Torque ripple elimination based on inverter voltage drop compensation for a three‐phase four‐switch inverter‐fed PMSM drive under low speeds

Abstract

Torque ripples under low speeds are a prominent issue of three-phase, four-switch (TPFS) inverter-fed permanent magnet synchronous motor (PMSM) drives. Due to the asymmetrical circuit topologies, the inverter voltage drops caused by semiconductor power switches have a unique influence on the performances of TPFS inverter-fed PMSM drives, producing unbalanced motor currents and pulsating torques. In this study, the effects of inverter voltage drops on the output voltages are investigated in depth through the proposed equivalent circuits of TPFS inverters, and the results indicate that each phase of stator voltages is distorted by unbalanced voltage harmonics, even including the phase directly connected to the midpoint of the DC link. The distorted stator voltages produce low-frequency current harmonics, resulting in first- and second-order torque harmonics. The torque ripples become tremendous when the PMSM is running at low speeds. According to the analytical results, a compensated modulation strategy is proposed to eliminate the torque ripples introduced by the inverter voltage drops. The experimental results demonstrate the effectiveness and feasibility of the proposed analysis and compensation method.