Pulsating Torque Harmonics in Electric Motors Driven by Carrier-Based PWM Multilevel Cascaded H-bridge Inverter

This paper investigates pulsating torque harmonics in the air gap of an electric motor driven by a three-phase cascaded H-bridge (CHB) multilevel inverter using single-carrier pulse width modulation (SC-PWM) techniques. SC-PWM methods are increasingly adopted in multilevel converter control due to their simplified digital implementation compared to conventional level-shifted and phase-shifted PWM approaches. However, their impact on motor drive systems, particularly air gap pulsating torque, remains largely unexplored. This study analyzes SC-PWM voltage waveforms and their influence on the air gap torque spectrum, identifying relevant electromagnetic torque harmonics using the Concordia transformation in the αβ-plane. Campbell diagrams assess potential interactions between SC-PWM-induced torsional stress and mechanical shaft resonances. Experimental validation with a 7-level CHB inverter confirms that SC-PWM without half-wave symmetry increases harmonic distortion in line-to-line voltages and phase currents, intensifying unwanted pulsating torque harmonics.