Low voltage modulation method in six-step operation of three phase inverter

This paper proposes the asymmetric six-step operation which can adjust the level of the phase voltage in three phase voltage source inverter. In high power systems, the switching frequency of the power switch is limited to a few hundred hertz. So, the conventional symmetric six-step operation is commonly used because of the maximized modulation index and the minimized number of switching. However, the modulation index cannot be adjusted in the fixed dc link voltage system. By modifying the duration of the turn on time asymmetrically, the modulation index can be changed even in the minimum number of switching. The modulation index is expressed as the function of the turn on time. Also, the harmonic components of the modulated phase voltage are calculated from the Fourier analysis. The proposed modulation is implemented at the induction machine drive and it can reduce the rms currents in the low torque conditions.