The new wave of copper rotor applications

18/08/2014 18
By Nuno Fernando

2nd September 2014

Copper rotors are now being used in different branches of industry, especially for applications where high efficiency is required. Industrial applications include motors for high-speed machining centers, pumps, drills, compressors and fans. Transport applications include asynchronous motors for electric traction, starter-alternators, generators, and electric power-steering.

An interesting new automotive application for copper rotor technology is in the Tesla Model S, which is being marketed as an electric family car with a range approaching that of a normal petrol or diesel car. The Tesla Model S can accelerate to 100 km/hr in 4.3 seconds. At its heart is a three-phase, four-pole AC induction motor with a copper rotor. Its lithium-ion battery spans 60-85 kWh and gives the car a range of around 480 km.

But why are motor manufacturers increasingly turning to copper, and turning away from aluminium? Isn’t this strange, considering that die casting copper is more expensive than die casting aluminium, due to copper’s much higher melting temperature? What benefits do copper rotors offer that are driving rotor manufacturers to produce copper rotors on a routine basis?


A lower carbon footprint

Businesses, governments and consumers are becoming increasingly conscious of the environmental impact of their operations and technology strategy decisions. They are thus continually seeking more energy-efficient materials and processes. Driven by high global energy costs and carbon taxes, R&D departments are constantly looking for ways to reduce the energy content of materials used in products, and the products’ use of energy during their operational life.

This is where copper scores extremely highly, because the energy required to produce copper is far less than many competing materials. Both aluminium and copper materials used in motor rotor production must come from high purity virgin sources (recycled copper can be used for electrical applications but requires significant refining). The table below shows global average energy requirements and the resulting carbon footprint for copper and aluminium production. As can be seen, aluminium production requires up to 6.3 times more energy than copper and can result in nearly seven times more CO2 emissions.

  • Copper (primary) :     11,137 kWh/ton     -       8,744 kg CO2/ton
  • Aluminium (primary) : 70,044 kWh/ton     -     55,185 kg CO2/ton

Source: Deutsches Kupferinstitut e.V.


Higher efficiency motors

The second major advantage of using a die cast copper rotor is that it provides a simple solution for achieving high motor efficiency in compliance with Class IE3 and occasionally even Class IE4 energy efficiency – and without altering the size.

Thanks to the superior conductivity of copper over aluminium, motor losses can be reduced by 14-23% and rotor I2R losses by 29-41%, leading to improvement of the overall efficiency of the motor. For example, an efficiency increase of 4.3% for a 3 kW motor is possible, compared to a motor with an aluminium rotor, without any change in motor size.

In addition to the inherent efficiency improvements, this also strengthens the arguments for copper rotors mentioned in the previous paragraph – the environmental benefits – because the higher the efficiency of a motor, the less energy it consumes and the less CO2 it generates.


Same or smaller size rotors

To increase the efficiency of an aluminium rotor means increasing rotor length, which immediately impacts the amount of electrical grade lamination steel required, the number of stampings, and consequently the cost of the rotor, the shaft and the housing. And of course a larger rotor size will influence the size and cost of other motor components.

Copper enables manufacturers to achieve up to 2.5% efficiency gains in current aluminium based products without increasing the size of the motor. This means that the investment incurred by manufacturers seeking to produce higher efficiency motors will be minimized.

If a higher efficiency rotor is not needed, copper leads to another interesting possibility. Compared to an aluminium rotor motor, a copper rotor of equivalent efficiency will be significantly smaller in size. For example, a 4 kW 4-pole aluminium rotor motor with a 200 mm rotor length is 2% less efficient than the same size copper rotor motor. And the smaller the size; the sleeker the design of the motor.


Diverse design options

In other words, introducing copper into the rotor opens up a range of interesting design options and benefits:

  • Reaching the next higher level of energy efficiency, possibly with the same motor size.
  • Maximizing energy efficiency to higher and even “super-premium” levels by redesigning both the rotor with copper conductors and the stator by increasing the volume of copper winding wire.
  • Maintaining a specific energy efficiency level while reducing motor size and weight.


Further benefits of copper

Other advantages offered by copper rotors including the following:

  • Cost reduction – The energy savings afforded by the high performance of copper allows a very quick return on investment. For example, when replacing the aluminium rotor by a copper rotor on a 3 kW motor, payback is less than seven months. Proper selection of diameter and size of other components reduces the overall cost of the motor by 7%.
  • Reduced operating temperature – A copper rotor operates at 10-12°C less than an aluminium rotor. This translates into longer service life of a motor, and doubles the life of the insulation.
  • Lower noise level – Since the motor runs cooler, a smaller – and thus quieter – cooling fan can be used.
  • Higher torque – At constant speed, using a copper rotor instead of an aluminium one allows the motor to develop a nominal torque around 60-80% higher.
  • Superior momentary overload – Compared to an aluminium rotor, a copper rotor can withstand higher momentary overloads due to copper’s inherent property of lower resistance.


Copper rotors in practice

German casting company Breuckmann is an example of a rotor manufacturer keen to pursue the advantages of copper in its die-casting manufacturing processes. As a result of their extensive R&D efforts, they are continually pushing back the boundaries and have a leading position among the die cast copper rotor manufacturers of large and special size rotors.

“The maximum frame size we used to cast last year was 200. After continuous development we can now easily die-cast frame sizes of 225, and are targeting 250 and even larger,” says the Sales Director at Breuckmann. “Recently we achieved something unique in the world. We rose to the challenge of die-casting a rotor ring with an outside diameter of 400 mm and an inside diameter of 300 mm. That’s an extremely large rotor that a few years ago would have been impossible to cast in copper.”

In addition to pushing back the boundaries as regards size of rotor, they are also seeking to increase the efficiency of their copper rotors. They recently conducted tests to compare their latest die-cast copper rotors with equivalent conventional copper bar inserted welded rotors.

“We observed a 1.5% nominative efficiency increase with the die-cast rotor,” says the Sales Director. “This might appear to be a small increase, but it’s very significant because at the higher efficiency ranges – IE3 and higher – small efficiency improvements can lead to motors which fulfil the next efficiency class up. Our die-cast copper rotors are thus suitable for IE4 efficiency class.” Breuckmann’s die-cast copper rotors have now been introduced into ATB motors incorporating Danfoss inverters.