Minimization of AC Copper Loss in Permanent Magnet Machines by Transposed Coil Connection

With the increasing demand for high power density and high-speed electrical machines, ac copper loss issue becomes more severe and poses a significant challenge for machine design. This article proposes a suppression technique by transposing the coil connection in the same branch to balance the inductances of parallel strands, thus reducing the circulating current ac loss. Compared to the commonly used transposition method implemented within one coil (e.g., Litz wire), the proposed method can be beneficial to reduce the transposition points and have no influence on the slot filling factor. The proposed idea is investigated by the field-circuit coupled finite element analysis (FEA), taking the bundle-level circulating current into account. It shows that the copper loss can be reduced by 17% compared to the nontransposed one at 400 Hz for a studied 12-slot 10-pole permanent magnet machine. The influence of winding layout and slot/pole combination on the effectiveness of the proposed transposition method is also investigated. Finally, experiments are implemented on a designed test-rig to validate the FEA method and the effectiveness of the new transposition.