A Quick Dynamic Torque Control for an Induction-Machine-Based Traction Drive during Square-Wave Mode of Operation

In high-power traction drives using induction motors, during operation above the base speed, it is generally preferred to drive the machine in the square-wave mode since it offers maximum dc-bus utilization and reduced switching loss. However, the conventional field-oriented control method cannot control the drive effectively in square-wave mode since independent control of torque and flux is difficult to achieve due to stator voltage saturation. This article proposes a new technique to control the drive in the square-wave mode where only 1 DOF is available for control. In the square-wave mode, the proposed method controls the torque of the machine by directly varying the stator voltage angle with respect to the rotor flux axis. The flux of the machine is kept at a required value below the saturation limit by appropriate selection of the operating point. The proposed control strategy also offers good torque dynamics in the square-wave operating mode. The control algorithm is experimentally verified on a 5.5-kW induction machine drive and the experimental results are presented.