There are many reasons for the torque fluctuation of Brushless DC motor, which can be divided into the following aspects: torque fluctuation caused by electromagnetic torque generation principle, torque fluctuation caused by current commutation, and torque fluctuation caused by cogging effect. In addition, there are torque fluctuations caused by armature reaction and motor process defects. They are described as follows:

Principle electromagnetic torque fluctuation caused by non ideal back EMF waveform

From the working principle, sine wave drive is a high-performance control mode. The current is continuous. The three-phase sine wave AC current and the three-phase sine wave back EMF in the three-phase winding work together to produce smooth and stable electromagnetic torque. Theoretically, a uniform output torque independent of rotation angle can be obtained, and a well-designed system can achieve a low ripple torque of less than 3%. The stator magnetic field of square wave drive is discontinuous and step-by-step rotation. From the principle of electromagnetic torque generation, the torque fluctuation of Brushless DC motor is much larger than that of sine wave drive. Although the back EMF is trapezoidal wave, the flat top width is 120 ° and the stator current is square wave, the electromagnetic torque will be constant without considering the commutation process, and there is no torque fluctuation in theory. However, in the actual motor, due to the reasons of design and manufacture, it is difficult to achieve that the back EMF is a trapezoidal wave with a flat top width of 120 ° electric angle; In fact, the back EMF waveform of most brushless DC motors can not be trapezoidal wave, but closer to sine wave; In this way, the current waveform will inevitably deviate from the square wave, and these non ideal conditions will lead to the principle fluctuation of its electromagnetic torque.