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

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, 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 (τRM S) 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., τ₆, τ₁₂, . . . , τ₆(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.

I. Introduction

Neutral-Point-Clamped (NPC) multilevel converters have been found to have excellent performance in terms of output voltage waveform quality, higher compatibility for medium-voltage applications, lower electromagnetic interference, reduced common mode voltage, lower voltage stress across the semiconductor devices, and improved line-current-distortion [1]– [10]. Due to aforementioned qualities, NPC inverters have found applications in wide areas including medium voltage drives [2], [3], renewable energy [3], [6], traction [3], [11], and marine propulsion [3]. Due to an additional switching level, the output waveform of comparable quality can be achieved with a lower switching frequency as compared to two-level inverter [7].