68 resistance to the flow of eddy currents in motor cores. The general trend in motor core production underscores the importance of steel laminations that typically range in thickness from 0.20 to 0.25 mm; however, even thinner laminations (<0.2 mm) are good for reducing eddy current losses at higher switching frequencies. Also, alloying elements such as Si, Al, Mn and P reduce electrical conductivity and, consequently, eddy current losses. The production and subsequent processing of very thin and highly alloyed steel sheets and lamination stacks present significant manufacturing challenges in terms of both time and complexity. For example, the stamping and joining of such thin materials represent major hurdles because such processes push traditional mechanical interlocking methods to their technical limits. Advanced lamination and coatings Interlocking electrical steel sheets by means of clinching can impair the magnetic properties of the motor by damaging the insulation between the sheets and the electrical steel itself. As an alternative, adhesive can be applied to the raw material before punching. The lamination stacks are then created by pressing the electrical sheets together, leaving the insulation undamaged. However, applying adhesive inside the punching tool can lead to a lower output rate and might be effective only in certain areas. Insulation coatings applied to electrical steel laminations play a crucial role in reducing eddy current losses and improving overall motor efficiency. By electrically isolating the individual laminations, these coatings prevent the flow of large inter-laminar eddy currents within the steel core. Focusing on thermoplastic insulation, a key area for efficiency gains, current manufacturing equipment is compatible with thermoplastic films for slot liners and wire coatings, despite their lower thicknesses, which eases adoption because no major investment is required. Beyond eddy current reduction, certain coatings, particularly self-bonding varnishes (face-bonding), offer additional benefits such as improved acoustic performance through vibration damping, enhanced stack stability by bonding the laminations together and the elimination of point contacts that could lead to short circuits. Furthermore, media-tight stacks achieved through adhesive bonding can facilitate more efficient cooling by preventing the escape of cooling media. The higher thermal conductivity of some bonding varnishes also contributes to heat dissipation. The self-bonding process (Backlack), for example, offers a comprehensive solution. In application of this technique, the electrical sheets are joined together to form a stack under pressure and temperature in a baking station. This creates face bonding, which not only enhances the structural integrity of the stack, thereby reducing vibrations and noise during motor operation, but also results in denser layering of the sheets. Furthermore, because the thermal conductivity of bonding varnish is higher than that of air, it is easier for heat to be removed from the motor core. Two-layer stamping represents another emerging technology aimed at addressing the challenges of joining thin laminations. Instead of stamping a single layer of the lamination with all its features (such as the outer shape, slots and holes) in one process through a series of dye stations, two-layer stamping involves simultaneously processing two layers of the electrical steel sheet in certain stages of the progressive dye. Likely advantages of this process include higher production efficiency and throughput, unit cost reduction, and (potentially) better stacking accuracy and material savings. Other manufacturing innovations are also crucial for effective application of advanced materials. One of these is the drive toward reducing overall material usage, which inherently shortens cycle times by minimising the number of processing steps. For stator windings, the integration of meter mix dispensing and curing systems for end winding potting offers a streamlined approach, particularly when considered early in the design phase, and it can enhance thermal management and electrical insulation. Winding materials For windings, copper provides better conductivity than alternatives such as aluminium, but it is both heavier and more expensive. Because aluminium conductors have to be larger in cross section than copper ones sized to carry the same maximum current, copper conductors tend to be more compact for a given performance. Development effort is going into increasing the conductivity of aluminium alloys to make them better substitutes for copper. While superconductors offer even higher conductivity, they must be maintained at very low temperatures, which is impractical for most e-mobility applications in the near term. Where weight and cost are high priorities in motor design, aluminium alloys with high conductivity are increasingly attractive. Examples include the high-purity 1xxx series Product focus | Motor materials May/June 2025 | E-Mobility Engineering Smartline EV 3.8 high speed blanking press (pictured) receives sheet electrical steel from a coil via a fine levelling machine, a strip thickness measuring instrument and a highperformance feeder to produce core laminations (Image courtesy of Andritz Schuler)
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