Adhesive bonding of steel laminations, as in this rotor, can be made media tight so that liquid cooling agents can circulate through motor cores without risk of leakage (Image courtesy of Voestalpine) 70 materials such as 1050 and 1350, which have an aluminium content of 99% or more. In addition to conductivity that is typically 61% of the International Annealed Copper Standard (IACS), they also offer good corrosion resistance and high ductility, but relatively low mechanical strength. The aluminium-magnesium-silicon alloys of the 6xxx series, such as 6101 and 6061, offer moderate conductivity of 40% to 50% of the IACS plus good mechanical strength through heat treatment and good resistance to corrosion. The more diverse 8xxx series includes some alloys tailored specifically for electrical conductivity, such as 8006 and 8011. While the conductivity is lower than that of the 1xxx alloys, they offer a useful combination of formability and strength. Beyond these alloys, ongoing r&d efforts are exploring advanced techniques to improve the conductivity of aluminium alloys, more of which later. Insulation materials for windings are another key area for r&d, with advanced thermoplastics providing alternatives to enamels, for example. Enhancements to wire enamels show marginal impact on insulation thickness compared with thermoplastic extruded solutions, which offer around 20% thickness reduction while maintaining superior properties after ageing (such as partial discharge inception voltage; PDIV). While polyimide, polyester-imide, and polyurethane-based wire enamels differ, all tend to lose thickness during ageing, degrading both the PDIV and the overall performance. Conversely, thermoplastic solutions demonstrate minimal performance drop, as highlighted in IEC 6000-34-21/41. Nanotechnology-based coatings and fillers in wire enamels show improved partial discharge resistance but still fall significantly short of automotive industry corona resistance requirements, which currently favour PD-free designs. These fillers can also introduce unacceptable brittleness. Ceramic bearings Despite their extra cost, ceramic bearings are attractive for premium and highperformance EVs, for example, where every fraction of a percent in terms of efficiency and reduction in parasitic losses contributes to extended range and overall driving experience. Here, ceramic bearings are used in traction motors, where their lower friction reduces energy consumption and heat generation, while their ability to withstand higher speeds supports high power outputs. Motors used in electric racing and high-performance road-going motorcycles often run at extremely high rotational speeds, evidencing how the enhanced stiffness of ceramic bearings enables more precise rotor dynamics. The reduced friction and increased durability of ceramic bearings can also benefit high-end e-bikes with powerful mid-drive motors, particularly in demanding off-road or high-speed applications. Additionally, the characteristics of low weight and high-temperature capability make ceramic bearings very attractive for eVTOL and electric fixed-wing aircraft, which are particularly sensitive to both weight and heat. Finally, EVs of many kinds often have auxiliary motors for various functions such as cooling pumps, compressors for air conditioning and power steering. In applications where these auxiliary motors operate at high speeds for extended periods, ceramic bearings can improve their efficiency and lifespan. Bearing material options Several types of ceramics are employed in bearings, each offering a specific set of properties. Silicon nitride (Si₃N₄), which is the ceramic used most commonly for high-performance bearings, combines high hardness, a high strength-toweight ratio, good wear resistance, a low coefficient of friction, stability at high temperatures, and good resistance to corrosion and other types of chemical attack. Silicon nitride bearings can operate with minimal or no lubrication in certain applications. Zirconia (ZrO₂) is endowed with high strength and toughness, together with robust resistance to wear. While generally not as hard as silicon nitride, it is better at resisting crack propagation. Zirconia bearings tend to be chosen for applications in which high load-carrying capacity and resistance to impact are important characteristics. Partially stabilised zirconia (PSZ) is a variant in common use. Alumina (Al₂O₃), which is significantly less expensive than either silicon nitride or zirconia, offers good hardness and May/June 2025 | E-Mobility Engineering
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