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Permanent Magnet Synchronous Motor: Structure and Advantages

Mar. 24, 2021
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Permanent Magnet Synchronous Motors have the advantages of simple structure, small size, high efficiency and high power factor. At present, permanent magnet synchronous motors have achieved performance in medium and low voltage motors in metallurgical, ceramic, rubber, petroleum, textile and other industries, and have gradually accumulated design and operating experience.


Permanent Magnet Synchronous Motors

Permanent Magnet Synchronous Motors



Structure


The permanent magnet synchronous motor is mainly composed of rotor, end cover and stator. The stator structure of a permanent magnet synchronous motor is very similar to the structure of an ordinary induction motor. The biggest difference between the rotor structure and the asynchronous motor is that high-quality permanent magnet poles are placed on the rotor. According to the position of the permanent magnet on the rotor, Permanent magnet synchronous motors are usually divided into surface rotor structure and built-in rotor structure.


The placement of the permanent magnet has a great influence on the performance of the motor. Surface type rotor structure-permanent magnets are located on the outer surface of the rotor core. This type of rotor has a simple structure, but the asynchronous torque generated is very small, which is only suitable for occasions with low starting requirements and is rarely used. Built-in rotor structure-the permanent magnet is located in the iron core between the squirrel cage guide bar and the rotating shaft, and the starting performance is good. Most of the current permanent magnet synchronous motors adopt this structure.


Working principle


The start and operation of the permanent magnet synchronous motor are formed by the interaction of the magnetic fields generated by the stator windings, the rotor squirrel cage windings and the permanent magnets. When the motor is stationary, a three-phase symmetrical current is applied to the stator winding to generate a stator rotating magnetic field. The stator rotating magnetic field generates current in the cage winding relative to the rotation of the rotor to form a rotor rotating magnetic field. The stator rotating magnetic field interacts with the rotor rotating magnetic field. The asynchronous torque causes the rotor to accelerate from a standstill. In this process, the permanent magnetic field of the rotor and the rotating magnetic field of the stator rotate at different speeds, which will produce alternating torque.


When the rotor accelerates to a speed close to the synchronous speed, the speed of the rotor permanent magnetic field and the stator rotating magnetic field are nearly equal, and the stator rotating magnetic field speed is slightly higher than the rotor permanent magnetic field. They interact to produce torque and pull the rotor into a synchronous operation state. In the synchronous operation state, no current is generated in the rotor winding. At this time, only the permanent magnet on the rotor generates a magnetic field, which interacts with the rotating magnetic field of the stator to generate a driving torque. It can be seen that the permanent magnet synchronous motor is started by the asynchronous torque of the rotor winding. After the start is completed, the rotor winding no longer functions, and the magnetic field generated by the permanent magnet and the stator winding interacts to generate a driving torque.


Advantage


1. Low loss and low temperature rise


Since the magnetic field of the permanent magnet synchronous motor is generated by the permanent magnet, the excitation loss caused by the magnetic field generated by the excitation current, that is, copper loss, is avoided; the rotor runs without current, which significantly reduces the temperature rise of the motor, and the temperature rise under the same load Lower than 20K.


2. High power factor


The permanent magnet synchronous motor has a high power factor and has nothing to do with the number of motor stages. The power factor of the motor is close to 1 when the motor is fully loaded. Compared with the asynchronous motor, the motor current is smaller, and the stator copper loss of the motor is correspondingly smaller and the efficiency is higher. . As the number of motor stages increases, asynchronous motors have lower and lower power factors. Moreover, because of the high power factor of the permanent magnet synchronous motor, the power supply capacity of the motor can theoretically be reduced, and at the same time, the specifications of the supporting switch equipment and cables can be reduced.


3. High efficiency


Compared with Asynchronous Motors, permanent magnet synchronous motors have a much higher efficiency value at light load, and have a wide range of high-efficiency operation. The efficiency is greater than 90% in the range of 25% to 120%. The rated efficiency of permanent magnet synchronous motors can reach 1 of the current national standard. Level energy efficiency requirements, which is its biggest advantage over asynchronous motors in terms of energy saving. In actual operation, the motor seldom runs at full power when driving a load. The reason is: On the one hand, when the designer selects the motor, the motor power is generally determined according to the extreme working conditions of the load, and the chance of extreme working conditions is very small. At the same time, in order to prevent burning under abnormal working conditions In order to ensure the reliability of the motor, the motor manufacturer will usually leave a certain power margin on the basis of the power required by the user in order to ensure the reliability of the motor. This leads to the actual operation of the motor, most of the work is below 70% of the rated power, especially for driving fans or pumps, the motor usually works in the light load area. For the asynchronous motor, its light load efficiency is very low, while the permanent magnet synchronous motor can still maintain a high efficiency in the light load area.


4. Other advantages


The permanent magnet synchronous motor also has the advantages of high starting torque, short starting time, and high overload capacity. It can reduce the installed capacity of the equipment drive motor according to the actual shaft power, save energy and reduce investment in fixed assets. The permanent magnet synchronous motor is easy to control, has a constant speed, does not change with load fluctuations or voltage fluctuations, only depends on the frequency, and runs smoothly and reliably. Because the speed is strictly synchronized, the dynamic response performance is good, and it is suitable for frequency conversion control. The installation dimensions of the permanent magnet synchronous motor conform to the IEC standard, which can directly replace the three-phase asynchronous motor, and the protection level can achieve IP54 and IP55. Some manufacturers also produce explosion-proof permanent magnet synchronous motors.


The company also provide Squirrel Cage Motors, please feel free to contact us if necessary


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