30 July/August 2026 | E-Mobility Engineering One of the most notable recent advancements in EV motor efficiency is linked to winding technology, as unravelled by Nick Flaherty It’s not a wind-up The shift from traditional round-wire windings to hairpin winding technology represents one of the most significant leaps in EV motor efficiency. By replacing bundles of copper wire with rigid, precision-engineered bars, manufacturers can pack more conductive material into the same space. Hairpin windings offer numerous advantages over conventional random windings, and they are increasingly being adopted by automotive manufacturers. The rectangular crosssectional geometry enables a higher slot fill factor, which improves stator slot utilisation and reduces the amount of insulation material required, thereby minimising losses. The robust design also supports higher current densities to increase the torque and power density, particularly during high-torque operations, without increasing the motor’s physical size. From a thermal management perspective, hairpin windings provide significant advantages. The compact structure and uniform end winding configuration enhance heat dissipation and integrate seamlessly with advanced cooling methods such as spray and jet cooling (see below), significantly improving the motor’s thermal performance. Hollow hairpins can also carry dielectric cooling fluids to further improve thermal efficiency, but they present more challenges in manufacturing. This trend underscores the potential for hairpin windings to become a dominant technology, from passenger vehicles to off-road and construction equipment. Unlike traditional random-wound motors that use bundles of thin, circular copper wires, hairpin windings use more heavy-duty copper bars with a variety of cross-sections. These bars are preformed into a U shape resembling a hairpin before being inserted axially into the slots of the motor’s stator. Once positioned, the open ends of these pins are mechanically twisted and laser-welded together to create a continuous, high-performance electrical circuit. This provides a higher fill factor to improve the efficiency of the motor, as well as improved thermal management and simpler mass production. Because the hairpins are rigid, the entire assembly process can be fully automated with high-speed robotics. This eliminates the variability of manual wire winding, ensuring the extreme consistency required for mass production of powertrains. The improved copper slot fill factor reduces the space dedicated to slots and enables potentially higher torque and power density, along with a reduction in DC resistance. Assuming the same stator volume, a hairpin winding machine can be less saturated than its stranded winding counterpart, leading to lower iron losses. This is a key advantage, especially for drive cycles. EV motors usually have a small number of series turns, which U-pin hairpins in a motor (Image: Jingawire) Comparison of hairpin and stranded windings (Image: University of Nottingham)
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