35 Hairpin & advanced windings | Tech focus targeted for next-generation traction propulsion applications. Overall, CHW presents potential for further reducing AC loss compared with that of other flat winding options. However, given the limited amount of literature on this topic, there is no absolute conclusion that CHW is the most advantageous solution in AC loss mitigation. The number of welds falls progressively from I-Pin and conventional hairpin, through diamondlap variants, to its lowest in CHW. Each reduction shortens cycle time, lowers capital expenditure and removes the thermal–mechanical hotspots that can trigger partial discharge. All flat-wire topologies already exceed stranded coils in dielectric endurance, but by eliminating every branch weld, CHW delivers the most uniform and reliable insulation system for high-voltage, inverter-fed motor drives. Welding can compromise insulation integrity, lengthen the process chain and enlarge the end winding envelope both radially and circumferentially. This restricts stator designs with many slots or radial layers. To combat the efficiency losses at high speeds, thin hairpin designs are using multiple layers of thinner bars instead of thick ones. Breaking the conductor into thinner slices means the skin effect is minimised, allowing the motor to remain efficient at 16,000+ rpm. Additionally, more layers also mean more surface area for heat dissipation. However, moving from four layers to 10 layers doubles the number of welds required, and a single weak weld can scrap the entire stator. Thinner pins also require more layers of enamel insulation, which can consume some of the space gains. Under present manufacturing practice, welded hairpin solutions struggle to exceed 10 layers and 96 slots. In contrast, established CHW lines already support 16 layer conductors in stators with more than 120 slots. This scalability is key to reducing the size of flat conductors, curbing conductor level AC losses, and increasing the exposed surface for effective cooling to meet the higher frequency and higher power density requirements of nextgeneration traction machines. However, there is a lack of comprehensive detail on CHW manufacturing techniques, as well as universally applicable design guidelines. In particular, there are extensive ongoing research elements in the winding layout theory of CHW, including critical areas of parallel branch and transposition designs. The industry trend is currently leaning toward multi-layer thin hairpins. While they are harder to weld, they provide the best balance for modern EVs that need to cruise efficiently at highway speeds while maintaining the torque density needed for quick acceleration. Hairpin winding technology has redefined EV motor design by replacing traditional ‘random-wound’ round wires with rigid, rectangular copper conductors. This transition isn’t just about shape – it’s a shift toward highprecision automated manufacturing. Design tools Manufacturing and assembly processes mean that the design of CHW is less flexible compared with that of a conventional hairpin. The selection of the parallel branches is limited by the wire numbers and the layer jumps of wires are fully determined by the winding pattern, while there is also layer asymmetry due to the radial shift. These constraints and features must be considered during the design stage. Suitable manufacturing processes should be selected based on the winding parameters and the application. For example, choosing the appropriate winding direction based on the required number of parallel branches can save unnecessary welding. Deciding whether to adopt a non-radial-shift pattern based on the motor’s operating conditions and efficiency requirements can also optimise the motor’s performance and cost. Reasonable and necessary slot transpositions should be used to achieve slot symmetry in parallel branches. Corresponding wires that exchange layer positions during the radial-shift region should be seriesconnected (if possible) to eliminate layer asymmetry. The ideal winding design should achieve strong symmetry to eliminate circulating current losses, but cost and manufacturing consistency should also be considered. Irregular slot transpositions usually increase wire stress and the end height of the wire mat, causing manufacturing complexity and inconsistency. While achieving symmetry design, irregular slot transpositions should be minimised and, if necessary, placed in different slots to reduce the impact of the increased wire mat height on the rolling up and expansion process. Design tools can automatically compute the hairpin winding wave pattern, dividing the scheme into different elementary windings. Each elementary winding is built to meet flux linkage balancing rules covering all the layers and all the slots per pole per phase, and represents a potential parallel path. Different parallel path combinations are possible by putting in series groups of elementary windings. A design comparison for a traction application motor is shown above. The two machines are designed for the same voltage, current and slot current density and have been optimised for the same output performance. E-Mobility Engineering | July/August 2026 Feature Round Wire Standard Hairpin S-Winding Multi-Layer (8+) Slot Fill Low (~45%) Very High (70%+) High (65%+) High (68%) High rpm Efficiency Good Poor Moderate Excellent Manufacturing Speed Slow Very Fast Moderate Fast (but complex) Reliability High Medium (Welding) High Medium (Welding)
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