Improved Direct Torque Control Strategies for Low Speed Range Operation of Induction Motor Drives

Abstract

Induction motors are the starting point to design an electrical drive system which is widely used in many
industrial applications. In modern control theory, different mathematical models describe induction motor according to
the employed control methods. The direct torque control (DTC) scheme being one of the most recent steps in this
direction. This scheme provides excellent properties of regulation without rotational speed feedback. In this control
scheme the electromagnetic torque and stator flux magnitude are estimated with only stator voltages and currents and
this estimation does not depend on motor parameters except for the stator resistance. Literature review has been done to
study the recent improvements in DTC scheme especially in low speed range operation to overcome the torque ripple
problem.
The conventional direct torque controlled scheme suffers from great torque ripple due to fast response of torque in
low speed. The motor speed is an important parameter which effects on the motor torque ripple in low speed, when
applying a forward nonzero voltage vector causes to jump up of motor torque and this leads to a positive ripple in torque
response. so we have to improve in look up table to reducing the torque ripple especially in low speed range. In thesis
Simulation of conventional DTC scheme and Modified DTC scheme is described. In modified DTC scheme use the 12
sector method and 5 level hysteresis band controller.