Laser technology provides a high level of flexibility and quality when processing hairpins for the electric motors. Article by TRUMPF.
Nowadays, manufacturers of electric motors are also looking for more productive processes for volume quantities, with the same high-quality requirements when it comes to the welding result. An important impetus here is achieving a higher degree of automation to be able to produce higher quantities.
One method in the construction of stators has therefore largely prevailed: Instead of winding copper wire around the individual stator grooves, as was often the case before, manufacturers have started embedding rectangular copper rods—called “hairpins” due to their shape—into the entire groove using compressed air. The typical edge lengths of the rectangular cross section of the hairpins is between 2 and 4 mm. The process achieves higher process speeds and can easily be automated. As the hairpins are stiffer than round wires, their alignment in the motor can be better controlled. The larger fill factor also results in a higher thermal load capacity, and higher motor power.
The copper rods are coated with an insulating layer, which requires ablation at both ends locally—called hairpin stripping—to enable contacting. Pulsed laser processing is suitable here to strip the hairpins. Compared to mechanical processes, such as planing and milling, laser processing is up to 80 percent more productive. Once the hairpins have been embedded in the grooves, protruding ends on the top and bottom of the stator are twisted together using a fixture (necking) or fixed in place, and then welded for contacting. The ends are not always ideally aligned to each other, however. If you use automated remote welding, a camera-based sensor system integrated in the laser optics helps achieve a reliable and reproducible result, and therefore the highest possible current flow.
Stripping of Hairpins
Common insulating layers for copper hairpins are polyamide-imides (PAI), polyether ether ketone (PEEK), and polyamide-imides with polyimide foil (PAI+FEP).
In the past, PAI coatings were almost exclusively prevalent in the industry, but we are now seeing a tendency towards a steady increase in PEEK and PAI+FEP. However, PAI coatings still have, and are likely to have in the future, the largest share by far.
All of these insulation coatings can be burnt away from the copper quickly and in a targeted manner using laser pulse processing. The laser light couples into the insulating layer, heats it up, and burns it off. PEEK behaves as a volume absorber for laser light, anyway; for PAI and PAI+FEP, it is recommended that the first run over is used to carbonize the material in order to increase the absorption. The copper discolours due to the heat influence during laser ablation. This is not relevant for the further processing, however, as the structure of the copper is not changed. Burrs also form at the boundaries to the coated copper, which, in unfavourable circumstances, could lead to the burr becoming stuck on a surrounding component or fixture. The formation of burrs and edges can be optimized, however, through reworking using another femtosecond-pulsed laser.
Hairpin stripping can be done with different types of TRUMPF ns laser. Most common is the TruMicro 7000 series with a laser power of up to 2 kW (pulse energy 100 mJ) at a pulse repetition rate of 5 to 250 kHz and a pulse duration of 30 ns.
There are two stripping processes for the hairpins depending on the coating type:
- For coatings which are transparent to the laser a “blast off” process can be used.
- For non-transparent coatings an evaporation process is needed.
Click here to read the full version of the article in the April 2020 issue of Asia Pacific Metalworking Equipment News.
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