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

because in any type of motor the operating frequency and induction level can be either low or high, according to the speciality alloys provider. That said, all the types of electrical machine referred to above have their own sensitivities to key parameters such as magnetic permeability and coercivity, electrical resistivity, thermal conductivity and so on, adding to the complexity of the issue. These differences do however allow some material selection heuristics to emerge, the stack producer notes. Materials and motor types For permanent magnet synchronous machines, eddy-current losses (Watt losses) are particularly important. Thin layers are needed to minimise them, along with the mechanical strength to secure magnets in rotors at high rotational speeds. Because asynchronous machines are not so sensitive to such losses, thicker material is used and laminations might typically measure 0.35 mm. Axial flux and transverse flux machines have different flux paths from those in more widely used radial flux machines, and sometimes the flux moves in three dimensions. In this case, the use of soft magnetic composite or other 3D flux materials is key, the alloys provider says. Induction machines require high magnetic permeability, the stack producer notes, and one important condition that the motor designer must make is whether to use the same or different materials – one optimised for the rotor, the other for the stator laminations. Using different materials can result in a lot of scrap during manufacturing, particularly with the stator, for which segmented assembly is a popular solution. The alternative of using the same material for both rotor and stator laminations requires a steel optimised for strength and with good – but less than ideal – magnetic properties. The rotor would be assembled from components made from this material without further property-changing treatment, says our second steel producer, while the stator laminations would be annealed at the customer’s facility to optimise their magnetic properties. Annealing is a heat-treatment process in which the steel is brought up to a specific temperature, held at that temperature to ‘soak’ and then cooled at a controlled rate. As well as enhancing the metal’s electrical and magnetic properties, annealing also softens it to make it easier to work when cold and improves its machinability. As well as the fundamental characteristics of the materials, engineers must also take economic and strategic factors into account when making their selections, the stack maker adds. These include the number of steel mills that can produce the steel to the right specification: the specifications are quite demanding, so the choice of available grades tends to be limited, and changing suppliers during a project is difficult. Processability must also be considered because, for example, the thinner the material the more laminations are required for a given stack height. All these factors lead to longer processing times and the need for more machines to stamp out the layers. With cost, power density and efficiency over a wide speed range in mind, says the speciality alloys producer, thin-gauge silicon-iron laminate seems to offer a good trade- off between cost and performance for now, with cobalt-iron materials becoming an interesting option for future high-performance applications. Demanding power and torque densities Motors and generators with very high power and torque densities are particularly demanding on laminations. A key reason for this is that this performance is typically designed into compact electrical machines that run at ever-higher rpm, resulting in higher internal switching frequencies along with larger mechanical stresses and stress fluctuations in the rotor, our electrical steel producer notes. The higher frequencies lead to larger losses that must be compensated for with better magnetic properties in the steel, while the higher mechanical stresses can lead to plastic deformation and/or fatigue, requiring stronger materials to resist these forces. Laminations for motors designed for high torque and power densities are typically made from electrical steels with high permeability. To achieve high permeability, the alloying content of the steel must be limited, and that can make it more challenging to keep power losses to a minimum. However, such losses can be kept down even in steels with lower alloy content by reducing lamination thickness, although high strengths are not possible, according to the stack manufacturer. The company also points out that the processability in terms of stamping behaviour of low-alloyed and high-alloyed materials are different. This is significant, because one potential means of increasing Focus | Motor laminations Demanding applications for electrical machines make soft magnetic iron-cobalt alloys an increasingly popular option in lamination stacks (Courtesy of Carpenter Electrification) 66 Winter 2021 | E-Mobility Engineering