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., τ
₆
, τ
₁₂
, . . . , τ
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.