Improved PWM control for GTO inverters with pulse number modulation

A pulse width modulated (PWM) inverter using GTOs is one of the promising power converters for AC motor drives. There are, however, still some unsolved problems with GTO inverters: (1) the modulation factor of conventional GTO inverters is limited to about 0.8 in order to maintain the minimum on-time of every GTO. If a modulation factor of 1.0 is possible in the bridge-type inverter, the output capacity would be larger in proportion to the modulation factor. The pulse number modulation (PNM) is proposed as an effective measure for this modulation factor improvement; and (2) the minimum on-time of a GTO brings about uncontrollability for a low voltage reference in the neutral point clamped (NPC) inverter. The authors propose a new procedure using the PNM technique to solve this voltage uncontrollability. The PNM method contributes to make an equivalent unity modulation factor in PWM inverters and to linearize voltage control characteristics in NPC inverters