1. In terms of motor structure and design

　　The brushless DC motor has the same structure as the permanent magnet synchronous motor, with a permanent magnet rotor and a stator structure similar to that of an AC motor. However, the permanent magnet synchronous motor requires a sinusoidal anti-electric waveform, so there are different considerations in the design. Its rotor design strives to obtain a sinusoidal air gap flux density distribution waveform. The brushless DC motor needs a trapezoidal back-EMF wave, so the rotor is usually designed according to the air gap flux density. Cooperate with the same purpose in winding design. In addition, BLDC control hopes to have a low-inductance winding to reduce the speed drop caused by the load, so the magnetic sheet surface-mounted rotor structure is usually used. The built-in permanent magnet rotor motor is not suitable for brushless DC motor control because of its high inductance. The IPM structure is often used in permanent magnet synchronous motors. Compared with the surface-mounted rotor structure, it can increase the torque of the motor by about 15%.

　　2. Torque fluctuation

　　Brushless DC motor and permanent magnet synchronous motor performance is the most noticeable difference in torque stability. Torque fluctuations during operation are caused by many different factors, the first is the existence of cogging torque. A variety of effective design measures to minimize cogging torque have been developed. For example, the stator skew or the rotor magnetic pole skew can reduce the cogging torque to less than 1% to 2% of the rated torque. In principle, there is not much difference between the cogging torque of a permanent magnet synchronous motor and a brushless DC motor.

　　This article is taken from ------"Permanent Magnet Brushless DC Motor Technology"