Research Of Three-state Markov Chain In Random SVPWM Technique

With the rapid development of the automatic control technique, the power electronic technique, the computer application technique and other related areas, the vector control theory establishes the important theoretical foundation for the research and development of the AC-DC-AC variable-frequency speed control system, and make the AC-DC-AC variable-frequency speed control system become a popuplar research topic. The ACDC-AC variable-frequency speed control system that be controled by the space vector pulse width modulation(SVPWM) technique has the high DC voltage utilization ration, the small swithing loss, the lower low-order harmonic content, the lower output torque ripple and the higher speed regulating performance as the system’s advantages, so that the SVPWM technique palys an irreplaceable role in AC-DC-AC variable-frequency speed control system. Because the swithing componets of the conventional SVPWM technique are turn on or turn off in a fixed way, so that the system generates a lot of higher harmonic components in the integral multiple times of the switching frequency, the system circuit causes the electromagnetic interference(EMI), the system motor produces the harsh vibration and some other problems. So that the efficient control performance of the system has been seriously affected. In order to solve these problems, the random SVPWM technique is propsed in this paper. The technique makes the discrete higher harmic components of the AC-DC-AC variable-frequency speed control system into harmonic componets that distribute continuously and uniformly, and which makes its power spectrum presents continuous spectrum characteristic. So as to reduce the high frequency noise, the mechanical vibration of the system and the output current ripple effect of the system.The paper designs the conventional random swithing frequency SVPWM technique base on the basic working principle of the random pulse width modulation(RPWM) technique and the basic working principle of the SVPWM technique, and the modulation technique is applied to the overall speed control system. But the multiple random numbers will inevitably less or greater than expected mathematical average value in conventional random number generation process of the modulation technique. The paper uses the Markov chain algorithm that be added to the random modulation technique to optimize the generated random numbers. So that the generated random numbers more evenly distribute in the up or down side of mathematical expectation.Based on the AC-DC-AC variable-frequency speed control system, this paper uses MATLAB/SIMULINK software simulation platform to build the simulation model of the single random SVPWM technique, the simulation model of the double random SVPWM technique, the simulation model of the double random SVPWM technique with two-state Markov chain and the simulation model of the double random SVPWM with three-state Markov chain. Through comparative analyzing the simulation experimental results of the simulation models, the double random SVPWM technique with three-state Markov chain can greater reduce the higher harmonic amplitude of the three-phase output current and the higher harmonic amplitude of the three-phase output voltage, and which makes the frequency spectrum more uniformly and more continuously distribute on the whole frequency band. The technique furthest reduces the circuit EMI and the high frequency noise of the system than other simulation models.