A Variable Structure Modulation Strategy to Suppress CMV and Reduce Switching Losses for UAV Electric Propulsion System

It is well known that the electromagnetic compatibility and efficiency of UAV electric propulsion systems are related to the weight of electromagnetic filter and heat sink so that the weight of devices can be reduced by suppressing common mode voltages (CMVs), and the reduction in switching losses is the benefit for improving system efficiency. The modulation strategies have been widely adopted to suppress CMV and switching losses, but most of the strategies have not considered the suppression of low-frequency CMV. This article proposes a variable structure modulation strategy (VSMS) to suppress high-frequency CMV, low-frequency CMV, and switching losses. First, this strategy adds the restriction condition that the sum of vectors with the same CMV amplitude is equal, that is, improved SPWM (ISPWM). It extends the round shape with the traditional modulation degree of 1 to a regular hexagon that is circumscribed to the round shape, achieving zero CMV within this modulation degree. The use of near-state PWM (NSPWM) outside the regular hexagonal area can effectively suppress CMV and switching losses. Different from the traditional combination method, which only selects different methods based on the modulation degree, VSMS can automatically switch ISPWM and NSPWM within the whole vector circle as the voltage rotates, thus achieving the integration of the two methods. Finally, the effectiveness of VSMS is verified by experimental results.