A Simple Duty Cycle Control Strategy to Reduce Torque Ripples and Improve Low-Speed Performance of a Three-Level Inverter Fed DTC IPMSM Drive

This paper presents a novel duty cycle-based direct torque control (DDTC) strategy for a three-level neutral point clamped inverter fed interior permanent magnet synchronous motor. DDTC methods proposed in the literature for three-level inverter fed DTC (3L-DTC) are complicated, parameter dependent, and pay little attention to inverter inherent switching constraints. In this paper, a simple yet robust duty ratio calculation method, which respects the inverter switching constraints, is proposed. Additionally, a novel switching strategy is adopted to minimize the effects of flux drooping phenomena seen in 3L-DTC drives during low-speed operations. The proposed 3L-DTC method is efficient in reducing both torque and flux ripples while operating at low switching frequencies. Therefore, it is suitable for medium-high power ac drives. Experimental results on a scaled down test rig and comparative analysis are provided to validate the effectiveness of the proposed strategy.