Optimal PWM Schemes in Wound Rotor Synchronous Machines and IPM Synchronous Machines for Maximum System Efficiency: A Comparative Study

Wound rotor synchronous machines (WRSM) without rare-earth magnets are becoming more popular for traction applications, but their potential in drive performance has not yet been fully explored. This paper presents a Pulse Width Modulation (PWM) scheme optimization procedure to minimize motor and inverter losses. It leverages different PWM schemes with different PWM switching strategies and switching frequencies. First, a generic PWM-induced motor loss calculation tool developed by BorgWarner is introduced. This tool iteratively calculates motor losses with PWM inputs across the entire operating map, significantly improving motor loss prediction accuracy. The inverter losses are then calculated analytically using motor and wide-bandgap (WBG) switching device characteristics. By quantifying these various scenarios, the optimal PWM scheme for achieving the best system efficiency across the entire operating map is obtained. The PWM-induced motor loss characteristics, the system loss tradeoffs, and the resulting optimal efficiency are discussed in detail. Finally, a comparison between IPMSMs and WRSMs in terms of system efficiency is presented. Discussions on the differences of PWM-induced losses and optimal modulation schemes between WRSMs and IPMSMs are provided to understand the tradeoffs in selecting different motor topologies.