Research On Torque Ripple Minimization Techniques For Permanent Magnet Brushless Dc Motor

With the development of Power Electronics and rare-earth permanent magnet materials, the advantage of permanent magnet brushless DC motor, which has wide speed adjustment range, high torsion/current ratio, high power density, has become more obvious. Therefore, it attracts more and more attention and applied more widely. The torque ripple problem, however, limits the application of permanent magnet brushless DC motor in the area which requires precise position and speed control. Due to the 120°and 2-phase conduction mode and the existence of the armature inductance, the torque ripple will be generated in the commutation period. The torque ripple due to phase commutation is the main reason for generating the torque ripple of the permanent magnet brushless DC motor and takes about 50% of the output torsion. The analysis and restraint of the torque ripple in the commutation period of the permanent magnet brushless DC motor has become the focus of the domestic and the international research. Focusing on the existing problem of running and controlling the permanent magnet brushless DC motor, this thesis studied the restraint strategy to restrain the torque ripple due to phase commutation of permanent magnet brushless DC motorThis thesis analyzed the working mechanism of permanent magnet brushless DC motor, built the mathematical model of permanent magnet brushless DC motor and proposed theoretical basis and analytical model for the new restraint strategy; analyzed the reason of the generation of the torque ripple due to phase commutation and obtained the analytical expression. Based on this expression, the running status of the motors is classified into two categories: low-speed and high-speed. At the low-speed mode, the effect of several generally used PWM modulation methods to the electromagnetic torque ripple and the restraining results of these modulation methods to low-speed torque ripple have been analyzed; at the high-speed mode, by studying the overlap phase commutation method, the mechanism and application range of the overlap phase commutation method restraining the torque ripple due to phase commutation was explained by details. Pointing against the undecidable overlap phase commutation delay, the required time of restraining torque ripple due to phase commutation by overlap phase commutation method was mathematically deduced and simplifed by Taylor's formula. This method figured out the undecidable overlap phase commutation delay, carried out simply, and could be embedded in the policy of phase commutation control. After passing the phase commutation time and coming back to the non- phase-changing status, the torque ripple which is generated by the current switch process is efficiently restrained.On the basis of the improved control strategy, the driving system design scheme based on DPS and FPGA was proposed; the hardware and software design of the driving system has been finished. By the comparison experiment, the new method's restraint of torque ripple due to phase commutation of the new method and its efficiency of improving the permanent magnet brushless DC motor 's function have been proved.The research of this thesis not only provided the theoretical basis for improving the permanent magnet brushless DC motor's operating function, but also proposed the existing problem and difficulties of restraining the torque ripple and finally pointed out the direction for the future research.