The electric cars should charge safely and quickly, and the electrical installations should be light and compact. Busbars offer these advantages over conventional cables, and electromagnetic pulse technology (EMPT) also provides a cost-effective manufacturing method suitable for large-scale production.
The driver of an e-mobile not only demands an acceptable range from his vehicle, but it should also be quick to charge. This is why car manufacturers want to work with 1000 A wherever possible. The cables heat up in the process and are also heavy and bulky. Busbars offer great advantages here.
Unlike a cable, a busbar always has its tested short-circuit resistance. Changing the direction of cables sometimes requires large bending radii. Conductor rails, on the other hand, can be bent with small radii. In addition, cables must be laid at a distance because of the possible waste heat. This is not necessary with busbars and their use significantly reduces the space required. And very important: conductor rails do not burn. In addition, the installation of busbars only takes around a third of the time of a cable installation, and in the case of a power distribution system it is even 70 percent faster, as the rigid busbars are easier to install automatically than the flexible cables.
Busbars also offer long-term reliability in harsh environments and can withstand operating temperatures from -40 to +125 °C. They can dissipate heat and thus help to prevent overheating. Thanks to lower inductance and higher capacity compared to cables, they make charging more efficient.
Joining aluminum and copper efficiently with EMPT
The most suitable materials for busbars are aluminum and copper. For price reasons, they want to make the majority of them from inexpensive aluminum, but expensive copper is needed for the contacts. However, when the two metals and condensation water come together, a violent electrochemical reaction starts: the more noble copper decomposes the less noble aluminum, contact resistance and temperature rise and, in the worst case, a fire occurs. Furthermore, the two metals are not easy to join.
“This is where EMPT technology comes into its own. It works quickly, can be easily automated and is safe. The weld seams produced are very stable and helium-tight, which means that no corrosive medium can penetrate. In addition, the restrictions regarding the geometries of the parts to be joined are less stringent than with other processes,” summarizes Dr. Ralph Schäfer, Head of Research and Development at PSTproducts.
The most important components of such an EMPT system are the pulse generator, control cabinet and, depending on the application, flat coils or field formers. A pulse generator consisting of capacitors connected in parallel supplies the currents required to generate the magnetic field. If the high-current switch between the coil and capacitors is closed, the current flows in pulses into the coil. Pulsed currents in the range of a few 100 kA to over 1000 kA can be generated. Coils and field formers are used to direct magnetic pressure onto electrically conductive workpieces. The coil consists of one or more electrical windings and is usually made of a high-strength copper or aluminum alloy. The coil cross-section is usually between 10 and several 100 mm2. The required mains connection power, on the other hand, is limited to 380V/64A due to the 3 to 8 seconds it takes to charge the capacitors, even with powerful systems. The power consumption is between 0.015 and 0.03 kWh per pulse, depending on the size of the system.
EMPT welding is based on an electromagnetic pulse, shorter than 100 µs. The pulsed current has a very high amplitude and frequency, typically several 100 kA and discharge frequencies between 10 and 50 kHz, and thus creates a strong magnetic field that generates an eddy current in one of the workpieces. The two workpieces are positioned overlapping, with an acceleration gap between them. The coil accelerates one of the two workpieces onto the joining partner. When this workpiece area strikes its contact partner, repulsive Lorentz forces and a high magnetic pressure are generated, which exceeds the yield strength of the material and causes the workpiece to strike a stationary joining partner at up to 500 m/s. This creates extremely high mechanical stresses and strains in the collision area.
The maximum stress occurs at the point of contact and creates a kind of bow wave in front of the joining area of the two workpieces. This plastic deformation breaks up the surface oxide layers of both contact partners and leaves behind a wavy structure. The air gap between the workpieces is compressed, blowing dirt and chipped oxide particles out of the joining area.
The two surfaces are pressed together under enormous pressure so that the atoms of the joining partners form a metallic bond. As the melting point of the joining partners is nowhere near reached, metals with different melting points can also be joined without distortion. The joining zone generally has a higher strength than the weaker base material. EMPT welding therefore works without increasing the temperature and therefore without changing the microstructure, i.e. without a weakening heat-affected zone.
Another major advantage is that the contact resistance is not increased when EMPT welding aluminum and copper and the good conductivity of the two metal partners is maintained across the joint. In addition, the joint has significantly better final resistance values than resistance or laser welding according to the standard test procedures, in particular thermal shock and vibration.
EMPT welding delivers high-quality weld seams without the use of shielding gases or filler metals. Pulse generators and coils from PSTproducts have now been optimized to such an extent that service lives of over two million pulses can be easily achieved before capacitors or parts of the coil have to be replaced.
EMPT welding has been used by international customers for many years for the production of components in large series quantities. A system typically performs 1 to 5 million welds per year. The process is also environmentally friendly, as neither smoke nor radiation are produced. The magnetic field and thus the welding parameters can be controlled very precisely, thus delivering constant, documentable quality.
With an EMPT system, up to ten busbar connections can be made per pulse with cycle times from 5 s. This makes this welding process commercially cost-efficient. This enables mass production in the tens of millions per month, with a material thickness of up to 4 mm. Material thicknesses of up to 8 mm are currently technologically possible.
EMPT welding suitable for large-scale production
“PSTproducts has extended the life expectancy of pulse generators and coils and increased the maintenance intervals to between 500,000 and 2,000,000 pulses by selecting suitable materials and optimizing the system technology. The joining costs have thus been reduced to just a few cents. The availability of EMPT systems meets today’s industrial requirements with 100% process control and proven use in fully automated production lines,” reports Ralph Schäfer.
Such busbars are an issue wherever large currents are transmitted, i.e. not only in vehicles, but also in wind power and electric ship propulsion systems.
PSTproducts offers prospective customers a detailed analysis, prototype and initial sample production for every application, as well as pre-series if the customer so wishes. The team advises on system construction, and later on the conversion, upgrading and/or modernization of existing systems.
Busbars will replace some of the wiring in e-mobility in the foreseeable future, making it safer. And they also offer great advantages for photovoltaic and wind power systems. And EMPT welding is a reliable process that can also join future materials, with just one restriction: the materials must be electrically conductive.
Author:
Dr. Barbara Stumpp, specialist journalist from Freiburg
Web:
www.pstproducts.com