Research On Direct Torque Control System Based On Space Vector Modulation

The issues of multi-level inverter control and application in AC drive system have been concerned by scientists around the world. In this paper, the analysis of the content and progress of research on multi-level inverter topology and space vector modulation technology has been executed. The solution of diode clamped inverter as the research carrier has been proposed. The way of three-level voltage space vector modulation and application in induction motor direct torque control system has been researched.Based on the analysis of the relationship between the voltage space vector and switching state in general condition. The mathematical model of the diode clamped three-level inverter has been established. After in-depth study on the fluctuation in the midpoint voltage of the inverter, the selection optimization program of voltage vector has been proposed in direct torque control system. It is the characteristic of the algorithm that the voltage vector synthesis, speed, torque and flux can be be taken into account. The reslult of simulation shows that it can play an important role in enhancing the performance of the system and achieve the desired effect.The result of the study suggests that one reason of causing torque ripple in low speed is that the fixed width of the torque hysteresis has been setted. To solve this problem, a fuzzy logic controller has been proposed which can dynamically adjust the width of torque hysteresis and improve stability of the system in the low speed. The inputs of the fuzzy logic controller are the signal of speed error and the rate of current change. The output is the amplitude increment of torque hysteresis. The demagnetization effects has been caused by stator resistance voltage drop, and it makes flux instability in the low speed. The five-layer fuzzy neural network has been created to identify the stator resistance. The operating current, speed and temperature of the stator winding are used as inputs of the fuzzy neural network. This method has been proven to be effective by the simulation in the three-level space vector modulation direct torque control system.Based on the analysis of the characteristics of three-level topology and space vector modulation, the solution of hardware design and the process of software design has been proposed for diode clamped three-level voltage space vector modulation in asynchronous motor direct torque digital control system. The improved algorithm has been executed on the experimental platform and the effectiveness has been proven.