In order to obtain the best possible performance from the motor windings, lead partner, COREteQ specified the use of Litz cable, a braid of 56 insulated copper strands woven into a rectangular cross-section.
TWI was tasked with finding a solution for joining the cable to the motor winding. Each winding had to be capable of carrying a 90A steady state current, with peaks to 100A. To achieve this without excessive self-heating, a low-resistance connecting joint to every wire was essential. The joints between the cable and the motor also needed to have high mechanical and electrical stability in order to withstand operating temperatures in excess of 110°C for long periods of time.
The specification required four steps to complete the process; stripping of the insulation to reveal the conductor, consolidating the wires to enable handling and joint formation, addition of a terminal tag and, finally, the use of a high integrity joining process.
TWI investigated indirect resistance welding and the commercial processes of ‘hot crimping’ and metal-flash welding. These were found to be unable to meet the specification and a new process was required.
This new process required the following attributes:
- Have the advantages of resistance welding in providing a fast energy pulse to limit thermal damage down the cable
- Use readily available low cost resistance welding equipment
- Make a joint to the cable surface without full stripping of the cable insulation
- Make a reliable low resistance joint to the cut end of the cable to connect every wire
- Not use low melting-point solders
- Use limited indentation deformation, so that the conductor cross section was maintained throughout the joint
- Controllable so that thinner walled tags could be used
- Take advantage of the metal-flash action and compatible with plated tags, which may be used in future
- Did not require fluxes.
Since traditional brazing methods require the use of a flux, which can lead to corrosion over time, it was decided that the connection would be made using no flux which would prevent corrosion and provide a longer life expectancy. This was achieved by applying heat using electrical resistance welding power supplies. In addition, this method allowed the connection of all 56 wires in the cable, whereas traditional binding methods could leave the wires at the core of the cable without proper connection and thereby reduce efficiency.
Initially, a fluxless reactive braze powder was tested as an insert between the cable end and the copper termination. However, the powder was found to be unnecessarily reactive with air enclosed in the terminal / cable during the process. Therefore, a lower cost, lower reactivity braze foil insert was found to be a good solution, given it was reactive enough and provided more metal than the powder to join the cable end to the terminal.
Ultimately, TWI was able to provide a solution for the cabling, which could be used in a variety of applications outside of that originally intended for this project.
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