asynchronous motors are motors that work on the principle of electromagnetic induction. Their rotors do not have any permanent magnets and are not directly connected to a power supply. It generates a rotating magnetic field on the stator through the alternating current input from the power supply, while the rotor of the asynchronous motor rotates by interacting with the rotating magnetic field. Common AC asynchronous motors include induction motors and induction generators. Permanent

magnet synchronous motors are motors that operate by using the magnetic field generated by permanent magnets on the rotor to interact with the rotating magnetic field on the stator. Since permanent magnet synchronous motors have permanent magnets on their rotors, they do not need to generate a magnetic field through the current input from the power supply. The operating speed of the permanent magnet synchronous motor is controlled by controlling the frequency and phase of the rotating magnetic field on the stator.

Below we will discuss in detail the reasons why AC asynchronous motors are less efficient than permanent magnet synchronous motors:

1. Different ways of generating magnetic fields:

AC asynchronous motors need to pass current to generate magnetic fields on the stator coils, while permanent magnet synchronous motors generate magnetic fields through permanent magnets. Since the magnetic field in AC asynchronous motors is generated by current, there will be resistance and electromagnetic induction losses in the current, so there will be additional energy losses. The magnetic field of the permanent magnet synchronous motor comes directly from the permanent magnet, without the need to input additional current, so it can reduce energy losses.

2. Different rotor losses:

The rotor in the AC asynchronous motor is an aluminum cage structure, commonly known as a "squirrel cage rotor". Due to the good electrical conductivity of aluminum, the current in the rotor will generate eddy current losses, resulting in energy losses. The permanent magnet synchronous motor has permanent magnets fixed on the rotor, which will not generate eddy current losses, reducing rotor losses.

3. Efficiency differences under rated load:

Under rated load, the efficiency of the permanent magnet synchronous motor is relatively high because of its magnetic field generation method and small rotor losses. The efficiency of the AC asynchronous motor is relatively low due to the magnetic field generated by the current and the existence of rotor losses.

However, despite its relatively low efficiency, the AC asynchronous motor is still widely used in many applications. The main reasons are as follows:

1. Strong starting ability:

AC asynchronous motors have good starting ability, especially when starting with a large load. Due to the rotor structure of the AC asynchronous motor, when the motor starts, due to the induction of the rotor, the motor will automatically reach the rotation speed of the rotor and can start quickly.

2. Long transmission distance:

AC asynchronous motors are suitable for scenarios with long transmission distances because their transmission losses are relatively small. Permanent magnet synchronous motors are generally suitable for scenarios with short transmission distances. When transmitting power over long distances, due to the resistance and electromagnetic induction losses of the cable, the transmission efficiency of AC asynchronous motors is higher.

3. Relatively low cost:

Due to the simple structure and mature manufacturing process of AC asynchronous motors, their manufacturing cost is relatively low. In contrast, permanent magnet synchronous motors require the use of permanent magnet materials and have a more complex control system, so their manufacturing cost is higher.

In summary, the main reasons for the lower efficiency of AC asynchronous motors compared to permanent magnet synchronous motors include different magnetic field generation methods, different rotor losses, and efficiency differences under rated loads. However, AC asynchronous motors still have certain advantages in many scenarios. Different motor types should be selected according to specific application scenarios and working requirements.