ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

in the electromagnetic, structural and thermal domains, a producer of high- performance speciality alloys points out. Which materials are best though depends heavily on the application. In most high-volume EV applications, motor laminations are made from silicon steel, says the alloys company. However, in EVs higher up the performance scale and in heavy- duty commercial vehicles, where the application requires greater torque and power density, high-induction and low- loss iron cobalt alloys come to the fore. Steel requirements Although the imperative to increase motor/generator performance while reducing size, weight and cost is a constant – and many different e-machine concepts are developed to meet that in their intended applications – the steels used in their rotors and stators must always fulfil the same set of requirements, the electrical steel producer says. These include supporting high magnetic flux, which depends on a high saturation level and good permeability. The material must also support rapid changes in the magnitude and direction of the magnetic field, for which materials with low coercivity tend to be preferred because they enable higher torque densities and lower losses. Higher electrical resistivity is also preferred because it limits the circulation of eddy currents and reduces the associated losses, particularly in high-speed applications. If the resistivity of the laminate stack as a whole is not the same in all directions (anisotropic) then steel with higher resistivity in one direction is also desirable, says the alloys producer. Thermally speaking, higher conductivity is usually an advantage. Most, if not all, motor components prefer to run cool, and overheating is a major constraint in motor design. Higher thermal conductivity in the laminations therefore speeds the transfer of heat from source to sink, and increases the motor’s power density. This property is subject to a trade-off though, in this case with resistivity, as it is not easy to optimise them both without compromising other properties. With these general requirements in mind, engineers looking for the best materials for their designs seek the right balance between thin, highly alloyed materials that minimise power losses for high-frequency applications, high permeability that enables higher torque outputs, and high yield strengths. Insulating coatings and the way laminates are joined together are also important. Managing interdependencies Striking the right balance is challenging, a second steel specialist we consulted points out, because these properties are not independent. For example, a higher percentage of alloying elements reduces power losses, which is an advantage, but it also decreases permeability, which is a disadvantage. This fundamental interdependence of key parameters is at the root of the complexity in e-motor design, which means it is not possible to define a standard set of simple rules for creating laminations for different types of motor/ generator. What’s more, lamination designs are heavily influenced by the type of motor for which they are intended, which can include DC and AC, synchronous and asynchronous, permanent magnet and induction, radial flux, axial flux and transverse flux machines. The geometry of their laminations is becoming increasingly complex in the drive to achieve ideal performance-to-volume- ratios and to incorporate features such as cooling passages, a provider of metal-forming tools and systems notes. The choice of material is also becoming more critical. In general, this depends more on the application and operating region than the motor type, which has a more indirect influence The standard way of minimising the problem is by constructing rotors and stators from layers of steel that are as thin as is practical, oriented to lie as parallel as possible with the lines of magnetic flux and separated by insulating layers. Thin layers limit the size of the current loops that can form and, because current is proportional to the area of the loop, minimising the area also minimises the amount of current that can flow. Because the power dissipated by an eddy-current loop is proportional to the square of the current, small reductions in current bring much larger ones in losses. Reaping benefits that are disproportionately greater than the magnitude of the reductions in laminate thickness is what drives the industry’s efforts to produce rotor and stator cores built from ever-thinner layers. That’s far from the whole story, however, as the magnetic and electrical properties of the materials are also critical. While laminations for industrial motors are typically made from steel between 0.35 and 0.5 mm thick, the trend in traction motors is to use much thinner gauges to drive down eddy-current losses – 0.2 mm is now considered practical for volume manufacturing, and still thinner layers are under development. Making a good electrical machine also requires balancing the demands Tata’s range of lamination steels is optimised for medium to fundamental frequencies of 200 to 2500 Hz and harmonics up to about 25 kHz (Courtesy of Tata Steel Europe) Winter 2021 | E-Mobility Engineering 65 Focus | Motor laminations