Second-order Sliding-mode Control Approaches to Improve Low-speed Operation of Induction Machine under Direct Torque Control

Direct torque control is a powerful approach widely used for the control of electrical machines. However, this approach is sensitive to several problems, such as the stator flux demagnetization phenomenon and the appearance of undesirable torque ripples, especially at low-speed operation. To overcome these problems, a variable structure control approach for a direct torque control–space vector modulation system, with a constant switching frequency, is proposed in this article. The proposed sliding-mode direct torque control–space vector modulation method can (i) enhance the system robustness, (ii) reduce the torque ripples, and (iii) prevent the demagnetization problem, which penalizes the stator flux regulation. To reduce the chattering phenomenon that appeared on the control law, a second-order sliding-mode approach, which is a new second-order procedure, has been proposed. Simulation results dealing with low-speed operation of the induction motor under variable structure control direct torque control–space vector modulation strategy is presented and compared to those yielded by the direct torque control–space vector modulation strategy using hysteresis controllers.