This paper describes a combination of direct torque control (DTC) and space vector modulation (SVM) for an adjustable speed sensor-less induction motor (IM) drive. The motor drive is supplied by a two-level SVPWM inverter.
The inverter reference voltage is obtained based on input-output feedback linearization control, using the IM model in the stator–axes reference frame with stator current and flux vectors components as state variables.
Moreover, a robust full-order adaptive stator flux observer is designed for a speed sensorless DTC-SVM system and a new speed-adaptive law is given. By designing the observer gain matrix based on state feedback control theory, the stability and robustness of the observer systems is ensured. Finally, the effectiveness and validity of the proposed control approach is verified by simulation results.
DTC abandons the stator current control philosophy, characteristic of field oriented control (FOC) and achieves bangbang torque and flux control by directly modifying the stator voltage in accordance with the torque and flux errors. So, it presents a good tracking for both electromagnetic torque and stator flux. DTC is characterized by fast dynamic response, structural simplicity, and strong robustness in the face of parameter uncertainties and perturbations.
One of the disadvantages of conventional DTC is high torque ripple. Several techniques have been developed to reduce the torque ripple. One of them is duty ratio control method. In duty ratio control, a selected output voltage vector is applied for a portion of one sampling period, and a zero voltage vector is applied for the rest of the period. The pulse duration of output voltage vector can be determined by a fuzzy logic controller.
In , torque-ripple minimum condition during one sampling period is obtained from instantaneous torque variation equations. The pulse duration of output voltage vector is determined by the torque-ripple minimum condition. These improvements can greatly reduce the torque ripple, but they increase the complexity of DTC algorithm. An alternative method to reduce the ripples is based on space vector modulation (SVM) technique.
Source: University of Constantine
Authors: Zhifeng Zhang | Renyuan Tang | Baodong Bai | Dexin Xie