An Optimal Switching Strategy Of Cascaded H-Bridge Multilevel Inverter For High-Performance Direct Torque Control Of Induction Machines

Direct Torque Control (DTC) of induction machine has received wide acceptance in many adjustable speed drive applications due to its simplicity and high performance torque control. However, the DTC using a common two-level inverter poses two major problems such as higher switching frequency (or power loss) and larger torque ripple. These problems are due to inappropriate voltage vectors which are selected among a limited number of voltage vectors available in two-level inverter. The proposed research aims to formulate an optimal switching strategy using three-level Cascaded H-Bridge Multilevel Inverter (CHMI) for high performances of Direct Torque Control (DTC) of induction machines. By using three-level CHMI, it provides greater number of voltage vectors which can offer more options to select the most appropriate voltage vectors. The most appropriate voltage vectors are identified as the vectors that can produce minimum torque slope but sufficient to satisfy torque demands. The identification is accomplished by using an equation of rate of change of torque which is derived from the induction machine equations. The proposed strategy also introduces a block of modification of torque error status which is responsible to modify the status such that it can determine the most optimal voltage vectors from a look-up table, according to motor operating conditions. Some improvements obtained in the proposed strategy were verified via simulations and experimentations, as well as comparison with the conventional DTC using a two-level inverter. The improvements obtained are as follows; 1) minimization of switching frequency which is expected to reduce power loss, and 2) reduction of torque ripple. These two improvements are important requirement