Quasi-Six-Step Operation to Improve Torque Capability of Inverter System With Bootstrap Circuit

This article proposes a control scheme for quasi-sixstep operation with a voltage constraint to improve the torque capability of low-cost drives with bootstrap circuits. Voltage utilization is limited for a low-cost drive system embedded with a bootstrap circuit, where only the linear modulation is considered. To improve torque capability, overmodulation can be achieved by shifting the voltage vector into the vertices of limited voltage hexagon. The voltage constraint is maintained by modifying the voltage references using the concept of offset voltage. Further, the beat phenomenon triggered by the overmodulation scheme is effectively suppressed by modifying the voltage references at step changes. Subsequently, the offset voltage is added to extend a charge time of the bootstrap circuit. The proposed carrier-based quasi-six-step techniques not only enhance torque capability but also decrease the subharmonics. Simulation and experimental results confirmed the effectiveness of the proposed method. The proposed method increases the base speed while minimizing the subharmonics compared to the conventional overmodulation method.