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Permanent magnet synchronous motor VS AC asynchronous motor

2021-08-12 16:18:02 Volcano Motor Read

In the power system, the expressions of permanent magnet synchronous motor and AC asynchronous motor are often heard, and permanent magnet synchronous motor occupies the vast majority in frequency. What are the differences between them, and what are their advantages and disadvantages?


Motor characteristics:


AC asynchronous motor

Power density



Speed adjustable range



Unit power cost



Demagnetization phenomenon

The temperature is high, will



Compared with AC asynchronous motor, permanent magnet synchronous motor has the following advantages:

1. Because the magnetic field of permanent magnet synchronous motor is generated by permanent magnet, the excitation loss caused by the magnetic field generated by excitation current is avoided.


2. Compared with asynchronous motor, the external characteristic efficiency curve of permanent magnet synchronous motor has higher efficiency at light load, which is also the biggest advantage of permanent magnet synchronous motor in energy saving.

Usually, when the motor controller is under load, it rarely runs at full power. This is because, on the one hand, the user usually determines the motor power according to the extreme working conditions of the load when selecting a motor, and the chance of extreme working conditions is very small. At the same time, in order to prevent the motor from burning under asynchronous conditions, the user will leave a certain amount of motor power allowance.

On the other hand, when designers design motors, in order to ensure the reliability of motors, users usually need a certain power margin on this basis, so the working power of more than 90% of the actual running motors is lower than 70% of the rated power, especially for motors driving fans or pumps. As a result, motors usually work in light load areas. For induction motor, its efficiency is very low under light load, while permanent magnet synchronous motor can still maintain high efficiency under light load.


3. Because of the high power factor of permanent magnet synchronous motor, its current is less than that of asynchronous motor, so the stator copper loss of motor is small and the efficiency is high.


4. High system efficiency. The parameters of permanent magnet motor, especially the power factor, are not affected by the number of motor poles, so it is easy to design multi-pole motor, which can make traditional load motor driven by gearbox become permanent magnet synchronous motor. The direct controller system driven by motor saves gearbox and improves transmission efficiency.


5. High power factor: the power factor of permanent magnet synchronous motor can be adjusted in design, even can be designed as 1, which has nothing to do with the number of poles.

However, due to its own excitation characteristics, asynchronous motor will increase with the increase of the number of poles, which will inevitably lead to the decrease of power factor. For example, an 8-pole motor, whose power factor is usually about 0.85, has the following advantages:

A. High power factor, low motor current, reduce copper consumption of motor stator and save energy.


B. The power factor is high, the power capacity of the motor can be reduced, other auxiliary facilities such as switches and cables can be smaller, and the corresponding cost is lower.


C. The power factor of permanent magnet synchronous motor is not affected by the number of poles. If the motor support system allows, the number of poles can be designed to be more, the corresponding motor is smaller and the direct material cost of the motor is lower.


6. The structure is simple and flexible: because the parameters of the permanent magnet synchronous motor are not affected by the number of poles of the motor, it is convenient for the motor to drive the load directly, and the gearbox with high noise and high failure rate is omitted.

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