Variable time step control with synchronous PWM in low frequency modulation index for AC machine drive

This paper proposes a variable time step controller with synchronous PWM. Since a frequency modulation index is low in an ultra-high speed or high power drive system, the modulated voltage by a fixed time step control includes large harmonic components. The optimal PWM method is one of the solution to minimize the harmonic components, but it needs many offline calculations and its structure is complicated. The proposed variable time step control has small calculation burden and it does not need offline calculations. Furthermore, the harmonic components of the modulated voltage is small because the frequency modulation index is kept as integer value in all speed conditions. Also, the transition strategy between synchronous PWM methods is proposed and it is verified with simulation and experimental results.