Direct Torque Control of Doubly Fed Induction Machine

The present paper proposes a novel Doubly Fed Induction Machine (DFIM) control strategy using Direct Torque Control (DTC) linked with two Switching Tables in order to control electromagnetic torque of DFIM via stator and rotor flux vector. The proposed scheme is described clearly and simulation results are reported to demonstrate its effectiveness. The entire control scheme is implemented with Matlab/Simulink. Due to its excellent speed controllability, reduced converter size, high performance and relatively low cost compared to other machines (2). Doubly-fed induction machines have been proposed in the literature, among other industrial applications as motor in high power applications such as traction, marine propulsion or as generator in wind energy conversion systems like wind turbine, or pumped storage systems. Several methods of control are used to control the doubly fed induction machine among which the vector control or field orientation control that allows a decoupling between the torque and the flux, in order to obtain an independent control of torque and the flux like DC motors (3). Therefore, decoupling the control scheme is required by compensation of the coupling effect between q- axis and d-axis current dynamics. It is well known that vector control needs quite complicated to on line coordinate transforms to decouple the interaction between flux control and torque control in order to provide a fast torque control. In recent years an innovative control method, called direct torque control has gained the attraction of researchers. Direct torque control (DTC) strategy was introduced by Takahashi to give a fast and good dynamic performance and can be considered as an alternative to the field oriented control (FOC) strategy (4). Therefore, in recent years the industrial application areas of the high performance AC drives based on DTC technique have gradually increased due to the following advantages over the field oriented control technique, such as Excellent dynamic performance, precise and quick control of stator flux and electromagnetic torque, absence of co-ordinate transformation, which reduce the complexity of algorithms involved in FOC, robust against machine parameters variations and no current control loop. In this study a new Dual-Direct Torque Control scheme is developed with flux model of DFIM. Two Switching Tables (ST) linked to VSI are defined for stator and rotor flux vector control. We propose separate control of stator and rotor flux. In fact, in order to applied DTC strategy to this configuration, we define a first switching table to control the stator flux vector, and a second switching table to control the rotor flux vector. The next part of the control strategy makes possible to control interaction between both flux vectors. Consequently, we are able to control the electromagnetic torque and to regulate the mechanical speed (3). The performance of such a scheme depends on the error band set between the desired and measured stator and rotor flux values. In this control scheme, the stator inverter switching frequency is changed according to the hysteresis bandwidth of stator flux and stator flux angular position, and the rotor inverter switching frequency is changed according to the hysteresis bandwidth of rotor flux and rotor flux angular position.