jackymechanical

The short answer is: Brushless DC motors and synchronous AC motors are similar in structure and operation. Some manufacturers and experts even group them together as similar technologies and place them under the "Permanent Magnet Synchronous Motor" column. However, their key difference lies in the stator coil winding and the back-EMF waveform. This gives them unique performance characteristics and stipulates their own drive technology.
 Structural similarity
Despite the particularity of their names, both brushless DC motors and synchronous AC motors are brushless, and both run at synchronous speed. Brushless means that they rely on electronic devices (typically Hall effect sensors) rather than mechanical brushes to control current to the windings. And synchronization means that their rotor and stator electromagnetic fields operate at a synchronous frequency, or synchronous speed.
Brushless DC motors and synchronous AC motors both have permanent magnets embedded in the rotor (typically four or more). The rotor magnet can be ferrite, which is cheaper, but has a lower magnetic flux density, or rare earth alloys (such as neodymium), which has a higher magnetic flux density, but in some applications, the cost is lower high. The stator is usually made of steel laminations, with windings placed on the slot shafts cut on the laminations.
The rotor permanent magnet generates a rotor magnetic flux, and the current supplied to the stator winding generates a magnetic pole. When the rotor position is such that the N pole of the rotor is close to the N pole of the stator, the two poles repel each other and generate torque.
The difference between operation and performance
In a brushless DC motor, the stator coil is trapezoidal around the city, and the generated back electromotive force has a trapezoidal ripple form. Because of their trapezoidal wave form, in order to obtain the best performance, the required DC can form a brushless DC motor. In contrast, a synchronous AC motor is wound by a sinusoid and generates a sinusoidal back electromotive force. Therefore, in order to obtain the best performance, they require a sinusoidal drive current.
 The type of drive current has an effect on the amount of noise generated by the motor. The trapezoidal drive current used by the brushless DC motor tends to produce an extremely large amount of auditory and electrical noise compared to a synchronous AC motor driven by a sinusoid.
Phase commutation, which is the behavior of switching the motor phase motor to drive the appropriate stator coil, which is determined by the rotor position. Inside the brushless DC motor, the rotor position is monitored by three Hall-effect sensors and commutators in six steps, or 60 electronic angles each. Because the commutation is not continuous, torque ripples are generated in each commutation.
Synchronous AC motors benefit from continuous monitoring of the rotor position through a single Hall-effect sensor combined with control logic and a rotary encoder. Because their commutation is continuous, synchronous AC motors can run without the torque ripple sent to the brushless DC motors. Sine commutation, however, requires more complex control algorithms than trapezoidal commutation.