Fuzzy Logic Control Based Direct Torque Control Scheme for a Four Switch Inverter-fed Induction Motor Drive

Direct Torque Control of induction motor fed drives has become popular and widely used in industries due to fast and good torque response. Induction motors (IM) are simple in construction and are less sensitive to the motor parameters compared to other vector control methods. A new torque and flux control scheme called the direct torque control (DTC) has been introduced for induction motors. In DTC the torque and flux of an induction motor can be controlled directly by applying a suitable voltage vector to the stator of an induction motor. This paper proposes a novel direct torque control (DTC) strategy for induction motor (IM) drives fed by a four switch three-phase inverter (FSTPI). The introduced strategy is based on the emulation of the operation of the conventional six switch three-phase inverter (SSTPI).Using the fuzzy controller for a nonlinear system allows for a reduction of uncertain effects in the system control and improve the efficiency. A method to achieve fastest dynamic performance by modifying the two leg inverter fed DTC of induction motor based on Fuzzy Logic Concept is used here. This has been achieved thanks to a suitable combination of the four unbalanced voltage vectors intrinsically generated by the FSTPI, leading to the synthesis of the six balanced voltage vectors of the SSTPI. Due to the usage of the Fuzzy logic concept, the reliability, efficiency and performance of ac drive increases. Initial torque peak and torque ripple are minimized in the four switch three phase inverter based DTC using Fuzzy Logic. By using the simulation results we can analyze the proposed method. INTRODUCTION Different techniques of induction machine drive have been introduced in order to ensure speed control at variable frequency. In this paper, a controller based on fuzzy logic is designed to improve the performance of DTC and reduce the torque and flux ripple. The major focused features are the uncontrolled switching frequency of the inverter and the high torque ripple resulting from the use of flux and torque hysteresis controllers. DTC is an efficient control technique used in AC drive systems to achieve high performance torque control and flux control Currently and more than two decades of investigation, several DTC strategies have been proposed so far [2]–[5]. These could be classified within four major categories: 1) strategies considering variable hysteresis band controllers [6]; 2) strategies with space vector modulation (SVM)based control of the switching frequency [7], [8]; 3) strategies using predictive control schemes [9]–[11]; and 4) strategies built around intelligent control approaches. Commonly, the voltage source inverter (VSI) feeding IM under DTC is the six-switch threephase inverter (SSTPI). This said, some applications such as electric and hybrid propulsion systems should be as reliable as possible. Within this requirement, the reconfiguration of the SSTPI into a four-switch three phase inverter (FSTPI), in case of a switch/leg failure, is currently given an increasing attention. A DTC strategy dedicated to FSTPI-fed IM drives has been proposed in [17]. In spite of its simplicity, this strategy is penalized by the low dynamic and the high ripple of the torque. This paper proposes a new Fuzzy logic into DTC strategy with a Four switch inverter fed to an induction motor. Fuzzy logic improves the overall performance of DTC controlled system [2]. Using an appropriate vector selection table and emulation of six switch inverter [1] an efficient method is implanted. Fig. 1. Implementation scheme of the DTC strategy dedicated to FSTPI-fed IM drives. The DTC scheme is very simple in function; in its basic configuration it consists of hysteresis INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VIII /Issue 3 / MAR 2017 IJPRES controllers, torque and flux estimator and a switching table. The basic concept of DTC is to control directly both the stator flux linkage (or rotor flux linkage, or magnetizing flux linkage) and electromagnetic torque of machine simultaneously by the selection of optimum inverter switching modes. The use of a switching table for voltage vector selection provides fast torque response, low inverter switching frequency and low harmonic losses without the complex field orientation by restricting the flux and torque errors within respective flux and torque hysteresis bands with the optimum selection being made. This paper proposes a new DTC strategy dedicated to FSTPI fed IM drives. It is based on the emulation of the SSTPI operation thanks to the synthesis of an appropriate vector selection table, which is addressed by hysteresis controllers. The conventional DTC is based on flux and torque hysteresis controllers. Induction motor is fed from a Four Switch Inverter generating the voltage vectors of the Six Switch Inverter by reconfiguration. Applying the most optimized voltage vector that produce fastest dynamic torque response during transient states. Fuzzy logic concept is a most efficient artificial integilence method which has high application in electric motor drives. A method to achieve fastest dynamic performance by modifying the two leg inverter fed DTC of induction motor based on Fuzzy Logic Concept is used here. DTC OF FSTPI-FED IM DRIVES: BACKGROUND A. DTC Basis The conventional DTC drive employs two level flux hysteresis controller and three level torque hysteresis controller and its outputs are flux error and torque error respectively. DTC strategies allow a direct control of the motor variables through an appropriate selection of the inverter control signals, in order to fulfill the requirements as whether the stator flux and torque need to be increased, decreased, or maintained. These decisions are achieved according to the output cφ of the flux hysteresis controller, the output of the torque hysteresis controller, and the angular displacement θs of the stator flux vector Φs in the Clarke (αβ) plane. The dynamic of Φs is governed by the stator voltage equation expressed in the stationary reference frame, as follows: