Torque Ripple Minimization in Neutral-Point-Clamped Three-Level Inverter Fed Induction Motor Drives Operated at Low-Switching-Frequency

Abstract

Neutral-point-clamped (NPC) multilevel inverter fed high power drives operating at low switching frequency are prone to low-order torque pulsations. For a case of $N$ switching angles per quarter, apart from maintaining the fundamental component, $(N-1)$ voltage harmonics can be eliminated. Consequently, ${\frac{N-1}{2}}$ or lower number of torque harmonics could be eliminated. This paper proposes an optimal pulse-width modulation (PWM), which minimizes the combined root mean square (RMS) value ( $\tau _{{\rm RMS}}$ ) of torque harmonics of order lower than $6N$ at any modulation index ( $M$ ) for an induction motor drive fed from an NPC three-level inverter. This paper considers cases of two, three, and four switching angles per quarter cycle of the pole voltage. Compared with synchronized sine-triangle PWM and selective harmonic elimination (SHE) PWM, the proposed PWM reduces the first ( $N-1$ ) torque harmonics (i.e., $\tau _6$ , $\tau _{12}, \ldots, \tau _{6(N-1)}$ ) over a wide speed range. Simulation and experimental results are presented on a 3.7-kW open-loop constant volts-per-Hertz induction motor drive.