Applications of high conductivity copper and alloys for electrical engineering
15 February 2017
The starting point of any discussion on copper is its conductivity. This is no surprise, because copper has the highest conductivity of any non-precious metal – conductivity being defined as a material’s current-carrying capacity for a given diameter. Owing to this superior conductivity, copper became the international standard to which all other electrical conductors are compared.
In 1913, the International Electrotechnical Commission defined the conductivity of commercially pure copper in its International Annealed Copper Standard as 100 per cent IACS. It then became normal practice to express the conductivity of other materials in terms of IACS.
For example, pure aluminium has a value of 61 per cent IACS, or 61 per cent of the conductivity of copper. This means that the cross-sectional area of an aluminium conductor must be 56 per cent larger than copper for the same current-carrying capability. This is the key reason why copper has become first choice as a conductor for electrical applications.
Furthermore, since 1913, processing technology has improved to the point where copper conductors in electrical wire applications can exceed a conductivity of 100% per cent IACS, even routinely reaching 102 per cent IACS.
Any additional percentage in IACS conductivity is invaluable for today’s electricity systems, since conductors typically remain in use for decades, and higher IACS values mean lower energy losses.
High-conductivity copper is an excellent material for electrical engineering applications for other reasons, such as a unique combination of strength and ductility. Usually, the stronger a metal, the less pliable it is. Not so with copper. Copper wires can be bent, twisted, tightened and pulled without stretching or breaking them.
So, when a copper wire is pulled through a conduit, it resists stretching, neck-down or breakage. It can be stripped and terminated during installation or service with far less danger of nicks or breaks. Moreover, copper’s inherent hardness and strength lead to high fatigue resistance.
|Material||Fatigue Strength Nmm²||No of Cycles x 10⁶|
|High conductivity aluminium||Annealed||20||50|
Table 1: Comparison of fatigue properties of high conductivity copper and aluminium
Copper resists creep until it reaches a temperature of 150°C, which is well above the usual operating temperature. Corrosion is not an issue for copper. It is resistant to most organic chemicals and can operate indefinitely in most industrial environments. Copper is also one of the easiest metals to solder, and thus is ideal for many applications where good joint integrity is essential.
High conductivity copper and energy transition
With sustainable-energy systems being increasingly installed throughout the world, copper’s use will continue to expand. This is because in renewable energy systems such as wind turbines, solar panels and electric vehicles, the properties already mentioned enable high conductivity copper to connect these systems to the grid and keep them running reliably.
In addition, another property of copper makes it the ideal partner for renewable energy systems: its recyclability. There is no difference between ‘primary’ copper produced from mining and ‘secondary’ copper produced from recycling so copper can be used and re-used again and again.
The recycling rate of copper, on the other hand, is higher than that of any other metal. Approximately nine million tonnes of copper are recycled every year; 40 per cent of the world’s demand is met by recycled copper. This nine million tonnes includes all copper. The use of recycled copper for electrical applications is limited to components such as busbars and cables.
Furthermore, according to the International Energy Agency (IEA), half of the actions needed to meet global climate change goals can be met through energy efficiency. As the best electrical conductor, the products that contain copper tend to operate more efficiently.
High conductivity copper is the best choice for bulk electrical conductors, such as cables, motor windings and busbars. In addition, there are many electrical accessories—including terminations, connectors, contactors and circuit breakers—where other material properties are equally or more important.
For these applications, a very wide range of copper alloys is available with, for example, enhanced strength, resistance to stress relaxation or creep, while retaining excellent conductivity.
Returning to sustainable energy systems, some figures clearly demonstrate the importance of high conductivity copper in these applications. A typical PV solar power system can contain around 5.5 tonnes of copper per MW, while a grid energy storage installation can need between 3 and 4 tonnes of copper per MW.
Electric vehicles use two-to-four times more copper than internal combustion engine vehicles. A mid-sized electric car contains about 1,500 copper wires, totaling 1.6 km and weighing around 90 kg. An electric bus might contain up to 1,000 kg of copper. The use of copper wiring, tubing, busbar, cable, bushings, bearings and myriad electrical and mechanical parts keeps these systems operating longer and at higher efficiencies.
Alloying of copper
The most common high conductivity copper for conducting electricity via wire, cables and busbars is Cu-ETP (Electrolytic-Tough-Pitch). It has a minimum conductivity rating of 100 per cent IACS, although most Cu-ETP sold today meets or exceeds the 101 per cent IACS specification. It has an oxygen content of 0.02 per cent to 0.04 per cent.
Small amounts of alloying elements are often added to copper to improve strength, hardness, machinability and heat resistance characteristics. As mechanical properties are enhanced by the addition of these alloying elements there is a trade off with a reduction in electrical conductivity. Here are some examples of common copper alloys:
- Small additions of alloying elements such as tin, magnesium, chromium, iron and zirconium increase the strength of copper at the expense of conductivity. Applications include overhead grooved contact wires for trams and railways and high-duty power cables;
- The addition of tin, silver, chromium, zirconium and iron can increase the softening resistance of copper for applications such as electric motors, generators, power cables and welding electrodes that run at high temperatures, whilst requiring excellent conductivity;
- The addition of small amounts of beryllium, nickel and silicon give heat treatable alloys of very high strength with good conductivity. Applications include contact springs, switchgear and stressed automobile components. These alloys also have the highest fatigue strength; and,
- If tellurium or sulphur is added to copper, the resulting alloys provide free machining properties needed for high precision CNC machining of components such as semiconductor mounts, vacuum interrupters, plasma nozzles and resistance welding tips.
Figure 1: Effect of various elements (impurities or intentional additions) on the conductivity of copper
‘High Conductivity Copper for Electrical Engineering’
Produced by the Copper Development Association and primarily, but not solely aimed at electrical engineers, ‘High Conductivity Copper for Electrical Engineering’ gives an overview of high conductivity coppers, their properties and applications.
“We recently updated this publication as it is established as the ‘go to’ reference book on this topic for electrical engineers,” said Angela Vessey, director of Copper Development Association. “It helps users to understand metallurgical properties and processing requirements of copper-based conductivity materials and covers a wealth of information in a well-structured and easily readable format.”
After a brief introduction, Section 2 covers the mechanical and physical properties of copper, and the effect of impurities and minor alloying additions on copper’s conductivity. It describes the various types of high conductivity copper in existence today, and looks at production methods with chapters on cathode copper and refinery shapes.
Section 3 describes the very wide variety of possibilities that exist for single and multiple additions of elements to attain properties suitable for different applications. It divides copper alloys into non heat-treatable or heat-treatable alloys.
The development of more advanced microchips has required the production of copper alloys as lead frame materials with properties that provide long reliable life at elevated temperatures. This is the focus of Section 4. It describes copper alloys for semiconductor lead frames, and also discusses the topics of oxidation and corrosion.
‘High Conductivity Copper for Electrical Engineering’ can be downloaded in full from Copper Development Association’s resource library: www.copperalliance.org.uk/hcc.
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