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

frequencies, and consequently on efficiency, but there are practical drawbacks, the electrical steel supplier adds. Thinner laminations are more difficult to stamp and assemble into stacks, which can cause quality problems and increase costs. The stack manufacturer points out that thin laminations are not necessarily better for permeability, as this desirable property is independent of thickness. What’s more, thin laminations don’t improve the stacking factor either. The stacking factor is the ratio between the actual mass of the stack, consisting of all the laminations and any material between them, and its theoretical mass if it were made of solid steel, which would be the ideal situation for amplifying the magnetic fields generated by the copper coils if it were not for the eddy-current issue. Very thin laminations mean that more of the stack’s volume is taken up by the bonding/insulating varnish between them. Practical limitations affect how thin the laminations can be made, however. In addition to the stacking factor, these include the effect on the capacity of existing production lines, which is reduced in relation to the thickness of the material they are set up to process and the relatively small number of mills equipped to turn out such a thin product, the second steel maker says. Furthermore, the thinnest grades are more difficult to process into stacks. For example, interlocking each layer with its neighbours is considered impossible at thicknesses of less than 0.25 mm, while many more laminations are needed for a given stack height, so more strokes are needed on the stamping line, and the thinner the material the more gently and carefully it must be handled, the company cautions. This highlights the trade-off facing motor designers between the benefits of lower losses that flow from thinner laminations. The speciality alloys provider emphasises that a motor/generator made with thinner laminations takes longer to manufacture and costs more than one built up from thicker ones. Stack-joining techniques are often chosen on the basis of the robustness required of the finished stack, which can differ between motor components – for example, stators are not subject to the same forces as rotors, the alloys company points out. The most appropriate joining technique can also be affected by the required production volume, with different methods typically used in mass production and prototyping. Manufacturing advances Lamination manufacturing technology has moved on, pushed by the demand for compactness in stacks along with reduced noise and vibration. Also, the geometrical accuracy of the stacks is increasingly important, emphasises the stacks manufacturer, which puts pressure on steel suppliers to provide material with highly consistent mechanical properties to meet the specifications. Different stacking methodologies have also been developed to maintain the electrical insulation between the laminations, with an emphasis on cost- effectiveness for volume production. Laminations for traction motors built in high volumes, for example, are typically cut and shaped from the sheet metal in stamping machines equipped with progressive die sets in which the material passes from one stage to the next, each stage performing a simple cutting or bending operation to produce the finished component. Thinner gauges and more complex lamination designs require more stages in the die sets, says the metal forming systems specialist, which therefore have to be longer and need larger presses to accommodate them. The company has extended the press bed of its latest machines dedicated to EV motor production, providing space for it to clamp the longer progressive dies needed for increasingly complex lamination geometries. These machines also feature a control mechanism developed in-house specifically to check the depth of die penetration at each stroke and adjust it in fractions of a second, to ensure accuracy in formation of the parts from the first stroke. To form the rotor or stator core, the resulting laminations can be loosely stacked, interlocked, welded or bonded with adhesives, or some combination of them, the electrical steel maker says, with cost and the required stack tolerances being the most important factors influencing the choice among them. Interlocking in the press, often supplemented by welding, is the most commonly used method, because Motors with high power and torque densities achieved at high speeds are tough on materials, so they need steels that combine good magnetic properties with resistance to stress (Courtesy of Tata Steel Europe) 70 Winter 2021 | E-Mobility Engineering