Research On Improved Direct Torque Control Of Switched Reluctance Motor

With the development of modern electronics technologies and control theories,switched reluctance motor(SRM)is considered to be one of the promising electric machines to compete with conventional ac motors in many applications such as electric vehicle,aviation and aerospace and home appliance.SRM is structurally robust and suitable to run in harsh environment due to its permanent magnet and winding free rotor configuration.It has strong fault tolerance capability because of each phase is excited independently.However,despite the many advantages of SRM,the inherent drawback such as high torque ripple and acoustic noise largely hinder its application in many areas.There are numerous researches that are trying to solve the torque ripple issue of SRM,from motor design aspect or control aspect,and direct torque control(DTC)is one of them.The idea of DTC implemented in SRM is borrowed from ac motors in spite of the many difference between SRM and ac motors.Except for the torque ripple is largely reduced by using DTC,the effectiveness of the method is still questionable due to its low efficiency and high current requirement.In this paper,the basic principle of the conventional DTC by using the half-bridge power converter configuration is introduced,and its unique characteristics which differ from those of other commonly used control methods are presented.By inspecting the reference flux,current and output torque relationship,it is suggested that the efficiency of the motor is largely rely on reference flux selection.Besides,the cause of the generation of the negative phase torque is explained,and it heavily deteriorate the output capability of the motor.These are the major disadvantages of conventional DTC.It is not a wise choice to directly apply the widely adopted circular flux trajectory in ac motor DTC to SRM DTC,since the SRM has very different structure and operation principles.Inspired by how to solve the drawbacks of conventional DTC,the research in this paper suggests a new switching sector definition which fully considers the inductance characteristics of SRM,and defines new voltage vectors and switching tables in different sectors such that the negative torque generation can be prevented.Besides,in order to maintain good efficiency and reduce copper loss in full load range,a simple reference flux generator is proposed.Another constant frequency direct torque control is also proposed to solve the issues of hysteresis controller in conventional method.Simulation model is built in MATLAB/Simulink in order to verify the effectiveness and practicability of the proposed DTC method and compared with the conventional one.Finally,experimental setup is built for a prototype three-phase 6/4 SRM and experimental results are provided which proves the validity of the proposed DTC method and its advantage over conventional method.