Improved switching-table-based DTC strategy for the post-fault three-level NPC inverter-fed induction motor drives

In this study, a direct torque control (DTC) strategy based on a novel switching table for induction motor drives fed by the eight-switch three-phase inverter (ESTPI), which is the post-fault reconfigured topology for the three-level neutral-point-clamped inverter with the open-circuit fault occurring in a leg, is proposed to reduce the torque ripple and suppress the dc-link capacitor voltages offset. The influence of each basic voltage vector provided by the ESTPI on the stator flux, the electromagnetic torque and the dc-link capacitor voltages is analysed in detail, and the causes of the torque ripple and the capacitor voltages offset are also revealed. To suppress the dc-link capacitor voltages offset, a hysteresis comparator is added to regulate the dc-link capacitor voltages. Based on that, an optimised switching table to achieve not only torque ripple reduction but also dc-link capacitor voltages offset suppression is proposed. The feasibility and the effectiveness of the proposed DTC strategy are verified by simulations and experimental results.