Direct Torque Control with Improved Switching for Induction Motor Drive System Fed by Indirect Matrix Converter V. Faraji, M. Aghasi, D. A. Khaburi, M. Kalantar Iran University of Science and Technology, Tehran, Iran vfaraji@elec.iust.ac.ir , maghasi@elec.iust.ac.ir Abstract The combination of indirect matrix converter with direct torque control is a way towards energy-saving of the world. This paper investigates the use of four- step switching in rectifier bridge of indirect matrix converters to reduce the electromagnetic torque ripple which appears when direct torque control technique is used in induction motors. By suitably selecting switching pattern, the electromagnetic torque ripple of the motor is effectively reduced. Indirect matrix converter has no electrolytic capacitors in the DC link, which results that volume and size of converter reduced and increased longevity. Using this switching strategy, the advantages of the DTC schemes and the benefits of the indirect matrix converters can be combined. The simulation results of DTC system based on IMC and the comparison of motor performance under the proposed control system with respect to those obtained under conventional DTC confirms its effectiveness and accuracy. 1. Introduction Direct Torque Control (DTC) is an optimized AC drives control principle where inverter switching directly controls the motor variables: flux and torque. DTC of induction motor (IM) drives has been introduced in the 1980s by Depenbrock, Takahashi and Noguchi as an alternative to field orientation control (FOC), with the twofold objective of simplifying the control algorithms and achieving similar or even better performance [2]-[4]. The main advantage of DTC is the high performance achieved (decoupled control of torque and stator flux, Minimal torque response time and robustness) together with the simplicity of the scheme (no need for coordinate transformation, modulation block and current regulation), Although, some disadvantages are present such as non accuracy of torque and flux estimators, and an inherent torque and flux ripples because of non optimal switching [5]. Many articles have been published to solve problems of DTC. Some of this articles, have tried to improve flux and torque estimators [6],[7]. Some else have focused on optimized and novel switching techniques [8]. DTC is commonly used with a voltage source inverter (VSI), where electrolytic capacitor is used on the dc link of the AC/DC/AC converter in order to smooth the dc voltage and store the energy recovered from the machine during regeneration braking. Large electrolytic capacitors in dc link causes that size and weight of converter considerably increased and longevity decreased [9]. In recent years research on direct frequency conversion using Matrix Converters (MC) has become popular. Matrix converters have many desirable feature compared to the conventional voltage or current source inverter such as [10]-[14]: compact size, Regeneration capability, operation with unity power factor for any load and Good voltage transfer ratio, unitary power factor for any load. Since the absence of a path for the inductive load current would result large overvoltages, so this converter require snubber circuit. But charge and discharge currents in snubber circuit result losses and disturb waveform of input currents [15]. Accordingly in this paper we use commutation strategy for snubberless matrix converter. There have been typical two current commutation methods proposed which are not required snubber circuits for a PWM rectifier of AC-to-AC converters without DC link components. The first method named rectifier zero current commutation and second method named rectifier four-step commutation [15]. In this paper we use the rectifier four-step commutation method in the rectifier stage, therefore the mechanisms involved in the commutation process are firstly described. The paper is organized as follows: in section II, a review of conventional DTC is presented; then, in section III, Indirect Matrix Converter (IMC) is introduced and its current commutation methods for rectifier stage (rectifier four-step commutation) is explained in section IV, in section V the DTC system based on IMC with optimum switching in rectifier bridge of IMC is modeled and explained, simulation results of proposed model and comparison by conventional DTC are available in section VI. Finally, the conclusions are exposed in section VII. 2. Direct Torque Control Principles The main idea of the DTC is to choose the best vector of voltage, which keeps flux and torque in allowed bandwidth with minimum ripple. The block diagram of the DTC scheme is shown in Fig.1. The flux and torque estimations which are performed by means of mathematical model of induction motor are needed for DTC. From figure2, voltage vectors can be expressed by: ( 1) 3 (1, 2,..., 6) Ji i dc U V e i π - = = (1) 309