Direct Torque Control for 2L-VSI PMSM Using Switching Instant Table

In this paper, a direct torque control (DTC) with a variable switching instant is proposed for a permanent magnet synchronous motor (PMSM) fed by a two-level voltage source inverter (2L-VSI). The variable switching instant allows the switching behavior to occur at any instant within one control interval instead of being fixed at the discrete sampling instant, which can result in more switching states within a control interval, rather than increasing the switching frequency. To realize the variable switching instant, the proposed approach overthrows the traditional structure of DTC. The binary logic signal in the switching table is replaced with the continuous evaluating function that can reflect the trend of change of torque and flux modulus generated by all available switching patterns with different switching instants. All evaluating functions can compose the switching instant table (SI-table) offline. Based on the SI-table, the optimization process is adopted to calculate the switching instant and allocate the switching sequence online, which can capture a quasi-optimal switching pattern to reduce both the torque and flux modulus ripples under a low switching frequency. Finally, the feasibility and effectiveness of the proposed approach are verified by both numerical simulations and experimental results.