Research On Regenerative Braking Of Brushless DC Motor For Electric Vehicle

At present,electric vehicles have realized the electrification of vehicle energy and power,effectively alleviating the status quo of global environmental pollution,and avoiding a series of problems caused by the strong dependence of the cost of fuel vehicles on international crude oil prices.Therefore,electric vehicles have become a new type of vehicle that has developed rapidly and is widely used in new energy vehicles.Brushless DC motors have the advantages of high power output and high efficiency,and are widely used in the field of electric vehicles.This article focuses on brushless DC motors for electric vehicles and explores some of the important issues related to their braking conditions.Firstly,based on the mathematical model of brushless DC motor,the commonly used braking methods and braking modulation methods are analyzed and compared.The reasons for the commutation torque ripple of the motor under braking conditions are discusserd,the unified mathematical expression of the commutation torque ripple under different regenerative braking modulation methods is deduced,and the effects of different modulation methods on the motor torque ripple and feedback are studied.The influence of braking provides a theoretical basis for seeking a better PWM modulation method.The PWM-OFF-PWM method does not have the non-commutation torque ripple caused by the non-conductive freewheeling,which can better realize the vehicle regenerative braking.However,this method has torque ripple during the commutation process,this paper proposes an improved PWM-OFF-PWM method.The formulas of the duty cycle of switching tube in the commutation period and that during non-commutation period is derived.When the duty cycle of the outgoing and incoming phases meets the derivation formula,the commutation torque ripple can be effectively suppressed,so as to improve the stability of the motor operation.Secondly,the principle of feedback braking is studied based on PWM-OFF-PWM modulation,and the mathematical model of Brushless DC motor braking system is established.Four common motor feedback braking strategies are analyzed and compared.The constant value braking current method can make the driver have a more comfortable control feeling and is widely used in the field of electric vehicles.In order to solve the problem that electric vehicle system parameters are easily disturbed,a constant braking current control strategy based on sliding mode control is proposed in this paper.This strategy can not only regenerate energy,but also precisely control the braking current,thereby increasing the safety of regenerative braking and the comfort of the car.Then,the system simulation model of the brushless DC motor for electric vehicles is built in the MATLAB/Simulink environment,and the modulation method selected in this paper,the proposed method to suppress the commutation torque ripple and the regenerative braking control strategy are verified.By comparing the current waveforms of PWM-OFF and PWM-OFF-PWM modulation,it is verified that there is no non-conducting phase freewheeling in the latter.By comparing the fluctuations of the non-commutation current and torque in the commutation interval of the PWM-OFF-PWM method before and after the improvement,the effectiveness of the proposed suppression method is verified.By comparing the two braking methods of regenerative braking and natural stop,it is verified that the electric vehicle equipped with regenerative braking system can realize energy recovery.By comparing the current waveforms of traditional PID and sliding mode control,it is verified that the control strategy in this paper has better stability and dynamic response speed.Finally,according to the functional requirements of the control system,its hardware circuit and software program are designed.An experimental platform for the regenerative braking control system for electric vehicles is built,and the method of suppressing commutation torque ripple and the strategy of constant braking current proposed in this paper are verified from different aspects.The experimental results are consistent with the theoretical analysis and simulation results,which prove the correctness and effectiveness of the method.