SAT Implementation in Direct Torque Control For Dynamic Response in Multi Level Inverter Induction Motor DRIVES

This paper presents the implementation of Direct Torque Control (DTC) w ith Sector Advancement Technique (SAT) algorithm for the control of a Hybrid Cascaded H-Bridge Multilevel Inverter Induction motor Drive. It is useful to keep the motor torque and stator flux and the inverter's neutral point potential w ithin given hysteresis bounds while reducing the average sw itching frequency of the inverter and overall computational time period comparison w ith the standard direct torque control (CDTC). This method also improving overall efficiency with Torque and Flux ripple reduction. In addition, the multilevel inverter can generate a high and fixed sw itching frequency output voltage with fewer sw itching losses, since only the small pow er cells of the inverter operate at a high sw itching rate. Therefore, a high performance and also efficient torque and flux controllers are obtained, enabling a DTC solution for multilevel-inverter-powered motor drives. ince its introduction, direct torque control (DTC) has become a powerful control scheme for the control of induction motor (IM) drives. The standard DTC scheme uses hysteresis comparators for the control of both stator flux magnitude and electromagnetic torque. This control structure ideally keeps both controlled parameters within the hysteresis bands and results in a non constant switching frequency. One of the methods that have been used by one major manufacturer in multilevel inverters is direct torque control (DTC), which is recognized today as a high- performance control strategy for ac drives. Several authors have addressed the problem of improving the behaviour of DTC ac motors, particularly by reducing the torque ripple. However, when the DTC scheme is used in a discrete imple- mentation, both torque and flux exceed the bands imposed by the hysteresis comparators, due to the fixed sampling frequen- cy. It is possible for the discrete scheme to operate as an anal- ogy one if the hysteresis bounds are chosen to be sufficiently large. On the contrary, when the width of the bands is compa- rable to the maximum torque and flux variations during one sampling period, the excursions will be relatively large, partly due to the time delay that is caused by the data processing. Therefore, the sampling period is an important factor deter- mining the control performance and switching frequency. To improve the performance of control operation, different approaches have been proposed: improving the loo- kup table; varying the hysteresis bandwidth of the torque con- troller; and using flux, torque, and speed observers. Although these approaches are well suitable for the classical two-level inverter, their extension to a greater number of levels is not easy. Throughout this paper, a theoretical background is used to design a simple and practical strategy that is compatible with hybrid cascaded H-bridge multilevel inverter. It allows not only controlling the electromagnetic state of the motor with improved performance (minimization of the torque rip- ple) but also reducing flux and current distortion caused by flux position sector change. To improve the flux waveform, ripple free torque, dynamics and efficiency of the drive and to enhance the quali- ty of stator currents in the motor. Sector Advancing Technique (SAT) is employed for reducing the response time of the drive to given torque command.