A fractal based space vector PWM scheme for general n- level inverters

This paper proposes a generalized pulse width modulation scheme to generate switching vectors for multilevel inverters. An approach based on fractal theory is used to find the sector, corresponding to the instantaneous reference space vector. The scheme uses a modified normalization factor for the DC link so that PWM signals for any n -level inverter, including even levels, can be generated. The scheme is presented for a 4-level inverter. 4-level inversion is achieved by connecting a 3-level inverter and a 2-level inverter in open-end winding configuration. The 3-level inverter is used to generate the vertex of the center of sub hexagon containing the tip of the instantaneous voltage reference vector. The 2-level inverter is driven by the PWM signals, which decide the duration of individual vectors. As the motor is fed by the inverters from both ends, it amounts by the addition of the PWM signal with the vertex of the sub hexagon center, realizing the actual switching vectors. The scheme is implemented and tested with a 2-HP induction motor drive and experimental results are presented.