Brushless DC motors usually use tile-type magnets for the rotor poles. After the magnetic circuit design, the air gap flux density of the trapezoidal wave can be obtained. Most of the stator windings use concentrated full-pitch windings, so the induced back electromotive force is also a trapezoidal wave. The control of the brushless DC motor requires position information feedback, and it must have a position sensor or adopt a position sensorless estimation technology to form a self-controlled speed regulation system. When controlling, the current of each phase is also controlled as a square wave as much as possible, and the output voltage of the inverter can be controlled in accordance with the PWM method of the brushed DC motor. In essence, the brushless DC motor is also a permanent magnet synchronous motor, and the speed regulation actually belongs to the category of variable voltage and frequency conversion speed regulation.
Generally speaking, the AC permanent magnet synchronous servo motor has a stator three-phase distributed winding and a permanent magnet rotor. In the magnetic circuit structure and winding distribution, it is ensured that the induced electromotive force waveform is sine, and the external stator voltage and current should also be sine waves, generally relying on AC the transformer frequency converter is provided. Permanent magnet synchronous motor control systems often use automatic control, and position feedback information is also required. Advanced control methods such as vector control (field-oriented control) or direct torque control can be used.
The difference between the two can be considered to be the different design concepts caused by the square wave and sine wave control. Finally, a concept is clarified. The so-called "DC frequency conversion" of brushless DC motors is essentially AC frequency conversion through inverters. In theory, brushless DC motors are similar to AC permanent magnet synchronous servo motors and should be classified as AC permanent magnet synchronous servo motor; but it is customarily classified as DC motor, because from the point of view of its control and drive power supply and control object, it is also suitable to call it a "brushless DC motor".
The inverter output voltage can be controlled in accordance with the method of a brushed DC motor. In essence, the brushless DC motor is also a permanent magnet synchronous motor, and the speed regulation actually belongs to the category of variable voltage and frequency conversion speed regulation.