Decoupled PWM Control of a Dual-Inverter Four-Level Five-Phase Drive

Abstract

This paper studies pulse-width modulation (PWM) techniques suitable for a four-level five-phase open-end winding (OeW) drive. The drive comprises a five-phase induction machine, supplied using two two-level voltage source inverters (VSIs) with isolated and unequal dc-link voltages, in the ratio 2:1. A decoupled carrier based (CB) PWM modulation strategy, based on unequal voltage reference sharing between the two converters, is introduced in this paper. The stability of dc-link voltages in OeW drives is investigated next, using a novel analysis technique. Several modulation methods are analysed and the results show that application of the coupled pulse width modulation technique, with carriers having in-phase disposition (PD), leads to overcharging of the capacitor in the dc-link of the inverter intended to operate with the lower dc-link voltage. On the other hand, the proposed decoupled CB PWM scheme naturally eliminates the dc-link capacitor overcharging problem. These findings are verified experimentally, using open-loop V/f control. Two different decoupled CB modulation methods are compared and the best performing modulation method is selected and incorporated further into an OeW drive with field-oriented control (FOC). The presented steady state and transient experimental results demonstrate that the decoupled CB PWM technique is suitable for high performance variable speed drive applications.