Reduced torque ripple and switching frequency using optimal DTC switching strategy for open-end winding induction machines

Direct Torque Control (DTC) of induction machine has received wide acceptance in many Variable Speed Drive (VSD) applications due to its simple control structure and excellent torque dynamic control performances. However, the DTC which employs a two-level inverter and hysteresis controllers produces two major drawbacks, namely, larger torque ripple and variable switching frequency, which might produce a very high switching frequency (or power loss), particularly at a very low speed operation. The root causes of the problems can be identified as follows; 1) delay actions in controlling the torque (which is commonly resulted in digital implementation of hysteresis controller) causes the torque cannot be exactly restricted within the hysteresis band, and hence produces a larger torque ripple 2) inappropriate selection of voltage vector (among a limited number of voltage vectors available in a two-level inverter) cannot restrict the increase of switching frequency in the hysteresis controller, as the torque slopes regulated in hysteresis bandwidth vary during operating conditions. This thesis proposes an optimal DTC switching strategy to reduce torque ripple and switching frequency for open-end winding induction machines. The open-end winding induction machine is supplied by a dual-inverter which can offer a greater number of voltage vectors and hence, gives more options to select the most optimal voltage vectors to minimize the problems. The most optimal voltage vectors for every speed range are identified as the vectors that can produce the minimum torque slopes. By minimizing the torque slopes, the torque ripple and switching frequency can be reduced. The identification is made by investigating the torque slope behaviours and torque control capabilities for every speed range. The selection of the most optimal voltage vectors is accomplished by using a modification of torque error status and a look-up table. To obtain a constant switching frequency, a Constant Switching Frequency Torque Controller (CSFTC) is proposed without the use of a PI controller and a knowledge of machine parameters. Some improvements obtained in the proposed strategy were verified via simulations and experimentations, as well as comparison with the conventional DTC. The improvements obtained are as follows; 1) reduction of torque ripple and switching frequency with the proposed optimal DTC switching strategy, 2) a constant switching frequency with the proposed CSFTC. The main benefit of the proposed strategy is its simplicity, where the DTC improvements can be obtained without the common approach, i.e. the use of Space Vector Modulation (SVM) which involves complex control algorithms. It also shown that the average improvement about 39% and 43% can be achieved toward reduction of torque ripple and switching frequency