A Modulation Strategy for a Three-Level Inverter Synchronous Reluctance Motor (SynRM) Drive

In this paper, two modulation strategies called Mod-1 and Mod-2 are compared for a three-level diode neutral point-clamped (NPC) inverter synchronous reluctance machine (SynRM) drive. These modulation strategies do not operate at a fixed switching frequency. A new modulation strategy which operates at a fixed switching frequency is proposed called Mod3. The SynRM suffers from a poor power factor which makes it harder to archive capacitor voltage balancing for a three-level NPC inverter. These modulation strategies are of the nearest three vector (NTV) family. Mod-1 and Mod-3 choose certain vector states to achieve capacitor voltage balancing, whereas Mod-2 strategy makes use of the inverter vector state's dwell time. The parameters of interest are the torque ripple, the neutral point (NP) voltage ripple, the inverter efficiency, and the d-axis current ripple. Though lower torque ripple and better power quality are registered on Mod-2 strategy. Mod-1 strategy has 14% improvement in inverter efficiency at lower modulation index (mi = 0.4). Mod-2 strategy has 11% more NP voltage ripple at a modulation index of 0.4. One of the disadvantages of Mod-1 strategy is the varying switching frequency. Mod-3 scheme produced higher inverter efficiency, better balancing of the capacitor voltages, and guaranteed fixed inverter switching frequency. The disadvantage with Mod-3 strategy is that it generated higher torque ripple, daxis current ripple, and current total harmonic distortion when compared to Mod-1 and Mod-2 at the same load conditions.