48 prevalent, as well as the hairpin type, with the most recent trends more towards the hairpin technology,” says Anders. Pros and cons Given the large number of options and extra complexity arising from the possibility of combining different windings, our experts have given guidance on some of the important pros and cons of various types. With concentrated windings and the fractional slot type, the key advantages are the shorter end-turns that lead to lower resistance and better utilisation of the available volume, along with less need for insulation because the end-turns do not cross, says Anders. On the minus side, sub- and higher harmonics of the air-gap flux density cause additional – and significant – eddy current losses, he notes. Further, concentrated windings may be more susceptible to coil instability or movement during operation. Distributed, random, lap and singlelayer windings have less harmonic content in the air-gap flux density, and because there is only one coil side per slot and less insulation, there is more space left for conductor material, Anders explains. Cons include longer end-turns and consequent higher resistance, and the need for more axial space to accommodate them. With double-layer windings, however, short-pitched schemes can help suppress unwanted harmonics, but the requirement for insulation is greater due to the presence of two coil sides of different phases in one slot. The main benefits of hairpin and wave configurations are the greater fill factor, and the consequent lower overall phase resistance they allow, somewhat offset by higher manufacturing costs, he says. Montonen concurs that the shortend winding types, despite their lower resistance, represent poor solutions for high-speed machines. Meanwhile, distributed windings benefit from mature manufacturing know-how, and single-layer windings lend themselves easily to automated manufacture, but they have longer end-windings that increase resistance. In terms of application, hairpin windings are now the most commonly used type in high-performance electric cars, Montonen says. They can also be found in large roadgoing EVs, such as trucks and buses, as can random distributed windings. In light EVs, where economy and energy efficiency have a higher priority than performance, hairpins and aluminium windings are also common choices. A major reason for the broad application of hairpin windings is their excellent thermal performance in comparison with other types. They are solid conductors in contrast with the stranded wire types and can therefore achieve a high fill factor, making them very attractive where high power density and efficiency are needed. They also lend themselves to highly automated manufacturing processes using advanced winding machines with different processes, whereby the hairpins are inserted either axially or radially into the slots. Typically, the conductor is bent into a U-shape and inserted into the stator slots, and the end is then twisted and welded to the neighbouring hairpin. There is an alternative process whereby the conductor is bent to shape the entire winding, allowing these pre-shaped conductors to be inserted into the stator slots without the need to weld them to their neighbours, according to Xue et al in their 2021 paper, ‘Optimisation of Hairpin Winding in Electric Traction Motor Applications’. A further advance in manufacturing has come with the introduction of technology that allows the use of square wires, providing a high slot fill, and lowering the resistance and consequent ohmic losses once more. Also, these thicker conductors may be made of hollow wires, allowing for direct liquid cooling, Anders points out. “However, this requires care as the larger conductor sizes open the door for induced eddy currents and proximity effects, which increase resistance, especially at higher speeds. The manufacturer must pay attention,” he cautions. Litz wire, skin and proximity When the highest power densities are required for electric aircraft applications, the conductors often take the form of Litz wire, Montonen May/June 2024 | E-Mobility Engineering Lap windings on the rotor of a DC motor. They are also often found in universal motors capable of operating on AC or DC power (Image courtesy of Amada Weld Tech)
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