What is the difference between DC brushless motors and permanent magnet synchronous motors?

DC brushless motor is a subspecies of permanent magnet synchronous motor with simplified structure and control. Its typical features are centralized winding, trapezoidal back electromotive force, and square wave drive. The square wave is mainly used for simpler control, turning to a certain position and directly switching phases based on Hall signals. The current amplitude under chopping wave control is completed, and there is no need for the control board to have many complex functions. Pairing square waves also requires trapezoidal back electromotive force, otherwise excess harmonics cannot be utilized and will only cause losses... But now the performance of the controller for DC brushless motors is no longer a problem,
FOC and sensorless control are very popular, and many DC brushless motors use sine back electromotive force combined with sine wave drive. These DC brushless motors are very standard permanent magnet synchronous motors with concentrated windings.

In brushless DC motors, the rotor magnetic poles are usually made of tile shaped magnetic steel. Through magnetic circuit design, the air gap magnetic density of trapezoidal waves can be obtained. The stator winding mostly uses concentrated full pitch winding, so the induced back electromotive force is also trapezoidal waves. The control of brushless DC motors requires position information feedback and must have position sensors or use sensorless estimation technology to form a self-control speed control system. When controlling, the current of each phase should also be controlled as a square wave as much as possible. The output voltage of the inverter should be controlled according to the PWM method of a brushed DC motor.
Essentially, brushless DC motors are also a type of permanent magnet synchronous motor, and their speed regulation actually belongs to the category of variable voltage variable frequency speed regulation. The difference between the two can be attributed to the different design concepts caused by square wave and sine wave control.