Methods for direct torque control of induction machines

High performance drives with induction machines require use of vector control. The direct torque control and field oriented control are nowadays two most applied vector control methods. The main advantage of the method for direct torque control of induction machines is its simplicity and therefore its robustness. Namely, control variables (the electromagnetic torque and stator flux vector) are estimated from stator quantities and the only induction machine parameter required is the stator resistance. The main disadvantages of the method are high torque ripple, variable switching frequency, difficulty to control the control variables in the very low speed range and lack of direct current control. Modified methods for direct torque control of induction machines are developed in order to either repeal or diminish an influence of the aforementioned disadvantages. In the paper, the conventional method for direct torque control is described and an overview of the modified methods categorized in few groups with some common features is presented.