Efficient production processes are crucial to the success of electromobility. While development progress on the battery is in the public spotlight, progress on electric motors is taking place more quietly.
Welding the fine hairpins with the laser requires extreme precision.
Trumpf
In the production of the component that is crucial for power and efficiency in the electric motor, the stator, hairpin technology with its plug-in coil structure made of solid copper wires is increasingly replacing conventional wire winding technology, reports Achim Kampker, who heads the Production Engineering of E-Mobility Components (PEM) chair at RWTH Aachen University. In this design, the motor's windings are realized using wire hangers with a rectangular cross-section, which are inserted into the stator by a robot and welded with a laser beam. "The wire hangers are reminiscent of a hairpin, hence the name hairpin technology," explains Gerhard Babiel in the german book chapter Drive Systems.
In contrast to the traditional structure of an electric motor, hairpin technology involves inserting solid, bend-resistant copper wires (usually copper flat wires) into the grooves of the stator laminated core, with the laminated core consisting of a number of layers of electrical sheet insulated from each other, each with a thickness of around 0.18 to 0.5 mm, as in conventional stators. "Hairpin assembly represents a counter design to classic winding technologies," state the authors of the german book chapter Development of Electric Vehicle Specific Systems. According to PEM, the advantages of hairpin technology, which originated in generator construction, over classic winding methods such as needle or flyer winding include in particular
- their suitability for the stator winding of both synchronous and asynchronous motors,
- a mechanical groove fill factor of up to 90 % (electrical: up to 70 %) and
- the automatic production of high volumes, which in the automotive industry range between 150,000 and 200,000 units per year.
Rising raw material prices and the need to minimize installation space are behind motor designers' efforts to keep the geometric dimensions of a coil as small as possible. Important factors here are the filling shape and the filling factor of a winding, where the mechanical filling factor describes the ratio of the total cross section of the windings to the cross section of the available winding space. For the electrical fill factor, this value is further reduced by the area of the insulation system, i.e. the coating on the wire. Since round wire winding always encloses spaces that cannot be filled, the fill factor is always less than one. For comparison: "Conventional round wire windings achieve about 40 percent," writes Springer author Gerhard Babiel.
Different Manufacturing Processes for Hairpin Stators
In their book article Comparison of Various Manufacturing Processes for Hairpin-Stators with Different Conductor Material, authors Andreas Riedel, M. Kneidl, J. Seefried, A. Kuehl and J. Franke provide an overview of various manufacturing processes for hairpin stators with different conductor materials and point out that increasing the electrical copper fill factor in particular improves the power density of the electrical machine. However, large conductor diameters lead to additional losses due to eddy current effects at high frequencies, such as those encountered in high-speed applications. According to the authors, these disadvantages can be avoided by replacing the solid rectangular conductor with a stranded wire. However, they say, this semi-finished product requires a more complex manufacturing process. In their book article, they present the process chain for manufacturing a hairpin stator from stranded wire and discuss the challenges in the process chain of a stranded conductor compared to a solid conductor.
In the "anfaHair" research project funded by the German Federal Ministry of Economics and Climate Protection (BMWK), scientists from the PEM at RWTH Aachen University investigated how the production of novel electric motors can be made more efficient on an industrial scale. To this end, the team worked with the Berlin-based mechanical engineering company Röscher to develop four potentially marketable joining technologies for the hairpin stator manufacturing process. "We have extensively tested the new joining processes and concretely demonstrated their applicability in hairpin stator production," reports Achim Kampker, and "in the process we have identified technologically and economically viable use cases for all processes." The PEM scientists have now summarized their findings on electric motor design in production guidelines.
Dual Optics Make E-Motor Manufacturing More Productive
In production, contacting the numerous hairpins with a laser is currently the method of choice. However, the Aachen scientists have discovered that the series production of these electric motors still has high reject rates. The problem: Welding the fine hairpins is very demanding. It is possible that the manufacturer Trumpf has now found a solution for improving quality with its dual optics (dual PFO) presented at the "Laser - World of Photonics" trade show in Munich at the end of June. The company's programmable focusing optics (PFO) are scanner optics for welding and cutting. Using two mirrors, the laser beam can be positioned at any predetermined position within the processing field or space or guided on any seam geometry without the workpiece or focusing optics moving during the welding process, according to the company. The Dual PFO doubles the working field compared to a normal PFO, making it particularly suitable for processing large components such as electric motors.
To achieve high manufacturing quality, a PFO always measures the position of the component first before the welding process begins. Unlike a normal PFO, the Dual PFO measures and welds in parallel. "Users save a lot of time and money as a result," says Matthias Beranek, automotive industry manager at Trumpf. He adds that the Dual Optics can also be combined with a new image processing system based on artificial intelligence. This further increases the productivity of the Dual PFO, he adds. "In this way," says Baranek, "cycle times for automated welding of hairpins for electric motors can be reduced by up to 30 percent."
