Application of a SHE-PWM modulation for a low switching frequency motor drive with harmonic investigation using the DTFT

Abstract

This paper presents a study related to an electric drive composed by an induction machine fed by a NPC (neutral point clamped) frequency inverter, extending also to ANPC (active neutral point clamped) topologies, and IGBTs commanded by a SHE-PWM (selective harmonic elimination pulse width modulation) logic pattern. This work also addresses a concern related to the number of dv/dts reproduced in a fundamental period of a waveform, which is related to the semiconductors switching frequency. These dv/dts cause degradations to electric motors and its mitigation contributes minimizing problems on windings, insulations and bearings erosion. A SHE-PWM pattern with very low switching frequency is presented with simulations and experimental results mitigating the 5th and 7th harmonics for the entire drive voltage index modulation range. These harmonics have high amplitude and low frequency, causing high drive losses, and can be reduced by a modulation with semiconductors switching in a frequency equivalent to 360Hz, with high quality and symmetrical voltage and currents waveforms. SHE-PWM was compared with POD-PWM at the same switching rate, through simulation and harmonic content investigated by the DTFT (Discrete Time Fourier Transform).