Minimized torque ripple in direct torque control of induction machines

Direct torque control (DTC) of an induction motor fed by a voltage source inverter is a simple scheme that does not need long computation time, can be implanted without mechanical speed sensors and is insensitive to parameter variations. In principle, the motor terminal voltages and currents are sampled and used to estimate the motor flux and torque. Based on estimates of the flux position and the instantaneous errors in torque and stator flux magnitude, a voltage vector is selected to restrict the torque and the flux errors within their respective torque and flux hysteresis bands. In conventional DTC, the selected voltage vector is applied for the whole switching period regardless of the magnitude of the torque error. This can result in high torque ripple. A better drive performance can be achieved by varying the duty ratio of the selected voltage vector during each switching period according to the magnitude of the torque error and position of the stator flux. A duty ratio control scheme for an inverter-fed induction machine using DTC method is presented in this paper. The use of the duty ratio control resulted in improved steady state torque response, with less torque ripple than the conventional DTC. Fuzzy logic control was used to implement the duty ratio controller. The effectiveness of the duty ratio method was verified by simulation using matlab SIMULINK^®.