AC Electric Powertrain without Power Electronics for Future Hybrid Electric Aircrafts: Architecture, Design and Stability Analysis

This paper proposes an electric powertrain architecture for future hybrid electric aircrafts which structure is only composed of permanent magnet synchronous machines for both generators (PMSG) and motors (PMSM). The direct connection through an AC bus of a PMSG with one or several PMSMs involves the suppression of power electronics usually embedded in electric or hybrid electric powertrains. The idea is clearly to simplify the architecture and to significantly reduce the weight of propulsive device, "weight being the prime enemy in aeronautics". However, the connection between power generation and propulsion devices through power electronics converters offers degrees of freedom allowing to control and stabilize the whole system. Contrarily, the direct connection between synchronous machines (PMSG-PMSM) sets a rigid link with non-linear behavior between both devices, causing complex stability issues that are analyzed. For that purpose, after having discussed the advantages and drawbacks of this powertrain by comparison with classical architectures, including power electronics, a set of models (analytic and simulation) and analysis tools (root locus, transient time simulation) is proposed. They are used in a theoretical approach to emphasize the stability issue and to assess parameter sensitivity. A reduced power scale test bench with a single-motor AC powertrain is presented: together with circuit simulation models, it is used to compare and validate the theoretical analysis results.