Improved performance of Direct Torque Control of induction machine utilizing 3-level Cascade H-Bridge Multilevel Inverter

This paper presents the advantages of using multilevel inverter which can provide greater number of voltage vector selection in minimizing the output torque ripple for Direct Torque Control (DTC) of induction machines. By employing a 3-level Cascaded H-Bridge Multilevel Inverter (CHMI), it provides 19 effective voltage vectors where each vector may give different effects on control performance for various operating conditions. This paper reports a simple switching strategy based on the hysteresis controllers and lookup table for DTC utilizing a 3-level CHMI. The torque hysteresis controller with 7-level of hysteresis band is introduced to determine the most appropriate voltage vectors which inherently suitable for different motor operating conditions. It can be shown that the selection of the proposed voltage vectors using multilevel inverter has better DTC performances in terms of torque ripple and quality of output waveforms (i.e. THD) as compared to the DTC with the conventional 2-level inverter.