| Compared with two-level inverters, multilevel inverters have the following characteristics such as output voltage more close to sinusoidal wave, small harmonics and dv/dt. It's suitable for high-voltage high-power variable speed system of AC motors. However, as a new energy converting technology, the control of machine based on multilevel inverters for high performance has still some problems that are worth to be researched and probed in theory and practices.As one of typical representatives of multilevel inverters, three-level inverter is of the simple topology, few devices and high reliability. The research has the actual significance on high performance AC variable speed system based on three-level inverter. Taking the diode clamped three-level inverter as the objective, and induction machine as the load, this thesis gives the theoretical study and analysis of the multilevel with induction machine in detail. The inverters modeling, the control technology of PWM, and direct torque control (DTC) technologies are studied. The simulation and experiment researches are performed.The thesis analyzes the development and research status of multi-level inverters firstly. Also the development of AC variable speed and the characteristic of direct torque control are discussed. Then the aim and significance for the research are demonstrated. The topology and operation principles of diode clamped three-level inverter are discussed, the mathematics model is built under theα,βtwo phase stationary coordinates, and based on the mathematics model the simulation system is built. All these lay the foundation of researching all kinds of control strategy of the induction machine supplied with three-level inverter.The operation principle of three-level inverter is analyzed for the pulse width modulation (PWM), including carrier-based PWM and space vector PWM (SVPWM). The realization method of SVPWM is mainly analyzed. The reason is also analyzed for the neutral-point voltage fluctuating. Neutral-point voltage model is built which provides the evidence to the control of neutral-point potential. For the above PWM methods, the simulation models are built. The simulation results verified the validity and feasibility of the theory analysis.This thesis discussed the principle of direct torque control of induction machine. An improved method of u-i integrator of stator flux-linkage observer is discussed. The problems of DTC in three-level inverters and the scheme of selecting vectors are analyzed. Then, conventional DTC based on hysteresis band including single vector and synthesized vectors are analyzed in detail. However, the torque ripple is large. Therefore, the precontrol of torque and flux-linkage is introduced, a new DTC algorithm based on precontrol of torque and fast space vector PWM is presented. The torque ripple is lower. It's easy for digital implementation.Due to the large number of vectors for multilevel inverter, vector selecting is complicated when conventional DTC is applied to three-level inverter. The selected vector acts on a sample period. This will result into torque fluctuation. Therefore, a fuzzy DTC is presented. After introducing fuzzy logic, the error of torque and flux-linkage is divided into different grades. According to the grades, the desired vector is selected. This method not only reduces the ripple of torque and flux-linkage, but also keeps and inherits the advantage of conventional direct torque control. It has desired robust.Taking the DSP TMS320F2812 as the controller, the experiment platform for the diode clamped three-level inverter is established. The relative experiment researches were completed for the control of SVPWM and DTC control of torque precontrol and fuzzy logic. The main results and conclusions are given. The experimental results verified the theoretical analysis and the control strategies proposed in this thesis.
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