Constant Frequency Torque Controller for DTC with Multilevel Inverter of Induction Machines

Abstract

Direct Torque Control using multilevel inverter (DTC-MLI) with hysteresis controller suffers from high torque and flux ripple and variable switching frequency. In this paper, a constant frequency torque controller is proposed to enhance the DTC-MLI performance. The operational concepts of the constant switching frequency torque controller of a DTC-MLI system followed by the simulation results and analysis are presented. The proposed system significantly improves the DTC drive in terms of dynamic performance, smaller torque and flux ripple, and retains a constant switching frequency. Keyword:Constant INTRODUCTIONSince it has been introduced in early 1980s [1], Direct Torque Control (DTC) has gained its popularity in electrical drives research area. Recently, the application of high-power medium voltage in AC drives has shown rapid development. Thus, the use of multilevel inverters in DTC scheme has become an important structure for further development and improvement. Numerous technical papers have shown a superior performance of DTC scheme using multilevel inverters .By utilizing the multilevel inverter in DTC scheme, the choices of voltage vectors that can be used to control the torque and flux are increased. As a result, more precise control can be obtained. Several control method have been proposed for DTC scheme using multilevel inverter (DTC-MLI); hysteresis-based and non-hysteresis-based such as predictive control strategy [11,15,17,18,20,22,26,27,30], space vector modulation strategy [2, 4, 8-10, 14, 21, 28, 29, 33] and fuzzy logic control strategy [3,5,7,19,31,[34][35][36].The employment of hysteresis-based control strategy in discrete implementation has led to high torque ripple even with a small hysteresis band. This is due to the delay in sampling time. Besides that, it created a variable switching frequency of the switching devices which leads to an unpredictable harmonics current.As a result, some researchers preferred to use non-hysteresis-based control strategy. Significant improvements are accomplished in terms of flux and torque ripple and devices switching frequency. However it involves a complex mathematical equations and algorithm which has led to the complexity of the DTC-MLI scheme and high computational burden particularly when the multilevel inverter's level is increased.