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Electric Motor Hairpin Welding: Comparing 3 Joining Technologies

Copper hairpin

Top of mind for today’s electric motor manufacturers is to build smaller, lighter, more powerful motors with ever-improved electro-conductivity. One key to achieving that goal is optimizing the amount of copper used in the stator core slots. And to do that, manufacturers are replacing the older types of heavy, round wire motor windings with “hairpins:” solid, preformed rectangular bars that are intertwined and locked into place. These formed copper bars (named for their resemblance to hairpins) improve stator cavity fill yet produce a smaller end-product that weighs less, withstands greater thermal stress, exhibits improved torque, higher power density, and produces less heat.

Now, the ends of these hairpins need to be joined to provide an electrical path. Successful hairpin welding can be achieved using a variety of technologies such as laser welding, micro TIG welding, or resistance brazing. Each method comes with its own set of manufacturing advantages and challenges. Here’s our unbiased take on each.

Steps for successfully assembling hairpin motors

First, let’s take a look at stator assembly, which is a multi-step process as shown in the following figure:

Hairpin electric motor stator assembly

The copper bar is typically coated with a polymer to protect it from oxidation. This must be removed. This can be done mechanically or by means of laser cleaning (see related blog on Laser Cleaning).

After stripping or ablating the polymer coating, the copper bars are bent and the ends are twisted placing them in close proximity for welding. The goal in hairpin welding itself is to achieve a high-quality joint with minimal heat input and no spatter. There are several challenges to achieving a high quality weld including hairpin alignment and delivering enough energy to melt the copper material, but not so much as to overheat the copper material and damage the coating lower down on the hairpin. That has to be balanced with putting the energy in too fast creating spatter.

Which Welding Method?

To decide whether to use laser welding, micro TIG welding, or resistance brazing for a particular application, manufacturers must consider:

  • Hairpin alignment
  • Fixture/tooling
  • Part access
  • Cycle time
  • Quality of the hairpin end trimming

Here is a brief overview of what each option has to offer.

The chart below summarizes how each welding technology measures up to the key hairpin welding considerations.

 

Laser welding

Alignment and tooling are the two primary difficulties encountered when laser welding hairpins. Custom tooling can resolve some alignment conditions, but may make joint access more difficult. Vision systems can be used to help identify the location of the weld joint and ensure the hairpins are aligned and within acceptable positional tolerance for processing. Vision systems can also be used to adjust the laser’s power, beam path and speed. Any parts that fall outside the acceptable welding tolerances can be flagged for repair. Because the laser interacts with the top surface layer of the part first, excessive variations in the end trim (burr) play a role in how the laser couples with the material and can cause spatter, resulting in inconsistent welds and potential short circuits.

Micro TIG welding

As with laser welding, successful micro TIG welding relies on well aligned parts with a negligible gap. While part fit up for this process may be more forgiving than laser welding, it is less forgiving than resistance brazing. Accessibility to the weld joint is from the top of the hairpins, making it a good process to consider when the spacing doesn’t allow for resistance brazing, pincer type heads. Because the micro TIG strikes an arc between the electrode and the top surface first, excessive variations in the end trim (burr) play a role in where the arc interacts with the material. This can cause spatter and the potential for inconsistent welds.

Resistance brazing

Resistance brazing can be an ideal solution for hairpin connections because the pincer action draws the parts together, correcting for many gap/alignment issues. However, it necessitates access to both sides of the hairpins. Proper tooling is required to insure that there is no X-Y-Z shift. As motors become smaller and more compact, the ability to access may become increasingly difficult. Creative tooling and the use of multiple heads can improve the overall cycle time for the resistance brazing process. The quality of the end trimming does not affect the brazing process results.

Summary

Successful electric motor stator manufacturing results from successful welding of the copper hairpins. Laser welding, micro TIG welding and resistance brazing are all viable technologies; the one selected is dependent on many factors including end-trim quality, alignment and access as well as manufacturing considerations, such as throughput and safety.