Research On Direct Torque Control Strategies Of Switched Reluctance Machines

Switched reluctance machines (SRM) have got significant developments in the last decades due to their simple structures, low costs, high reliability and wide speed range. As a time-varying, nonlinear and strongly coupled system, it is hard to achieve good performance with conventional control methods. Torque ripple is one of the main disadvantages of SRM, and can cause speed ripple and noise, so it is necessary to reduce ripple.In this paper, direct torque control strategies of SRM are chose to reduce torque ripple. Two direct torque control methods are illustrated:one is direct torque and flux control and the other is direct instantaneous torque control. A new direct instantaneous control method is also proposed. The followings are discussed in this paper:(1) The basic principle, mathematic model and conventional control methods (Current chopped control, Angular position control and PMW control) of SRM are illustrated. Current research status of SRM is discussed and some existing control methods are analyzed and compared.(2) The direct torque and flux control of SRM is illustrated. The direct torque and flux control is similar to direct torque control (DTC) of conventional AC motors, the torque and amplitude of flux linkage vector are both controlled by choosing proper voltage vector. There are no turn-on and turn-off angles, which is different from conventional control methods. Due to no restriction on phase current position, negative torque are generated which reduce SRM’s efficiency.(3) The direct instantaneous torque control of SRM is introduced. With this method, the converter of SRM is controlled according to rotor position and the error of expected and actual torque. Compared to direct torque and flux control, there are some obvious differences:simpler structure with no flux vector amplitude closed-loop; determine the sectors by rotor position; turn-on and turn-off angle restrict the phase current in positive torque region to avoid generating negative torque. The direct instantaneous torque control is very practical because it is simpler, easy to use, and has good performance.(4) A new direct instantaneous torque control is proposed. This method has a very simple vector table, and can improve torque response. The principle of the new direct instantaneous torque control is illustrated, and a way to determine whether the proposed method can keep the torque at expected value is also proposed.(5) A nonlinear model of SRM is built to estimate torque value. Off-line and on-line parameter identification methods are illustrated. Two torque estimate methods based on this nonlinear model are proposed:one method estimates torque value by current and rotor position, and the other one estimates torque by current and flux linkage.