Research Of Direct Torque Control System Based On Three-Level Inverter

The traditional two-level inverter has been difficult to meet the requirements of the high-voltage and high-power applications with industrial development, power and voltage levels improvement. The high-voltage high power AC motors variable speed techniques based on multilevel converter have become one of research hot points now in the field of power electronics and power drives. As described in many literatures, by using multilevel technology, the voltage stress on switches will be reduced, the shape of output waveform will be improved and the rate of voltage and power can be increased too. Direct Torque Contro(lDTC)is an advanced AC drives control principle where inverter switching directly controls the motor variables: flux and torque DTC technology has been widely researched for the characteristics of briefness, robust, excellent dynamic response. This paper will focus on the study of direct torque control of induction motor drive based on diode-clamped three-level inverters.The paper firstly analyses the evolution and research status of multilevel converter topologies, the development of AC variable speed techniques and the characteristic of DTC are discussed, and also, the aim and significance for the research are presented. The paper discusses the three-level inverter math model and the three-level inverter space vector modulation of the operation principle and implementation method. The paper analyses impact of the middle and small vector on the midpoint voltage, symmetric SVPWM of seventh and the mid-point voltage control method of hysteresis are used. Theoretical analysis and simulation study results show that space vector modulation method the mid-point hysteresis voltage-controlled can effectively suppress the mid-point voltage fluctuations.This paper describes direct torque control of the basic principles and analysis the main issues of three-level direct torque control. The paper separately proposes the improved virtual vector synthesis method and the SVPWM vector based PI to solve the problem of direct torque vector selection. The simulation results validate the correctness of the methods.This paper summarizes the current identification methods of speed and analyzes through the model reference adaptive system(MRAS)for speed estimation. The method is improved, last the method is verified by simulation.At last, based on microprocessor TMS320LF2812 and CPLD, a prototype of three-level inverter is manufactured and tested. The experimental results prove the feasibility of control methods.