66 development. The temperature stability they need to withstand high operating temperatures comes from the use of heavy rare earth elements such as dysprosium and terbium. Whilst firstgeneration high-temperature sintered NdFeB magnets had a heavy rare earth content of between 8% and 10% by weight, the latest grain boundary diffusion treated magnets contain much less, typically between 2% and 3%. Hot-deformed NdFeB magnets provide an alternative without requiring heavy rare earths, achieving temperature stability equal to that of most sintered magnet grades but with better corrosion resistance. Production of hot-deformed magnets is simpler than that of sintered magnets formed from highly pyrophoric (igniting spontaneously in air) powders, and recycled powders are being developed for hot-deformed magnet production. Grains, crystals and orientation Besides permanent magnets, motors also rely on electromagnets, formed by current flowing through conductors wound around cores of electrical steel. Electrical steel is a crystalline material composed of grains, which are regions where the iron atoms are arranged with a consistent structure. The interfaces between these grains are known as grain boundaries. Certain grain orientations offer lower resistance to changes in the direction of magnetisation (magnetically favourable textures), while others are less conducive (magnetically unfavourable textures). Grain boundaries themselves act as obstacles to the movement of magnetic domains, which are regions within the material where the magnetic moments of the atoms are aligned. A smaller number of grain boundaries generally makes reversal of magnetic polarity easier, leading to reduced core losses. This is why electrical steel undergoes high-temperature processing to promote grain growth, resulting in larger grains. However, in very high-frequency applications, excessively large grains can lead to increased eddy current losses. One important distinction is between electrical steels that are grain-oriented and those that are not. Grain-oriented electrical steel exhibits superior magnetic properties, specifically in the rolling direction, owing to its specially aligned crystallographic texture, but exhibits poor properties perpendicular to the rolling direction. This brings advantages for use in segmented stator stacks where this directional property can be used effectively. However, processing grain-oriented steel into such segmented stacks is more complex and, importantly, it is unsuitable for use in the rotor. Non-grain-oriented electrical steel, on the other hand, possesses good magnetic properties both longitudinally and transversely to the rolling direction. This isotropic behaviour makes it more universally applicable for various motor components, including the stator in non-segmented designs and the rotor, because rotors are not normally segmented. Extra ingredients Alloying elements such as silicon (Si), aluminium (Al), manganese (Mn) and phosphorus (P) are added to electrical steel to modify its properties; specifically, to increase its electrical resistivity. This is crucial for reducing eddy current losses, which become particularly dominant in high-speed machines and at higher operating frequencies. Consequently, electrical steel grades used in e-mobility applications typically have higher alloy content. However, these alloying elements tend to diminish the polarisation and saturation magnetisation of the steel, which can reduce the motor’s torque density, but increasing the alloy content often leads to increases in the mechanical strength of the steel. Minimising core losses Efforts to improve electrical steels are focused on reducing core losses at high switching frequencies, increasing permeability, developing cost-effective manufacturing processes for thin laminations and improving mechanical strength. High permeability is a measure of its ability to conduct magnetic flux. The higher the permeability, the higher the torque and power of a motor. Core losses affect the energy consumption of a motor, and the aim is to minimise losses caused by electrical eddy currents and hysteresis. Hysteresis is the phenomenon of the magnetic flux density in a motor’s iron core (stator and rotor) lagging behind the cyclic magnetising force produced by the current in the windings. This ‘magnetic lag’ results in energy loss. Together, hysteresis losses and eddy current losses are known as iron losses. Achieving optimal performance at the high switching frequencies used to drive motors by inverters based on silicon carbide (SiC) transistors requires thin laminations with insulating coatings, which increase manufacturing complexity and cost. The thickness of the electrical steel laminations also has an impact on motor efficiency, with thinner laminations increasing Product focus | Motor materials May/June 2025 | E-Mobility Engineering Electrical steels formed into increasingly thin laminations and separated by layers of insulating compounds form the cores of rotors and stators, the latter sometimes using grainoriented steel (Image courtesy of Voestalpine)
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