Three-phase Four Bridges Inverter's Control Method And Realization In Unbalanced Loads

With the development of electric power and electron technology, that adopting the PWM three- phase three bridges inverter to making DC to AC for three-phase loads is in common used at present. For it has not neutral line, only symmetrical loads of three-phase (such as AC Motor) can get the symmetrical three-phase voltage. When the three-phase loads are asymmetric, three- phase voltage is unbalanced. For that reason, it is needed that using neutral line to assure three-phase voltage is balanced. In recent years, three-phase four bridges inverter is paid close attention. This inverter has the merit of compact structure, small bulk factor and efficient. Three-phase four bridges inverter added a bridge based on three-phase three bridges inverter, has a more complex control method.Based on analyzing other control methods, this paper designed three dimension space vector PWM algorithm. Compared with other ways, this method has some merits of high efficiency, low IGBT frequency, easy to control and digitalization.During designing three-dimension SVPWM, this paper narrate which district does the sample vector belong to, how to calculate the time of eigenvector working and the sequence of IGBT's turn on or turn off. Building the model to three-phase four bridges inverter in three dimension space and analysis it, this paper use double loop regulator that is voltage negative feedback and current positive feedback to control this inverter. Thinking that this algorithm is nonlinear and the advantage of artificial intelligence, this paper use fuzzy logic to design its regulator. Simulated this model with Matlab, the result proved the three-dimension SVPWM was fit for controlling three-phase four bridges inverter.The control circuit was designed using TMS320LF2407A DSP. By the help of this chip, it realized the algorithm. Use the chip's protected input to realize the protect of main circuit's big current. Use the chip's A/D module to sample the phase voltage and phase current. Use the chip's Event A module to realize the eight PWMs output. Base on the principle of fuzzy logic control, this paper made a table to calculate every input and every output. It is simple to realize using assemble language.The main circuit, protective circuit and drive circuit are designed in this paper in detail. By debugging the hardware and software, experiment results expressed that all the design are reasonable.