Elimination of low-order harmonics using a modified SHE-PWM technique for medium voltage induction motor applications

In high power applications the inverter of a motor drive should have a switching frequency as low as possible in order to reduce the switching and snubber losses. An effective pulse width modulation (PWM) approach that can be utilized successfully with high control accuracy is combination of selective harmonic elimination and pulse width modulation (SHEPWM). This technique offers many advantages other PWM techniques including direct control over output waveform harmonics, and the ability to neglect triplen harmonics in three-phase inverter systems. SHEPWM can optimize PWM output waveforms for selected harmonic elimination or to minimize total harmonic distortion (THD). This technique is used with voltage source inverters (VSI) and current source inverters (CSI) as well. This paper presents a novel technique that uses one inverter (the reference inverter) utilizing a conventional SHE-PWM scheme to eliminate a number of specific low order harmonics in the induction motor load. The first inverter directly feeds the induction motor load. The second inverter is phase shifted with a pre-calculated angle to eliminate the first significant surplus harmonic. This set of two inverters is connected to a second similar set of two inverters, with a second pre-calculated phase shift, to eliminate the second significant surplus harmonic and so on. Harmonic currents profiles using MATLAB and PSpice simulation are compared for a three-phase high-power medium voltage induction motor load, with and without a phase-shifting transformer.