Several Causes of Torque Ripple in Brushless DC Motor

1. Torque ripple caused by electromagnetic factors

This is the torque ripple produced by the interaction between the stator current and the rotor magnetic field. It is directly related to the distribution of current waveform, induced electromotive force waveform, and air gap magnetic flux density. Due to design and manufacturing reasons. It is difficult to keep the induced electromotive force as a trapezoidal wave, or the flat top width is not 120° electrical angle; or the induced electromotive force and the current cannot be strictly synchronized due to the insufficient accuracy of the rotor position detection and control system; or the current waveform deviates from the square wave and can only be approximated The ground changes according to the trapezoidal wave and so on. The existence of these factors will lead to the generation of electromagnetic torque ripple. Methods to suppress torque ripple caused by electromagnetic factors include optimization design method, optimal opening angle method, harmonic elimination method, torque feedback method, and so on.

2. Torque ripple caused by current commutation

When the DC brushless motor is working. The stator windings commutate in a certain order. Because of the inductance of each phase winding. Obstruct the instantaneous change of current. Every time through a magnetic state, the current in the armature winding will cause electromagnetic torque pulsation when switching from one phase to another phase. Methods to suppress torque ripple caused by current commutation include current feedback method, hysteresis current method, overlap commutation method, pulse width modulation (PwM) chopping method and so on.

3. Torque ripple caused by cogging

In order to place the stator windings, the stator core of a brushless DC motor must have teeth and implants. Due to the existence of stator cogging, the air gap is uneven, and the magnetic flux within a tooth pitch is relatively concentrated on the teeth. Make the air gap permeability not constant. When the rotor rotates. The air gap magnetic field is about to change, generating cogging torque. The cogging torque is related to the position of the rotor, which causes torque pulsation. The cause of cogging torque is different from the aforementioned two causes of torque ripple. The aforementioned two causes of torque ripple are caused by the interaction between the stator current and the rotor magnetic field, and the cogging moment is generated by the interaction between the stator core and the rotor magnetic field. The most common method to reduce the cogging torque ripple is the stator skew. Or the oblique pole of the rotor.

4. Torque pulsation caused by armature reaction

The influence of armature magnetomotive force on the main magnetic field of the air gap. It is called the armature reaction. The armature reaction of the brushless DC motor is more complicated. According to the nature of the armature reaction, the magnetomotive force of the armature reaction can be decomposed into a quadrature axis component and a direct axis component. The direct-axis armature reacts to the magnetomotive force to demagnetize the main magnetic field first and then increase the magnetization during the rotation of the rotor, so that the total magnetic flux of each pole of the load changes in the vicinity of the total magnetic flux of each pole of the load. In this way, the induced electromotive force and electromagnetic torque will also change, but the effect is not large. The permeability of the permanent magnetic material is very close to that of the air, which makes the magnetic resistance of the quadrature armature reaction magnetic circuit very close. Large, the magnetic flux reacted by the quadrature armature is very small. Its influence on the main magnetic field of the air gap is negligible.