Random Pulse-Width Modulated Neutral-Point-Clamped Inverter With Reduced Common-Mode Switching

This paper proposes the systematic integration of dual-random and reduced common-mode pulse-width modulation (PWM) techniques for controlling the popularly adopted three-level neutral-point-clamped inverter. Dual-random modulation intelligently combines the unique spread spectral merits of random switching frequency and random pulse placement to achieve a better distribution of harmonic energy over a specified frequency range without affecting the fundamental voltage component, while reduced common-mode modulation nearly eliminates the inverter generated common-mode voltage. The integration of both techniques therefore results in a system with the desired spread spectrum needed for reducing the emitted acoustic noise, electromagnetic interference, and other electrical and mechanical complications of an inverter-fed ac motor drives. For efficient implementation using a digital signal processor with embedded PWM peripheral, the paper examines in particular appropriate switching range selection, digital sampling phenomena, equivalent null vector placement, carrier-based PWM formulation and digital reduced common-mode logic mapping, which have not been collectively analyzed in the literature. Simulation and experimental results are presented to validate the theoretical modulation concepts developed in the paper.