Hot Crimping Magnet Wire to Cable Shoes or Terminals
Hot crimping of cable shoes is a metal joining technology that uses electrical current and mechanical pressure to connect electrical conductive wires. Hot crimping is a type of resistance welding. Hot crimping differs from other types of resistance welding because it uses a conductive metal sleeve to pass the current, generate the heat and hold the wires together. This sleeve fuses the wires and makes one compacted metal part. The sleeve can be a cable shoe, a terminal or a simple round sleeve.
The joint is formed using a process that is called diffusion welding. Diffusion welding means the copper is joined because the copper wires and the copper connector fuse together at near-to melt temperature. The material stays in solid phase and never enters the liquid phase.
Similar to resistance welding, hot crimping relies on heat generated by the electrical resistance of the material being welded and the force used to hold the materials together during welding. It produces electrical connections with a very high conductivity, nearly zero contact resistance and free of any significant voltage drop. The hot-crimped joint has very high tensile strength and will not weaken over time due to vibration or temperature, as it is one solid piece of metal. Due to the heating the metal anneals and mechanical stress in the joint is relaxed, thus minimizing stress on the copper in the wire and terminal after the joining process.
In many cases, these magnet wires are electrical isolated. The insulation can be enamel or a polymer film like polyurethane, polyamide, polyester or polyimide. Due to the intense heat, hot crimping vaporizes the insulation during the crimping. By eliminating the production step of stripping the isolation, the productivity of making the electrical connections is increased. As there is no stripping, no metal is removed, so maximum strength is guaranteed.
Examples of Hot Crimping Applications
Hot Crimping is being used by major European electric-motor manufacturers such as Bosch, Siemens and Volkswagen since 2011, making it a relatively new, but very rewarding technology. For decades, these companies have used machines or chemicals to strip the enamel coating off the ends of magnet wires before crimping them to copper ring terminals or sleeves. As stated before, in contrast this, hot crimping vaporizes the insulation during the crimping of the wire and terminal or sleeve. By eliminating this stripping step, hot crimping enables these manufacturers to increase productivity.
Magnet wire is an insulated copper (or aluminium) electrical conductor used in electromagnetic equipment like motors and transformers. The wire is wound in coils to generate an electromagnetic field. A copper terminal, which can be coated with tin, nickel or silver, is attached to the free end of the wire so it can be connected to a source of electrical power. Hot crimping can join standard magnet wires, high-frequency magnet wire (for frequencies above 10 kilohertz) and uninsulated copper wires. Hot crimping can weld individual magnet wires as thin as 30 AWG (0,05 mm2) or a wire bundle as thick as 400 square millimetres (stranded or braided).
Most magnet wire is made of fully annealed, electrolytic refined copper. Aluminium magnet wire is less common. This is because it has a lower electrical conductivity, and its cross-sectional area must be 1.6 times wider than copper wire to achieve comparable DC resistance.
Smaller-diameter magnet wire (20 to 36 AWG) tends to have a round cross-section. Thicker wire is often rectangular (with rounded corners) or square to more maximize use of available winding space. These wires are also referred to as "hairclips" referring to their bending form in electrical motors.
Hot Crimping has seen a large growth in the automotive market due to the electrification of the powertrain. High current connections from the battery to the inverter, from the inverter to the electrical motor and inside these components can all use Hot Crimping as a joining technology. The low electrical resistance will lead to lower temperatures for the parts in operation, increased reliability and higher electrical efficiency of the parts, compared to conventional joining technologies.
Another growth market for Hot Crimping is the renewable energy, where often high electrical currents are generated, for example in windmills, water turbines, PV installations, etc.
Improvement over Conventional Cold Crimping of Cable Shoes
Normal “cold” crimping makes a connection to the terminal or cable shoe by only using a mechanical clamping force. This can cause a deterioration of the contact to stress relaxation. “Cold” crimping can also suffer from cavity corrosion. Both problems are non-existent with a good-quality hot crimping.
Equipment & Operation
With a benchtop welder, the worker places the coil wound with magnet wire into the machine, inserts open wire ends in the terminal and activates the weld cycle. Within milliseconds, the welding heads release enough current to produce a temperature of about 700°C at the terminal. This temperature is hot enough to vaporize the insulation and weld the wire ends to each other and the terminal. A MIYACHI EAPRO fume extractor exhausts the vapours.
Once welding is complete, a two-stage cooling down of the terminal begins. Within a few seconds the terminal cools down to about 100 °C. After a few more seconds, it cools to 50° C and can be manually removed from the machine.
Semi-automated systems have a small pallet that moves the component into and out of the hot-crimping machine. This can be inline, or with a rotary table. The worker inserts the wire ends into the terminal, activates the machine and removes the component from the pallet after it cools.
All welding data is stored in the machine and can be easily accessed. Manufacturers can use this data to quickly verify or change welding parameters, or for product traceability.