49 Coil windings | Insight points out. Litz wire consists of multiple, individually insulated strands that are woven together. By distributing the current across many strands, Litz wire helps to mitigate the skin and proximity effects, and the consequent resistance and power losses at the high switching frequencies common in high-performance motors. AC tends to flow more towards the outer surface of a conductor rather than being evenly distributed throughout its cross-section. This is the skin effect, and it increases with switching frequency. Similarly, the proximity effect tends to concentrate current flow through a conductor towards the surfaces facing neighbouring conductors. Both phenomena reduce the effective cross-section of the conductor, increasing resistance. As well as reducing these undesirable effects, Litz wire’s multi-strand construction allows it to maintain flexibility while carrying high currents, which can be crucial in applications where space is limited. This construction also permits better heat dissipation, compared with monolithic conductors. Together, these characteristics enable Litz wire to carry high currents more efficiently in a cross-sectional area, contributing to higher power densities, reduced weight and, consequently, a greater payload in aircraft. Integrating this kind of wire into e-machines brings design and assembly challenges. For example, ensuring proper termination and insulation of each individual strand within the motor’s winding can be difficult and may require custom solutions. Further, Litz wire’s construction from individually insulated strands may need extra space in the form of larger slots or more winding layers. To take advantage of Litz wire’s good heat dissipation characteristics demands careful thermal design and may also require improved ventilation or cooling channels in the motor structure. Originally developed to carry AC in communication and radar applications, Litz wire designs tend to be optimised for specific AC frequencies. Designing conductors to perform well over the wide range of frequencies encountered in variable-speed propulsion motors can be complex, and it may require sophisticated modelling and simulation tools. Also, weaving together multiple, individually insulated strands to make Litz wire is an intricate process that can be labour-intensive or require specialised machinery, leading to higher production costs, although there have been recent advances in this area. One of these is formed Litz wire winding (FLW), consisting of bars made by compressing twisted bundles of parallel, connected strands and compacting them to achieve a high slot fill factor. According to Dimier et al in their 2020 paper, Comparison of Stator Winding Technologies for High-Speed Motors in Electric Propulsion Systems, the individually insulated strands are continuously transposed along the axial direction of the motor in predefined positions. Skin and proximity effects are minimised by the use of strands with small cross-sections. “The axial transposition ensures a balanced thermal behaviour of the FLW bar. A thin-strand insulation is sufficient since the voltage difference between parallel strands is small. The insulation to the stator or other phases is realised for the whole bar,” the researchers say. One FLW manufacturer in the automotive sector, Hofer Powertrain, claims that losses in the motor are reduced by 25% and that hairpinwinding production lines can be adapted to FLW easily. Conductor and core Material selection is also a key winding-related decision that affects performance in terms of electrical and thermal conductivity, and cost, Montonen notes. The mainstream options for e-mobility applications are copper, aluminium and alloys that mix copper with aluminium or silicon. As Anders explains, aluminium has a lower conductivity than copper, and therefore more space needs to be provided for the wires. If the winding losses must be identical to those of copper windings, this leads to slots with bigger cross-sections and an increase in total machine volume. Otherwise, efficiency will decrease with slot cross-sections that remain the same, which might be acceptable if cost is prioritised. Another important point from a manufacturing point of view is that the tensile strength of copper is significantly higher than that of aluminium. This may cause problems with aluminium during the manufacturing process, and possibly make the conductor more susceptible to fatigue and limit the overall lifetime of the machine, he cautions. The choice of winding material can also have an impact on the choice of core/lamination material, which E-Mobility Engineering | May/June 2024 Rectangular section hairpin windings, showing how they fit into the slots of an e-machine stator. Hairpin windings are increasingly used in permanent magnet synchronous machines (Image courtesy of Zeiss)
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