Flying Capacitor-Assisted Two-Leg Inverter for Permanent Magnet Synchronous Motor Drive

This article proposes an inverter configuration for a high-power-density permanent magnet synchronous motor (PMSM) drive. The proposed inverter consists of a two-leg voltage source inverter (VSI) and a flying capacitor. The VSI and the capacitor are connected to two of the three-phase stator windings of the PMSM, sharing one phase. Although the VSI controls the motor torque or speed, the flying capacitor not only boosts the stator voltage above the dc link voltage through resonance with stator inductances but also mitigates power imbalance through a phase shift. Despite the use of a two-leg inverter, such a combination enables PMSM drive with the high power density and low torque ripple. Unlike conventional boost inverters, the voltage stress of the switches is maintained at the dc-link voltage regardless of the boost ratio. It also allows PMSMs to have higher power capability while using fewer active devices than conventional three-phase VSIs. This article presents the operating principle and area of the proposed PMSM drive. A control strategy that accurately controls average torque and speed while minimizing copper loss and limiting inverter voltage and torque ripple is also proposed. The performance of the proposed PMSM drive is demonstrated with both steady-state and transient experiments.