Introduction and Working Principle of Brushless DC Motor

I. What is a Brushless DC Motor  

A brushless DC motor consists of a main motor body and a driver. It is a typical example of an integrated mechanical and electrical product. Since brushless DC motors operate in a self-controlled manner, they do not require additional starting windings on the rotor, unlike synchronous motors under variable-frequency speed control during heavy-load startups. Moreover, brushless DC motors do not experience oscillations or loss of synchronization when subjected to sudden load changes.  

For small and medium-capacity brushless DC motors, the permanent magnets are now often made from high magnetic energy product materials such as rare-earth neodymium-iron-boron (Nd-Fe-B). As a result, the size of rare-earth permanent magnet brushless DC motors is reduced by one frame size compared to three-phase asynchronous motors of the same capacity.  

II. Working Principle of Brushless DC Motors  

Brushless DC motors use semiconductor switching devices to achieve electronic commutation, meaning that electronic switching devices replace traditional contact-type commutators and brushes. Brushless DC motors offer advantages such as high reliability, no commutation sparks, and low mechanical noise. They are widely used in high-end tape decks, video recorders, electronic instruments, and automated office equipment.  

A brushless DC motor is composed of a permanent magnet rotor, a multi-pole winding stator, and a position sensor. Based on changes in the rotor's position, the position sensor commutates the current in the stator windings in a specific sequence (i.e., it detects the position of the rotor's magnetic poles relative to the stator windings and generates a position sensing signal at specific locations. This signal is processed by a signal conversion circuit to control the power switching circuit, which switches the winding current according to a certain logical relationship). The operating voltage of the stator windings is provided by an electronic switching circuit controlled by the output of the position sensor.  

There are three types of position sensors used in brushless DC motors: magnetic-sensitive, photoelectric, and electromagnetic.  

- In brushless DC motors using magnetic-sensitive position sensors, magnetic-sensitive devices (such as Hall elements, magnetic-sensitive diodes, magnetic-sensitive transistors, magnetic-sensitive resistors, or specialized integrated circuits) are installed on the stator assembly to detect changes in the magnetic field generated by the rotation of the permanent magnet rotor.  

- In brushless DC motors using photoelectric position sensors, photoelectric sensor devices are arranged in specific positions on the stator assembly. The rotor is equipped with a shading plate, and the light source is a light-emitting diode or a small bulb. When the rotor rotates, the shading plate causes the photosensitive components on the stator to generate pulse signals intermittently at a certain frequency.  

- In brushless DC motors using electromagnetic position sensors, electromagnetic sensor components (such as coupling transformers, proximity switches, LC resonant circuits, etc.) are installed on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect causes the electromagnetic sensor to generate a high-frequency modulated signal (the amplitude of which varies with the rotor's position).