Research On High Performance Speed Sensorless Direct Torque Control System

Application of conventional direct torque control (CDTC) system is limited by the problems of large ripple of torque and flux, high harmonic content of current and poor accuracy of flux observer in the steady state operation. In this dissertation, speed sensorless direct torque control system of induction machine is researched profoundly in order to solve the problems existing in CDTC and improve the performance of observer for flux and speed when speed sensorless technique is applied at low speeds.Firstly, the principle of DTC is researched, then the block diagram of DTC system of induction machine is described, and the CDTC system of induction machine based on circle track control of flux is simulated in the two case of high speed and low speed by using MATLAB/Simulink simulation software, and simulation results are presented.A novel sliding mode variable structure controller of flux and torque is proposed to replace the conventional hysteresis controller in order to solve the problems of large ripple of flux and torque in the steady state condition of CDTC system. In addition, space vector pulse width modulation (SVPWM) technique is combined with obtaining circle flux track as a goal. Simulation results demonstrate that the proposed novel sliding mode variable structure controller keeps the fast reaction of CDTC, while the ripples of flux and torque are reduced greatly, and the switching frequency of inverter is maintained to be constant in steady state condition.In order to eliminate the speed feedback information typical for most known observers, flux observer is structured by using two reference frames:the stationary frame for the stator equation, and the rotary frame for the rotor equation. The influence of inaccurate speed information on observers is eliminated due to the absence of the rotor speed adaptation in this observer, resulting in improving accuracy and increasing robustness of this flux observer.Model reference adaptive observer and novel SMO are simulated respectively by using MATLAB/Simulink simulation software. Simulation results show that this novel SMO can improve the observation accuracy for the stator flux and speed at low speed and increase the system robustness effectively, resulting in widening the speed regulation range of the speed sensorless DTC system and guaranteeing the high-performance control system operation in the whole speed range.