Study On Variable-frequency Speed Regulation System Based On Three-level Inverter

In speed regulation of AC motor, because of high performances and wide applications, variable-frequency speed regulation is considering as the best speed regulation method. Meanwhile, due to advantages such as less on-off frequency and harmonic of the output voltage, three-level inverter should become the preferred scheme in the main circuit of variable-requency speed regulation system. Therefore, this thesis focus on studying of the diode-clamped three-level inverter system.Comparing lots of control strategies, the space vector modulation (SVPWM) has larger modulation range and the advantages of higher DC voltage utilization, can optimize output voltage waveform, easy-to-digital implementation, and thus become the focus of research.Firstly, three-level inverter topology, control strategies and so on were reviewed in the thesis, on this basis, SVPWM algorithm of three-level inverter is analysised and simulated, meanwhile, the hazards, as well as the generation mechanism of unbalanced neutral-point voltage of three-level inverter is discussed and detailed analysised. And, gives mathematical solutions both on the respect of hardware and software, among these, the software control strategy keep the neutral-point voltage balanced by optimizing the modulation algorithm of SVPWM, which adjusts the work time of active and negative small vectors by using the opposite effects of neutral-point voltage made by active and negative small vectors.Secondly, in this thesis the study of the speed-sensorless vector control theory of three-level inverter has been made, the design figure of system as a whole is built, and, the corresponding simulation has been established. The simulation result agrees with the theoretical predictions.At the last, the design of the hardware, the software flow chart has been present in the thesis. Among, the hardware part of the system includes the main circuit(rectifier circuit, soft starting circuit and inverter circuit), driving circuit of IGBT, sampling circuit(voltage, current and speed sampling circuit) and protection circuit. The software part of the system includes the main program, subprogram of vector control interrupt service, subprogram of open or close the motor, subprogram of fault interrupt service, and SVPWM program of control module. The control circuit of the system has used DSP+FPGA structure, and regarded DSP as a major. Thus it could realize controlling the three-level inverter effectively and accurately. At the same time, there were a series of experiments made in this three-level inverter with the algorithm which has been mentioned above, and the experimental waveform also were given. The experimental results demonstrate that the algorithm is correct, and the three-level variable-frequency speed regulation system could be operation reliably.