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. This paper considers a specific case of four switching angles per quarter cycle of the pole voltage. For such cases, apart from maintaining the fundamental component, three voltage harmonics can be eliminated. Consequently, only one torque harmonic could be eliminated. This paper proposes an optimal pulse width modulation (PWM) which minimizes the combined RMS value (tau(RMS)) of 6th (tau(6)), 12th (tau(12)) and 18th (tau(18)) harmonic torques at any modulation index (M) for an induction motor drive fed from an NPC three-level inverter. Compared to synchronized sine-triangle PWM and selective harmonic elimination (SHE) PWM, the proposed PWM reduces tau(6), tau(12) and tau(18) over a wide speed range. Simulation and experimental results are presented on a 3.7kW open-loop constant volts-per-Hz induction motor drive