Active Torque Capability Determination Under Unity Power Factor Operation of Biaxial Excitation Synchronous Machines

Rare-earth free biaxial excited synchronous machines (BESMs) are best suited for achieving unity power factor (UPF) at their base speed, facilitating high-power density and cost efficiency in traction motor drive systems. Due to the presence of dual excitation sources, the rotor field current can be adjusted to increase the electromagnetic torque capability of the machine. This paper presents a novel algorithm to determine the maximum torque capability of BESMs while ensuring that stator and rotor voltage constraints are met and maintaining UPF across the entire operating range of the motor drive system. The proposed method is validated through extensive simulations under varying stator and rotor DC link voltage conditions, confirming consistent UPF operation. Additionally, a detailed analysis explores the trade-offs associated with different stator and rotor voltage levels, providing insights into optimizing BESM performance for traction applications.