Switching losses minimization by using direct torque control of induction motor

Abstract

Direct torque control is becoming the industrial standards for induction motor torque control. This paper presents switching loss minimization technique of improved direct torque control (DTC) of induction motor. Direct torque control (DTC) of an induction motor supplied by a voltage source inverter is a simple scheme that does not need long computation time, can be implanted without speed sensors and is insensitive to parameter variations. In principle, the motor terminal voltages and currents are used to estimate the motor flux and torque. Based on the instantaneous errors in torque and stator flux magnitude and estimates of the flux position, a voltage vector is selected to limit the flux and torque errors within their flux and torque hysteresis bands. In the conventional DTC, the selected voltage vector applies for the whole switching period, irrespective of the magnitude of the torque error. DTC drive gives variable switching frequency and high torque ripple. DTC gives torque and flux ripples because no any VSI states are capable to generate the exact voltage vector from switching table required to make zero both the torque electromagnetic error and the stator flux error. To minimize this problem, a torque hysteresis band with variable amplitude fuzzy logic controller is proposed. The fuzzy logic controller is used to reduce the flux and torque ripples and it improves performance DTC especially at low speed. A duty ratio control scheme for an inverter-fed induction machine using DTC method is presented in this article. The use of the duty ratio control gives improved steady state torque response, with less torque ripple than the conventional DTC. Fuzzy logic control (FLC) used to implement the duty ratio controller. Total harmonic distortion (THD) calculation of electromagnetic torque, rotor speed and stator current of DTC and DTC with fuzzy has done successfully in this article. With the help of FLC with duty ratio, 8% THD in torque, speed and stator current have minimized compared with DTC (Uddin and Hafeez, 2012). In this paper, switching loses minimization technique through THD minimization. Switching losses are minimized because the transistors are only switched when it is needed to keep torque and flux within their hysteresis bounds, improve efficiency & reduced losses. Direct torque control with the fuzzy logic controller has verified by MATLAB SIMULINK and experimentally.