must have compatible magnetic properties to ensure efficient energy conversion and minimise losses. For example, copper windings have electrical conductivity and lower resistivity, so a core material with high magnetic permeability, such as silicon steel, is typically chosen to maximise magnetic-flux density and minimise core losses. Anders notes that as aluminium wires require more space than copper ones, it might be an option to use a lamination material that allows for higher flux densities to compensate, but then the price advantage might be in question as better lamination materials come at a price premium. Montonen points out that the conductivity of the winding material dictates the shape of the slots in the core, while the saturation levels of the selected lamination materials affect the thickness of stator teeth. “It is the combination of these two and an iterative design process that enable the maximum amount of winding material to be fitted into the slot,” he adds. Winding and core materials should also have similar coefficients of thermal expansion to minimise the risk of excessive mechanical stresses from thermal cycling in operation, which can cause fatigue damage over time. The higher resistivity of windings made from aluminium or its alloys may generate more heat than copper windings, so core materials with higher thermal conductivity, perhaps combined with improved thermal management features such as active cooling may be needed. There are more exotic options in winding materials for specialised tasks. Silver, for example, offers the highest electrical conductivity among common metals, making it well suited to high-performance applications where minimising electrical losses is critical. If that need is extreme, superconducting materials such as niobium-titanium alloys, for example, exhibit zero resistance at cryogenic temperatures at the cost of a very complex and costly cooling system. Other specialised requirements, such as improved thermal stability or resistance to vibration and mechanical stress, can be met by composite conductors made from reinforced resins that incorporate conductive particles such as carbon nanotubes. Future directions Danfoss, which specialises in distributed and concentrated non-overlapping windings, has incorporated lessons from prototype-level studies of different windings optimised with cooling for high torque density use, Montonen says. Looking to the foreseeable future, he considers that Litz wire could be a revolutionary challenger to solid, copper hairpins, even though it has mainly been used in research projects and aerospace applications so far. Finally, he anticipates future use of oxidised copper, which has an oxide layer on the surface of the conductor. This has a number of benefits, including the prevention of further oxidation (corrosion) in general, but particularly at high temperatures, enhancing its thermal stability and extending its service life, while also mitigating the skin effect. Further, oxidised copper windings can be produced using environmentally-friendly processes that minimise the use of hazardous chemicals and reduce waste. Insight | Coil windings May/June 2024 | E-Mobility Engineering This Lucid integrated motor, inverter and transmission includes a stator with high-voltage continuous wave windings (Image courtesy of Lucid) 50
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