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Analysis on the Knowledge Points of Permanent Magnet Motor

[Guide] The DC permanent magnet motor removes the excitation winding and magnetic poles of a DC excitation motor or a conventional DC motor and replaces it with a permanent magnet. The rest of the main structure such as armature, brush and commutator remains unchanged.



In fact, permanent magnet motors are not just a permanent magnet synchronous motor, there are three basic types of DC permanent magnet, brushless DC permanent magnet and permanent magnet synchronous motor.



DC permanent magnet motor


The DC permanent magnet motor removes the excitation windings and magnetic poles of a DC excitation motor or a conventional DC motor and replaces them with permanent magnets. The remaining main structures such as armatures, brushes and commutators remain unchanged. For example, most toy motors and electric shavers use miniature DC permanent magnet motors as drive motors. The structure is very simple. The stator is made of iron shell with tile-shaped magnets pasted on the inner surface, and the rotor is a "commutator + electric "Pivot", carbon brush, brush holder and wiring system are integrated on the shell. Compared with the electrically excited DC motor, the DC permanent magnet motor's excitation field is not adjustable, and the speed can only be adjusted by adjusting the armature voltage.

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Brushless DC permanent magnet motor


Although the DC permanent magnet motor dispenses with the electric excitation system, due to the use of the "brush/commutator" mechanical contact mechanism, problems such as commutation sparks, electromagnetic interference, short life and reliability still exist, which greatly limit its scope of use.


With the progress and development of microelectronic devices and power electronic devices, the technology of replacing mechanical commutation with electronic commutation has become increasingly mature, and brushless DC permanent magnet motors have developed rapidly. For example, the electric motorcycles and electric bicycles that can be seen almost everywhere in daily life all adopt the outer rotor brushless DC permanent magnet motor.

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Brushless DC motor and brushed DC motor


As mentioned earlier, the brushless DC permanent magnet motor is developed on the basis of the brushed DC motor. The operation mechanism of the two is the same, but there are certain differences in performance.


In terms of operating mechanism, the voltage applied to the motor is a constant DC voltage, and the current input to the motor is a DC current. The polarity of the voltage acting on the armature coil (also known as the component) and the current passing through the armature coil The directions are all alternating, the waveforms of the induced electromotive force in the armature coil are basically similar, and the directions are all alternating. Therefore, brushless DC permanent magnet motors can directly follow the design ideas and design methods of brushed DC motors, but some specific calculation formulas are slightly different.


In terms of operating performance, the number of components of the armature winding of the brushed DC motor and the number of commutating pieces of the commutator are more than the number of phases of the armature winding of the brushless DC motor. There is a big difference during the operation: the magnetic pole field of the brushed DC motor The magnetic field and the armature are always orthogonal, while the magnetic pole field and the armature magnetic field of the brushless DC motor fluctuate within a certain angle range. The orthogonal state is only one of the instantaneous positions. Therefore, under the same other conditions, during operation, the torque ripple of the brushless DC motor is greater than that of the brushed DC motor, and the electromagnetic torque of the brushless DC motor is smaller than the electromagnetic torque of the brushed DC motor.

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Permanent magnet synchronous motor


The operating principle of a permanent magnet synchronous motor is the same as that of a traditional electrically excited synchronous motor. Because no excitation winding and DC excitation power supply are needed, the slip ring and brush device that are prone to problems are eliminated, and a brushless motor is formed.

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●Permanent magnet generator


After the permanent magnet generator is manufactured, it is difficult to adjust the magnetic field to control its output voltage and power factor, which limits its use range. For example, direct-drive permanent magnet wind turbines must exchange energy with the grid through frequency converters, and cannot be directly connected.

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●Permanent magnet synchronous motor


The permanent magnet synchronous motor powered by the frequency converter and the rotor position closed-loop control system form a self-synchronous permanent magnet motor. It not only has the excellent speed regulation characteristics of an electrically excited DC motor, but also achieves brushlessness, which requires high control accuracy and high reliability. The occasions, such as aviation, aerospace, CNC machine tools, machining centers, robots, electric vehicles, computer peripherals and household appliances, have been widely used. Generally, a motor whose back EMF and power supply current waveforms are both rectangular waves is called a brushless DC permanent magnet motor; a motor whose back EMF and power supply current waveforms are both sine waves is called a permanent magnet synchronous motor.

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Permanent magnet synchronous motor and brushless DC permanent magnet motor


Both the permanent magnet synchronous motor and the brushless DC permanent magnet motor are powered by the frequency converter, and the voltage and supply current that are finally added to the armature windings are synchronously alternating with the rotor speed, so they are often called permanent magnet synchronous indiscriminately Motor. However, as far as the internal electromagnetic process is concerned, the permanent magnet synchronous motor and the electrically excited synchronous motor are exactly the same, and the brushless DC permanent magnet motor is basically the same as the electrically excited DC motor. There is a big difference in the mechanism: The pivot magnetic field is a three-phase composite rotating magnetic potential wave, and the rotation speed of the permanent magnet is strictly synchronized with the rotating magnetic potential wave; the armature magnetic field of the brushless DC permanent magnet motor is a jumping magnetic potential wave controlled by a power switch device, and the permanent magnet rotation speed and the magnetic potential wave Macroscopically synchronized.



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