Feasibility of Ferrite Magnet Replacements in Neodymium EV Motors

Fig. 1: Neodymium metals, pictured, produce the strongest permanent magnets, with fields of 1.3 Tesla in most cases. [13] (Source: Wikimedia Commons.)

Rare Earth Elements (REEs), are a group of 17 lanthanides prized for their magnetic properties. Outside numerous industrial and consumer applications, rare earths are crucial to the electric motors of electric vehicles and wind turbines. Neodymium-Iron-Boron (NdFeB) magnets are most commonly used in these two applications due to neodymium's high remanence (Fig. 1). The magnets produce torque, decreasing the amount of electrical power required to provide rotational movement.

Given its cruciality, securing rare earth magnets has become an enormous resource security concern for the US in recent years. While REEs are far more abundant than their collective name implies, they are found in low concentrations, and thus require heavy refining to produce usable ores. The minerals bastnsite (RCO₃F, where R is a mixture of REEs) and monazite (XPO₄, where X is a mixture of REEs) are most often mined, and both require intense organic solvents, strong sulphuric or nitric acids, and extremely high temperatures (620°C or greater, depending on the precise method). [1] Because this refinement process is both energy-intensive and pollutive, China has become REE refinement specialists as high costs (and high EPA standards) pushed production out of the US. Now, China refines between 85-90% of the world's REEs, a figure that is unrivalled in market concentration by any other of the energy metals. [2]

China's command over the REE industry has significant impact on US procurement. In the past, China has halved REE exports to the US over political disputes and COVID-19 lockdowns, each time sending REE prices skyrocketing. It has even completely halted REE exports to Japan a number of times, due to South China Sea island disputes. [3] To hedge against price shocks, major industrial companies have been incentivised to study REE substitutes. Consensus among industry reporters and academics is that ferrite (iron-based) magnets are the likeliest replacement because it has similar price-to-performance ratios as some REE magnets, and because it's a proven technology: they were used in General Motors' second-generation Voltec powertrains, for instance. [4,5] Using Tesla's motor as a small case study, I'd like to consider the feasibility of such a switch to ferrite magnets, focusing on the weight penalty that aforementioned industry reporters emphasise.

As mentioned, the most common REE magnet used in EVs are made from an alloy of Nd₂Fe₁₄B (neodymium, iron, and boron) Nd, Fe, and B are melted together, cooled into a mass, pulverised, sintered, and finally magnetised. [6] Central to the engineering task of substituting REEs is that ferrite magnets have lower magnetic flux than NdFeB magnets. Compared to NdFeBs 1.4 T, ferrite magnets typically have a magnetic flux density of 0.4 T. [7] To achieve the same output in electrical power, manufacturers will have to use a larger ferrite magnet.

One paper with a reliable simulation method designed a ferrite flux switching generator, a form that could be used in EVs. Using the same input mechanical power, the authors found that their ferrite motor required a 2.4x increase in mass to reach the equivalent electrical power output from an REE counterpart. This multiple takes into account the larger, heavier magnet itself, as well as the mass of other components, like improved rotor cores and additional copper wiring. [8]

I'll apply this weight change to the motor in the Tesla Model S. I chose this model for no other reason than because there was a greater number of reliable sources with the model's subsystem specifications. The Model S weighs 2068.835 kg, which includes a front and rear motor, each originally weighing 90 kg with REE magnets. [9] Because it's dual-motored, I'm applying the 2.4x weight multiple twice, resulting in a ~12.2% increase in total vehicle weight with ferrite magnets:

The Model S also carries a 75 kWh battery, which the EPA rated to have a range of 475 km (295 miles) for the car. [10] It has an estimated aerodynamic drag coefficient (C_d ) of 0.24, and both cars would be travelling at the same velocity. [9] Since all of these variables are kept constant between the two cars, we don't need to use numeric values as the loss in total range that I'm looking for should be proportional to the change in weight.

The range can be estimated using the following formula:

Assuming that rolling resistance remains constant and is not influenced by weight, this equation now focuses on the impact of weight on aerodynamic drag. The aerodynamic drag force (F) can be expressed as [11]

where

Since the velocity and other factors remain constant, they can be disregarded so that aerodynamic drag is directly proportional to weight. Thus, if rolling resistance is constant, total energy consumption is also directly proportional to weight, making the EV range inversely proportional to it:

There are a few other variables we can consider. The larger motor size may change the shape of the car, and thus the aerodynamic coefficient used in the equation. With all variables constant other than weight, mass and range are inversely proportional, but this is less true in regimes where the drag constant is significant. Since I think it's probable that EV makers would prefer to reduce trunk space than increase overall drag, I'll ignore this variable.

Though significant, I don't think an 11% range penalty is an unsurmountable technical challenge. While ferrites are not the ideal material for strong EV motors, they have an ideal supply chain. It's worth noting that, surprisingly, the simulation from which we used the 2.4x weight multiple (Prakht et al.) found the heavier ferrite motor to be slightly more expensive than the REE motor (by < 3%). [8] Though ferrites were the cheaper material, a manufacturer would need to buy 2.4x the quantity of REEs as well as steel stator and rotor cores that are double the size/weight of their REE motor counterparts. At the time of writing, the authors assumed a price of $126.6/kg for NdFeB magnets. While NdPr typically is lower-priced than NdFeB, NdPr makes up the major cost of NdFeB magnets, so are a good proxy. $126.6/kg is high relative to NdPr's median prices, though these prices have reached $143/kg (Nd) and $140/kg (Pr) in the last two years. [12] At this higher price, ferrite magnet motors become economical, but it's difficult to underwrite an upfront capital investment into a ferrite magnet manufactory on this volatility.

In conclusion, though ferrite magnets don't seem to face massive technical barriers to investment as many imply, they do have surprisingly meaningful economic barriers owing to the weight penalty of their adoption. At the very least, ferrite motors can reach economic parity with NdFeB motors in some price scenarios, which may increase US procurement companies' bargaining power on the Chinese REE market.

© Kate Bradley. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

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[3] Z.-Z. Li et al., "How Do Rare Earth Prices Respond to Economic and Geopolitical Factors?" Resour. Policy, 85, 103853 (2023).

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[8] V. Prakht et al., "Comparison Between Rare-Earth and Ferrite Permanent Magnet Flux-Switching Generators For Gearless Wind Turbines," Energy Rep. 6, Suppl. 9, 1365 (2020).

[9] G. Sieklucki, "An Investigation into the Induction Motor of Tesla Model S Vehicle," 2018 International Symposium on Electrical Machines, IEEE 8442648, 10 Jun 18.

[10] A. Sharma et al., "Enabling the Electric Future of Mobility: Robotic Automation for Electric Vehicle Battery Assembly," IEEE Access 7, 170961 (2019).

[11] S. J. Ling, J. Sonny, and W. Moebs, University Physics, Vol 1 (Samurai Media, 2017).

[12] D. Gielen and M. Lyons, "Critical Materials for the Energy Transition: Rare Earth Elements," International Renewable Energy Agency, 2022.

[13] M. Kumada et al., "The Strongest Permanent Dipole Magnet," Proc. 2003 IEEE Particle Accelerator Conference, IEEE 1288751, 12 May 03.