Improved DTC Technique for THL-NPC VSI Fed Five-Phase Induction Motor Drive Based on VVs Assessment Over a Wide Speed Range

A three-level neutral point clamped voltage source inverter fed five-phase induction motor (FPIM) drive is assessed by analyzing common-mode voltage (CMV). The presence of CMV leads to motor bearing fault and phase winding insulation failure due to additional voltage stress. To nullify the effect of CMV, this article proposes a modified direct torque control (DTC) based on the virtual vector (VV) concept. The proposed DTC utilizes the appropriate nonvertex voltage vectors to form VVs that neutralize the average volt-second in the xy plane, maintain dc-link capacitor voltage balancing, and limits the switching voltage stress. The theoretical analysis is carried out to investigate the VVs effect on change in flux and torque response. Based on this assessment, a modified lookup table is designed for a wide range of speed operations. For the low-speed operation, i.e., below 25% of rated speed, the small VVs provide better drive performance instead of zero VV. The proposed DTC method is verified at steady-state and dynamic conditions through experimental investigations. Comparative results are provided for the assessment of the proposed DTC method in comparison to existed methods. It is concluded from the findings that the proposed DTC method is a promising approach to improving FPIM performance at high- and low-speed operations with zero CMV output.