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

it provides a good combination of tolerances and cost. With interlocking progressive tooling, small portions of each single lamination are pressed down into the one underneath it in the die. Known as clinching, this forms tiny depressions called dots that keep the stack together. Interlocking is a highly productive and well-proven technology, the metal forming systems provider emphasises. It offers high output, easy stack handling and a wide choice of suppliers of production equipment such as tools and dies. However, one increasingly important disadvantage is that the clinching dots create shortcuts between the layers that are detrimental to the motors’ magnetic properties. Using very thin layers of steel though makes it difficult to form these clinching dots, resulting in small joining forces, low stack density and sometimes unsecured stacks. For this reason, interlocking is reaching its limits as far as stamping advanced traction motor cores is concerned, with laminates 0.2 mm thick already proving a challenge for the technique. Gluing involves applying an adhesive in the stamping tool before the rotor and stator parts are cut out, then pressing the layers together with the glue in between. In this way, the electrical isolation coating on the steel remains undamaged, which is not the case with interlocking. This is an attractive alternative to interlocking, because both interlocking and welding increase eddy-current losses in the stack, so there are opportunities to increase quality by using advanced bonding techniques, says the electrical steel maker. Two main adhesive bonding technologies are used: dotted gluing and full-face bonding. Because (as the term suggests) dotted gluing only fixes each lamination to its neighbours in the locations where the dots are, it is not as strong or as stable a method as full-face bonding, which can produce stacks that exhibit very fine tolerances and high mechanical stability. Backlack and full-face bonding At this point it is worth examining in more detail full-face bonding using backlack. The process is said to contribute substantially to the behaviour of the steel inside an electrical machine, and is described as the least damaging and most flexible means of joining laminations to form a coherent stack. Well-suited to the use of very thin steel, it also allows for more intricate shapes in the laminates, the metal forming specialist says. After the laminations have been punched out of backlack-coated sheet steel feedstock, they are thermally bonded in two steps. In the first, the applied heat softens the varnish and then melts it after completion of a chemical reaction that imparts high bonding strength along with high resistance to various media. In the second stage, a further increase in temperature hardens the self-bonding varnish and transforms it into a stable, highly crosslinked, high- viscosity polymer. This material holds the laminations together and fills the gaps between them. The technique provides a great deal of design freedom, allowing for the thinnest webs and tightest tolerances without having to consider bonding points such as welding seams and clamps. Without imperfections such as welding seams, for example, which could change the properties of the base material, the steel also retains its desirable magnetic properties. Another benefit is mechanical stability, as the process prevents the laminations from fanning out and produces a compact stack that can be easily milled to accept the magnets. There are also acoustic advantages, the stack manufacturer notes, as the varnish between the layers of steel has a strong damping effect that reduces noise and vibration. With additive manufacturing (AM) techniques such as 3D printing making inroads into many areas of industry, it is natural to look at electrical machine laminations in the light of the technology. So far, its impact has been very small, because it faces a fundamental difficulty when it comes to reducing eddy-current losses in the final rotor or stator, and production of rotors and stators as single pieces by AM would lead to poor magnetic behaviour. Theoretically, laminations could be produced, but only as proof-of-concept or prototype parts, says the For interlocked motor laminations, this high-speed press can process a strip width of up to 600 mm and a material thickness of down to 0.2 mm (Courtesy of Schuler Group) Winter 2021 | E-Mobility Engineering 71 Focus | Motor laminations