A Comparison of Symmetrical and Asymmetrical Three-Phase Cascaded Multilevel Inverter Using Flexible Control Technique for DTC Induction Motor Drives

There are problems in the limitations of conventional inverters, especially in high-voltage and high-power applications. Recently multilevel inverters are becoming increasingly popular for high-power applications due to their improved harmonic profile and increased power ratings. Several studies have been reported on multilevel inverters topologies, control techniques, and applications. However, there are few studies that actually discuss or evaluate the performance of induction motor drives associated with three-phase multilevel inverter. This paper presents a comparison study of symmetrical and asymmetrical for a cascaded H-bridge multilevel using flexible control technique and also direct torque control (DTC) for induction motor drive. In this case, symmetrical and asymmetrical arrangements of five-level, seven-level, nine-level and eleven H-bridge inverters are compared using flexible control technique vector space in order to find an optimum arrangement with lower switching losses and optimized output voltage quality. So, as to decrease the THD value (total harmonic distortion) the number of levels is increased. The experiments show that an asymmetrical configuration provides nearly sinusoidal voltages with very low distortion, using less switching devices. Moreover, torque ripples are greatly reduced by increasing number of levels we can bring step waveform to nearly sinusoidal waveform by decreasing the THD value. Thus ripples are reduced and efficiency is increased.