Rising Copper Demand in an Evolving Electric Traction Motor Industry
Mar 26, 2020 Luke Gear, Dr Richard Collins
Commercial electric traction motors predominantly use copper due to its usefulness as magnet wire, which (at minimum) is used for the electromagnets on the stator. While several motor technologies have been adopted, each has a slightly different method of producing torque, and the material makeup of the rotor can vary from no magnets, to no copper, to neither magnets nor copper. The following article outlines which electric traction motor technologies are winning in electric cars, with a view to how this will impact overall copper demand by 2030.
Permanent Magnet Synchronous Motors (PMSMs) employ permanent magnets on the rotor to interact with the stator field and generate a torque. The advantages are high efficiency (< 95%), power density, torque density, compactness, and that they are relatively easy to control. Their Achilles heel is the reliance on Neodymium permanent magnets, which, due to high localisation, are vulnerable to supply shortages and price hikes, similar to the volatility seen from cobalt in batteries. While prices are currently at an acceptable level for PMSM adoption, they are rising steadily in real terms. There is roughly 5kg of copper present in the PMSM of a typical pure electric passenger car, from its stator.
In contrast, the AC Induction Motor (ACIM) has a rotor with loops of copper wiring wound in silicon-steel ('slip-ring') or copper bars with end rings ('squirrel cage'). The rotating field from the stator induces a current and electromagnet in the closed loops of copper, which interacts back with the stator field to produce a torque. Because the rotor is 'dragged' by the stator, but never quite catches it, the rotor and stator move at different speeds and the system is asynchronous.
The obvious advantage to ACIMs are no permanent magnets, but they are also relatively simple to construct and scale (no small permanent magnets to glue on and assemble), are reliable, robust and can 'overvoltage' for things like ludicrous mode (no risk of denaturing any magnets). The disadvantages are lower efficiency < 93% (less range) and power density than PMSMs. The presence of copper on both the stator and rotor means copper intensity is higher, at an average of around 10 - 15kg for a pure electric passenger car.
The third traction motor becoming important for electric cars is Permanent Magnet Switched Reluctance motors (PMSRs). On their own, Switched Reluctance Motors (SRMs) are fundamentally different to other motor types since there is no magnetic field on the rotor; the rotor is simply laminated silicon-steel and torque is generated by making the rotor constantly move to minimise its reluctance ('magnetic resistance'). Although very robust and simple to manufacture, pure SRMs are not used in cars because they are noisy, less efficient, experience torque ripple and are relatively difficult to control. However, a hybrid of permanent magnets and switched reluctance motors can reduce the required magnetic material and improve efficiency (hence 'PMSR'). Indeed, Tesla's Model 3 is reported to incorporate PMSRs with < 97% efficiency; BMW is also a notable adopter of this motor type. Like the PMSM, there is around 5kg of copper in a PMSR for a pure electric passenger car from the stator.
So far, automakers have tended to converge (or are converging) on permanent magnet motors (PMSM or PMSR) for their efficiency and power density, at the expense of ACIMs (shown below). But this could change quickly if the price of permanent magnets jumps too high. Most electric vehicles use one electric traction motor, but there is also a trend towards dual (Tesla, BYD), or even quad motors (Rivian) per vehicle. This brings efficiency, overall power output and redundancy improvements, with the overall result that the market for electric traction motors grows faster than the electric vehicle market.
As part of research commissioned by the International Copper Association (ICA) IDTechEx has found that these trends will culminate in more than 250,000 tonnes per annum of copper demand from electric traction motors of on-road electric vehicles by 2030. This figure incorporates the traction motors of pure electric, plug-in hybrid and strong hybrid passenger cars, buses, vans, trucks, two and three wheelers.
Logos from Wikipedia. Source: IDTechEx