Analysis and Mitigation of Phase Imbalance in Sub-Harmonically Excited Brushless Wound-Field Synchronous Machines

Sub-harmonically excited brushless wound-field synchronous machines (BL-WFSMs) excited by three-phase voltage source inverters inherently suffer from phase current imbalances due to asymmetric electromagnetic armature phase coupling. These imbalances deteriorate torque characteristics, increase losses, and limit the practical deployment of such machines. This paper presents a detailed analysis and mitigation of armature phase imbalance phenomena in sub-harmonically excited BL-WFSMs, focusing primarily on their electromagnetic origin, electromagnetically consistent modeling, and practical suppression techniques. First, conventional sub-harmonically excited BL-WFSM topologies are analyzed, and the root causes of phase asymmetry are investigated using two-dimensional finite element analysis (2-D FEA). The results demonstrate that inherent asymmetries in the magnetic field distribution and phase coupling introduce unequal phase inductance under inverter excitation. Based on these insights, a fractional-slot concentrated winding (FSCW) machine design featuring structurally and magnetically symmetric characteristics is proposed to eliminate the inherent imbalance in the armature phases. The proposed design is then optimized using a Kriging surrogate model and a genetic algorithm to achieve low torque ripple and high average torque in the steady-state operating region. A prototype is manufactured considering the optimized design parameters, and experimental validation is carried out using a three-phase voltage source inverter. The consistency of the 2-D FEM simulations and experimental results confirmed a substantial reduction in phase current imbalance, thereby validating the effectiveness of the proposed mitigation approach for sub-harmonically excited BL-WFSMs.