Brushless DC motors are actually permanent magnet synchronous motors. The main source of force is the distortion of magnetic field lines. Simply put, if the magnetic field lines of the stator and rotor magnetic fields are aligned, the lines will be smooth and harmonious, resulting in no torque. However, if the rotor and stator magnetic fields are not aligned, the distribution of magnetic field lines will be uneven. In this case, the magnetic fields will interact to align the stator and rotor magnetic fields, thus generating torque. Therefore, for a brushless DC motor, if the stator magnetic field rotates at a certain speed in space, this anti-distortion force of the magnetic field lines will cause the rotor magnetic field to rotate at the same speed as the stator magnetic field, thus achieving permanent magnet synchronization. You can imagine that if the load torque is 0, the directions of the stator and rotor magnetic fields will inevitably approach coincide during rotation. If there is a constant load torque, the magnetic fields between the stator and rotor will have a fixed angle, the magnitude of which is the same as the magnitude of the load torque.
Brushless DC motor principle
The stator of a brushless DC motor is a coil winding armature, and the rotor is a permanent magnet. If only a fixed DC current is supplied to the motor, the motor can only generate a constant magnetic field and cannot rotate. Only by detecting the position of the motor rotor in real time and supplying corresponding currents to different phases of the motor according to the rotor position, so that the stator generates a rotating magnetic field with a uniformly changing direction, can the motor rotate with the magnetic field.
The diagram illustrates the rotation principle of a brushless DC motor. For ease of description, the center tap of the stator coil is connected to the motor power supply (POWER), and the terminals of each phase are connected to power transistors. When the position sensors are turned on, the gate (G) terminal of the power transistor is connected to 12V, energizing the corresponding phase coil. As the rotor rotates, the three position sensors sequentially turn on, energizing the corresponding phase coils in turn. This causes the direction of the magnetic field generated by the stator to continuously change, causing the rotor to rotate. This is the basic rotation principle of a brushless DC motor: detecting the rotor's position and sequentially energizing each phase to continuously and uniformly change the direction of the magnetic field generated by the stator.
Figure: Schematic diagram of the rotation principle of a brushless DC motor
Modern control methods
The definition of a brushless DC motor is currently unclear. It is generally believed that BLDC brushless DC motors use square wave control, while PMSM permanent magnet synchronous motors use sinusoidal wave control (vector control). However, from the perspective of the motor itself, the only difference between them is the shape of the air gap magnetic field. A pure square wave magnetic field is basically impossible to design; they are all trapezoidal waves.
For BLDC, the control uses a six-phase commutation cycle and Hall effect sensors. Some people use sine waves (interpolation) to reduce torque ripple, but BLDC's low-speed characteristics are poor. There's no such thing as a "modern" control method; we can only say what stage of research has been reached, such as its sensorless algorithm. BLDC's current control algorithm is relatively outdated, used in older products or ultra-high-speed motors (where the number of switching cycles in a fundamental cycle is extremely small). For general applications, PMSM vector control is more widely studied. We'll discuss PMSM below.
For PMSM, conventional vector control is mature, and several research directions exist: sensorless control (back EMF observation, including model reference adaptation, sliding mode, Karl slack, and high-frequency injection methods suitable for low speeds), MTPA maximum torque-to-current ratio control (optimal copper loss, aiming for a universal algorithm that doesn't require motor parameters, but currently only perturbation-based or parameter-based methods exist), field weakening control, online parameter identification, offline parameter identification, controller parameter self-tuning, online temperature identification, direct torque control (DTC), and bandwidth-optimized controllers for various servos—too many to go into detail. Current development trends are: high performance, versatility, intelligence, and adaptability.
