Advanced Direct Torque Control of Induction Machine

Abstract

The induction motor is referred to as the main work-horse of the industries. So control of induction motor is most precisely required in many high performance applications. With the development in power electronic field various control methods for control of induction motor have been developed like Scalar control, Vector control (Direct and Indirect) and Direct torque control. Among these all methods, Direct torque control(DTC) seems to be particularly interesting being independent of machine rotor parameters and requiring no speed or position sensors. In spite of its simplicity, DTC allows a good torque control in steady state and transient operating conditions to be obtained. However, the presence of hysteresis controllers for flux and torque could determine torque and flux ripple and results into variable switching frequency operation of voltage source inverter. In order to overcome these problems, various methods have been proposed by several researchers like variable hysteresis band comparators[5], predictive control schemes [6], space vector modulation [7] and intellegent control techniques [8]. However these methods have deminished the main feature of DTC, that is simple control structure. This report presents constant switching frequency based torque and flux controllers to replace conventional hysteresis based controllers where almost fixed switching frequency with reduced torque and flux ripple is obtained by comparing the triangular waveforms with the compensated error signals.