A review of structural design for hairpin windings and ultra-high slot fill factor in electric motors for new energy vehicles

Hairpin winding technology has emerged as a transformative structural design paradigm for new energy vehicle traction motors, enabling slot fill factors exceeding 70% compared to 35%–45% for conventional round-wire windings, thereby delivering significant improvements in power density, thermal management, and manufacturing automation. This review systematically examines the structural design aspects of hairpin, X-pin, continuous wave, and I-pin winding configurations, encompassing conductor geometry optimization, end-winding shortening strategies, AC loss mitigation, and manufacturing process control. Particular emphasis is placed on the electromagnetic and mechanical trade-offs associated with rectangular conductors, including skin and proximity effects, welding quality control, and insulation integrity under high-voltage pulse-width modulation stress. The paper synthesizes recent advances in laser welding technology, variable cross-section conductors, and model-based defect monitoring systems. By analyzing comparative performance metrics across different flat-wire architectures and identifying persistent technical barriers including manufacturing complexity, AC loss management, and slot opening design, this review aims to provide a comprehensive reference for researchers and engineers engaged in next-generation high-efficiency electric motor development.