Research On Control And Commutation Torque Ripple Suppression Strategy Of Three-phase Four Switch Brushless DC Motor

The difference between the three-phase four switch and the three-phase six switch is that two equivalent capacitors in series are used to replace the bridge arm of one phase of the threephase six switch.This inverter topology reduces the number of switching tubes,which is equivalent to reducing the switching loss.For cost saving and fault-tolerant control,it is necessary to study the control of three-phase four-switch BRUShless DC motor.But if the topology of the brushless dc motor with dual polarity modulation strategy,when the capacitance of a phase of three-phase(abbreviation for capacitance under)as a conduction phase,the phase of the stator winding is formed by two phase winding capacitance and other circuit,on the contrary,under the help of electromotive force,the distortion makes capacitance current,it will affect the stability of the motor output torque.In addition,due to the topological characteristics of the three-phase four-switch inverter,the voltage of the two phase lines is not uniform,which will affect the phase current change rate and cause the problem of commutation torque ripple.Firstly,this paper introduces the mathematical model of brushless DC motor and the change of three-phase current and opposite electromotive force with Hall signal when the electric Angle is 120°,and analyzes the reason of current distortion when the traditional bipolar modulation method is applied to the three-phase four-switch brushless DC motor by mathematical analysis.In order to improve the current distortion,a method of controlling the capacitor phase current indirectly by modulating the other two currents independently according to the corresponding current waveform function is introduced,and the simulation software is used to verify that the control method can solve the above current problems.However,this method cannot solve the commutation torque ripple problem when the capacitor phase is non-commutation phase.Through simulation observation and mathematical demonstration,it is found that the torque pulsation is caused by the difference between the rising speed of the open phase current and the falling speed of the off phase current during commutation.However,both the direct current control method and the improved method considering commutation torque pulsation control strategy are hysteresis PWM control,switching frequency is not fixed,in order to obtain better control effect,the method needs to increase the control frequency to achieve,which has high performance requirements for the controller.Therefore,the introduction of the low required to control the frequency,and can achieve good control effect of current model predictive control method,the method by determining the commutation time and commutation time modulation strategy,establish the corresponding current prediction model and the optimization function were determined,ultimately through the output,the optimal duty cycle means to achieve stable control.The simulation model is established by using the simulation software,and it is proved that this method can avoid the current fluctuation and commutation torque ripple,the switching frequency is smaller,the motor speed has a smaller tracking static error control effect.Finally,the experimental platform of brushless DC motor is used to test the above method,and the experimental waveform and data under each control strategy are compared to verify the control effect of current model predictive control.