Welding Automotive Power Train Stators –Technologies and Challenges
Resistance and laser solutions to solve space and accessibility issues As the use of electric motors in automobile power trains has increased, stator welding has become an established manufacturing process. One interesting example is a system Amada Miyachi Europe supplied for Formula One racing cars that use electrical motors to provide a boost. Development of a robust and stable stator welding process is critical since the many connections between stator coils and the “outside world” are essential for reliable engine performance. Since the F1 manufacturer builds only about 20 cars per year, though, the stator welding system differs significantly from that of a passenger car that may number in the hundreds of thousands. Depending upon the geometry of the space, end users can choose from a variety of resistance welding and laser welding equipment options. Stator welding in electrification of the automotive power train The stator is the stationary part of an electric motor and is used primarily to keep the electrical field-aligned. The stator used in an electric motor is an electromagnet, with a coil that energizes it, when an electrical current is applied. This coil may be either iron core or aluminum. To reduce loading losses in motors, manufacturers usually use copper as the conducting material in the windings. [1] There are many types of stators, but most electric motor stator applications call for connection of insulated copper wire to electronic components outside the motor. Figure 1 shows a typical example of the stator, including the ring of coils and the housing to which electronics will be connected. (This is what will be visible in the assembly factory.) In this example, each of the coil wires is insulated using a lacquer coating. The coil wire is put into a copper fork (Figure 2) or hook (Figure 3). Using resistance welding, two electrodes pass current through the fork or hook, thereby heating up the coil wire to a temperature of 200-300°C. The lacquer burns away, making the electrical contact possible. More current is then applied to close the fork or hook around the copper wire, making the connection to a copper buss bar. Although not directly related to the main engine power train, stator welding is as important, if not more so. For example, electric pumps are used for cooling fluids and air conditioning. A typical high-end car may have dozens of small electrical motors—in automatic seats, car windows, windscreen wipers, and vents—in addition to the main motor. Traditionally, these were connected to a petrol engine by a fixed connection belt. They are now frequently driven by an electric motor. Recent trends have seen a proliferation of smaller motors used in electric and hybrid vehicles. Meeting stator welding manufacturing challenges By far the overriding manufacturing challenge for stator welding is finding a process and equipment that guarantees 100 percent process quality. Stability of the process is key. It is very easy to make a connection in a lab, but it is another thing altogether to guarantee the connection in a full production line. At the beginning of the development process, stator welding application engineers focus on the mechanics of making one good connection. The next step, always done in close cooperation with the end user, is a full development process. For example, Amada Miyachi Europe often builds laboratory test equipment with all the future production line’s primary functionality and then makes a small quantity of parts. Using this approach, experts test the equipment and develop the process in iterative steps. The next stage is building a production system. Some electric motor manufacturers have already developed their own stator welding process and are just looking for a welding module, power supply, or laser to do the job. Or they may work with a stator welding vendor that can develop a process module built into a complete production line built by an equipment system integrator. This may include components that perform required mechanical movements and measurements. Alternatively, they may purchase a fully automated process, including vision, quality check, and all other components needed for the joining process. Advantages and disadvantages of stator welding technologies for automotive power train Several technologies may be used for automotive power train stator welding, including resistance welding, which is often coupled with brazing, hot crimping, TIG or laser welding. All of which are often used with the space saving hairclip technique. The advantage of using resistance welding is that electrodes are actually in contact with the parts and compress the product during the process whilst the movement and force are precisely measured. This gives the end user improved process control and the ability to conduct quality checks. This is a big plus—the system keeps track of what is happening and users can log and store data, then go back to that data if something goes wrong. Recent advancements in resistance welding technology, particularly by replacing older AC equipment with DC inverter technology, provide even better process control. Using resistance welding with hot crimping technology efficiently produces high-quality joints that are effectively sealed from the environment, and are therefore more durable than those produced by other joining methods. Increased electrification in vehicle design has spurred a recent surge in use of the hot crimping process. Hot crimping makes it easier to join wires to one another and to the terminals with a strong mechanical connection that will withstand the stress placed on them during use. Figure 4 shows a few hot crimping stator application photos. The basic hot crimping process consists of placing a conductive metal sleeve, hook, fork, or other shaped object around the copper wire or wires that are to be joined. The sleeve or other metal object is then placed between the two electrodes of a resistance welding system. A large amount of current is passed through the sleeve, causing it to heat to approximately 500˚C. This temperature is adequate to burn away the enamel and allow gasses to escape through the metal sleeve. Once the enamel coating has burned away,
