31 E-Mobility Engineering | July/August 2026 Hairpin & advanced windings | Tech focus numbers. The direct side effect was an explosion in the quantity of welding points, increased end winding mass/ volume and higher risk of insulation failure, leading to the development of different types of hairpin. Types of hairpin As well as the basic U-pin hairpin, the winding forms of flat wire motors mainly include the I-pin, X-pin and S-winding, or the continuous hairpin winding (CHW). In the U-pin, the hairpin is first formed, then inserted and then welded on one side. It is the most widely used form of flat wire winding. The process begins with enamelled copper wire being straightened, cut and bent into a U-shaped form. These pins are inserted axially into the stator slots, which have been pre-lined with insulation paper. After insertion, the open ends of the U are twisted – a mechanical process where a nesting tool grabs the ends and rotates them to a specific pitch. The final and most critical step is the laser welding. Each pair of pins must be precisely aligned and welded to complete the circuit. This requires high-end 3D vision systems to guide the laser because even a micrometrescale offset can cause a cold weld or excessive spatter, leading to motor failure. The high fill factor (of up to 75%) significantly reduces DC resistance, but the thick conductors suffer from the skin effect at high frequencies, where current migrates to the surface, increasing AC losses at high rpm. I-pin I-pins are simplified versions of the hairpin where the conductor is a straight bar rather than a pre-bent U and achieve a ‘slot fill’ of over 70%, significantly reducing resistance. The I-pin winding is inserted directly and then soldered on both sides. There is no need for single-slot assembly, further reducing the space reserved for winding assembly. The disadvantages are that the welding process is cumbersome and the end size is large. Unlike U-pins, which only require welding on one side of the stator, I-pins require welding on both sides. This doubles the number of welding points, which traditionally increases the risk of manufacturing defects. However, the technical advantage lies in the insertion process. Because there is no pre-bent crown, I-pins can be inserted into much tighter slots without risk of the crown catching or the insulation scraping. This allows for even more aggressive electromagnetic designs. To mitigate the double-welding drawback, a specialised bridge or busbar connector is included on one side to streamline the electrical interconnections. The flat surfaces of the bars means a large number of strands in hand might be necessary, causing significant AC loss. A proper hairpin winding combination of layers and parallel paths can significantly reduce this loss. Owing to the lower DC resistance, the hairpin machine clearly has higher efficiency in the base point and a significantly larger high-efficiency region. This is also due to lower AC losses in the hairpin winding machine. Stranded winding requires bundles of parallel wires in the slot to meet the voltage requirement and reach a good copper slot fill factor. These bundles, which represent the effective series turns, are particularly prone to high AC losses due to the circulating currents in the parallel wires. The chosen hairpin winding design with eight layers has significantly less AC loss as a result of the lower slot height. However, there are several variations of the hairpin, each balancing manufacturing complexity against electromagnetic performance. The flat-wire hairpin winding for automotive applications was introduced in 1999 with a segmented winding embedded into the stator slot providing a direct heat path to the laminations. However, this also brought massive end winding welding joints, which required costly CNC bending equipment, a succession of stripping-, aligning-, twisting- and laser-welding steps for every segment, longer cycle times and higher likelihood of insulation damage. Subsequent developments focused on trimming the segment count, simplifying welding operations and balancing multiple parallel paths to avoid circulating currents and losses. Ambitious performance roadmaps for electric ground vehicles and aircraft powertrains are pushing traction motor designs to higher speed and fundamental operating frequency. To mitigate AC winding Joule losses, designers progressively reduced individual conductor dimensions by increasing both the stator slots and layer U-pins used in commercial EV motors (Image: Jaguar Land Rover)
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