49 E-Mobility Engineering | July/August 2026 E-motor production technology | Deep insight One of the main cost drivers is the process step of twisting in the production of hairpin stators for electric motors. This requires special tools with high demands on tolerance, accuracy and strength. The corresponding investment for a tool set is therefore in the range of €100,000 to €200,000, which is a considerable hurdle in terms of flexibility, especially in the prototype area. The production tools for creating and twisting increasingly complex hairpins face considerable challenges. The hairpins can be complex shapes and some can even be hollow, meaning that the tools that turn the wires into the right shape for the motor are complex. Systems such as SpeedFormer from Wafios can support various parts of the production cycle, running different stators on one line. The tools have evolved from pipe-bending systems with 170 CNC axes to adjust itself, feed the wires and not need a tool change with three different wires, and can switch the wires in 60 seconds while the wire guides are switched automatically. The graphical programming system allows automatic correction of the geometric features of the hairpin while the aim for the next generation of tools is to provide fully automatic correction and set-up from design. “Wire straightening is also important. As every coil of wire is different, you need to straighten the wire on the bending machine every time and we have AI tools that simplify the straightening with the servos,” says Martin Bauer, industry manager for e-mobility at Wafios. “It can take two hours to straighten the wire with an experienced operator and a third of the line can be just bending, then cutting and welding.” To address the cost issue, the variTwist joint project by Production Engineering of E-Mobility Components (PEM) of RWTH Aachen University and Berlin-based industrial partner Röscher developed a flexible tooling system for twisting flat copper conductors. The system is adaptable to the different variant drivers in hairpin stator production, and thus will bear the advantage of a massive reduction in tool costs as well as in delivery times, especially in prototype and small series production. “The twisting step is a particular cost driver in the manufacture of hairpin stators for electric motors,” says Prof Achim Kampker, director of PEM. “Especially in the prototype and preseries phase, the machine and tool costs exceed those of the product many times over and thus constitute a major obstacle in terms of flexibility.” The process station developed by PEM in the variTwist project enables the costeffective implementation of a flexible twisting process with tooling costs of less than €1000. The completed stator undergoes trickling or vacuum pressure impregnation with a high-grade resin to eliminate air voids, thereby enhancing dielectric strength and thermal conductivity. For an interior permanent magnet motor, the rotor core follows a similar lamination stacking process but introduces intense mechanical and magnetic complexity. The entire rotor assembly is placed into a high-energy post-magnetisation fixture, where massive electromagnetic pulses permanently align the magnetic domains of the Nd-Fe-B inserts. The rotor stack is fitted onto the rotor shaft using thermal shrinking (by heating the core and cooling the shaft via liquid nitrogen). Because traction motors operate from 16,000 up to 20,000 rpm, the assembly undergoes dynamic balancing. Material is precisely milled away or balancing weights are added to reduce residual unbalance to the ISO G1.0 standard (ISO 21940-11) or better, suppressing high-frequency harmonics and bearing wear. “The other element that is needed is good simulation to deal with the manufacturing tolerances on the durability and NVH side to produce the system. For high-speed concepts, 30,000 rpm is a sweet spot. Speeds below that increase the use of the materials, while speeds above this need non-conventional bearings and carbon on the outside of the rotor, which increase the product cost,” says Meister at AVL. “The production processes are mostly the same,” he continues. “One difference with the high-speed concept with a permanent magnet is that The variTwist process (Image: RWTH Aachen University)
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