Post-fault direct torque control strategy for eight-switch three-phase NPC inverter-fed induction motor drives

Purpose This work aims to propose a novel post-fault direct torque control (DTC) scheme for induction motor drives supplied by an eight-switch three-phase inverter (ESTPI), implemented through the design of three new look-up tables. Design/methodology/approach A comprehensive analysis is performed to evaluate the effect of each voltage vector generated by the ESTPI on the stator flux, electromagnetic torque, DC-link capacitor voltages and common-mode voltage (CMV). To mitigate the capacitor voltage imbalance, a dedicated hysteresis comparator is incorporated to regulate the voltage difference between the DC-link capacitors. Torque ripple is reduced by eliminating vectors responsible for excessive torque pulsations, while CMV variations are minimized through the creation of two virtual vectors that replace those generating high CMV levels, effectively confining CMV within ± Vdc/6. Findings The validity and effectiveness of the proposed DTC strategy are confirmed through extensive simulation studies. By introducing three redesigned look-up tables and incorporating dedicated control mechanisms, namely, a hysteresis comparator for neutral point voltage (NPV) balance, the removal of torque-inducing vectors and the use of virtual vectors to constrain CMV, the proposed approach significantly enhances the drive’s post-fault performance. Originality/value The ESTPI, obtained by reconfiguring a faulty three-level NPC inverter, enables robust post-fault operation by maintaining balanced power delivery, reducing circuit complexity and preserving acceptable output voltage levels. However, when feeding induction motor drives, the performance of the ESTPI can be hindered by several critical challenges, including DC-link capacitor voltage imbalance, elevated torque ripple and fluctuations in CMV. Simultaneously addressing these issues is crucial to ensure stable, efficient and reliable motor-drive operation under fault conditions.