Rare Earth Permanent Magnet Motors

As early as the s, the world's first motor appeared, and the rotor of this motor is a permanent magnet to generate an excitation magnetic field. However, the permanent magnet material used at that time was natural magnetite (Fe3O4), which had a very low magnetic energy density. The motor made from it was bulky and was soon replaced by an electric excitation motor. With the development of technology, there have been many choices of permanent magnet materials. The motors using rare earth permanent magnet materials are also called rare earth permanent magnet motors.

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Rare Earth Permanent Magnet Motors

The Structure of Rare Earth Permanent Magnet Motors

The rare earth permanent magnet synchronous motor is mainly composed of rotor, end cover, stator, and other components. Generally speaking, the stator structure of a permanent magnet synchronous motor is very similar to that of an ordinary induction motor, but the rotor of a permanent magnet synchronous motor has a unique structure.

Problems with Rare Earth Permanent Magnet Motors

1. Difficult to Adjust and Control the Magnetic Field

The permanent magnet motor can maintain its magnetic field after it is manufactured, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. It is difficult for a permanent magnet generator to adjust its output voltage and power factor from the outside, and a permanent magnet DC motor can no longer adjust its speed by changing the excitation.

2. High in Price 

As the current price of rare earth permanent magnets is still relatively expensive, the cost of rare earth permanent magnet motors is generally higher than that of electric excitation motors. 

3. 

Demagnetization Problem

Rare earth permanent magnet motors have strict requirements for the working environment. When the temperature exceeds 180°C, some rare earth permanent magnet materials will experience irreversible demagnetization. Moreover, they are prone to breakage under severe vibration or large temperature differences. Rare earth permanent magnet materials are easy to be oxidized and corroded, so it is best to perform surface treatment on them before use. In addition, rare earth permanent magnet motors are very sensitive to overload. Once overloaded, permanent magnet materials will be demagnetized. At the same time, the electromagnetic load of the rare earth permanent magnet motor is very high, the magnetic field is difficult to adjust after being made, and its power control system is much more complicated than that of an induction motor. 

Conclusion 

Thank you for reading our article and we hope it can help you to have a better understanding of the rare earth permanent magnet motors. If you want to learn more about magnets, we would like to advise you to visit Stanford Magnets for more information. As a leading magnet supplier across the world, Stanford Magnets has been involved in R&D, manufacturing, and sales of magnets since the s. It provides customers with high-quality permanent magnets like SmCo magnets, neodymium magnets, AlNiCo magnets, and ferrite magnets (ceramic magnets) at a very competitive price.

Abstract

In this review article, we focus on the relationship between permanent magnets and the electric motor, as this relationship has not been covered in a review paper before. With the increasing focus on battery research, other parts of the electric system have been neglected. To make electrification a smooth transition, as has been promised by governing bodies, we need to understand and improve the electric motor and its main component, the magnet. Today's review papers cover only the engineering perspective of the electric motor or the material-science perspective of the magnetic material, but not both together, which is a crucial part of understanding the needs of electric-motor design and the possibilities that a magnet can give them. We review the road that leads to today's state-of-the-art in electric motors and magnet design and give possible future roads to tackle the obstacles ahead and reach the goals of a fully electric transportation system. With new technologies now available, like additive manufacturing and artificial intelligence, electric motor designers have not yet exploited the possibilities the new freedom of design brings. New out-of-the-box designs will have to emerge to realize the full potential of the new technology. We also focus on the rare-earth crisis and how future price fluctuations can be avoided. Recycling plays a huge role in this, and developing a self-sustained circular economy will be critical, but the road to it is still very steep, as ongoing projects show.

Keywords:

permanent magnets, rare-earth elements, critical raw materials, electric motor, recycling, additive manufacturing

1. Introduction

The world is shifting to combustion-free transport. New research shows that in , an estimated 6.5 million electric vehicles (EVs) will be sold worldwide. Half of this number has been sold in China alone (an increase of 160% to the year ), which makes it the world's largest electric vehicle (EV) market in less than a decade. Europe is heading in the same direction, selling over 2.3 million EVs in , which represents 19% of total car sales in [1]. To achieve its target of net-zero greenhouse-gas emissions by , set in its December 'Green Deal' to transform its transport sector, a lot more EVs have to 'hit the roads'. This raises the question of the raw materials needed for such an attempt [2].

Most public and scientific interest has been focused on how we will store the energy that is produced by renewable sources and how we will be able to harvest that stored energy. Batteries will probably be the main energy-storage option, although hydrogen could be a viable and possibly even better option. In either case, the efficiency of electric motors that act as converters of energy into mechanical motion will be one of the most important considerations.

All the early inventors used permanent magnets in their previously called electrical rotating machines. However, the early motors were very different from the motors of today. The first electrical motor using permanent magnets was constructed by Michael Faraday in [3]. He adopted ideas that were previously presented by Hans Christian Oersted [4] and William Wollaston [5]. Faraday's device was very simplistic and did not look like an electric motor, but with the use of permanent magnets, a bowl of mercury, and a battery, he generated an electromagnetic field that produced mechanical motion. This triggered many new modifications to the idea, changing it to the design we know today. However, the first patent for the electric motor was not granted until to Thomas Davenport [6]. Because he used low-quality permanent magnets in his design, which produced a power output of 4.5 W, they did not sell. This made many future inventors switch to electromagnets, which were more suitable for the job at the time. Not until new types of magnetic materials, such as carbon, cobalt, and tungsten steel, were invented almost 100 years later did inventors use permanent magnets in their designs. But the real breakthrough came with the discovery of Al-Ni-Co magnets [7], where permanent magnets were able to replace electromagnets in electric motors and the development of permanent-magnet motors began.

The most efficient electric motor is a permanent-magnet synchronous motor [8]. Their efficiency makes them popular for drive motors, power steering, stop-start motors, and regenerative braking generators. These motors use permanent magnets based on rare-earth elements (REEs), in particular neodymium-iron-boron (Nd-Fe-B) and samarium-cobalt (Sm-Co), because of their high maximum energy product (BH)max (a measure of the magnet's performance), which is needed for the high efficiency and the high resistance to demagnetization. But there are still some challenges and gaps in their performance and application, like:

• Rare Earth Material Dependence:

Many high-performance permanent magnets, particularly those based on neodymium, rely on rare earth elements. The mining and processing of these materials can be environmentally damaging and subject to supply chain issues. As a result, REEs are considered by the European Commission to be the most critical raw materials in terms of their economic importance and supply risk [9]. Research is ongoing to develop alternative magnet materials that reduce dependence on rare earth elements. Another aspect is recycling, where a lot of research is conducted to improve the recyclability of permanent magnets.

• Temperature Sensitivity:

Permanent magnets can lose their magnetic properties at high temperatures. This limits the operating temperature range of motors and can be a concern in applications where motors are exposed to elevated temperatures or require high-temperature resistance.

• Demagnetization Risk:

Permanent magnets are susceptible to demagnetization under certain conditions, such as high temperatures or excessive magnetic fields. This can result in a loss of motor performance and reliability.

• Cost of Materials:

High-performance permanent magnets can be expensive due to the cost of rare earth elements. Reducing the cost of materials while maintaining or improving performance is a key challenge in making permanent-magnet-based motors more cost-effective. For less-demanding electric motors, where size does not matter, ferrites can be used. These magnets are abundant, cheap, and have the largest share of the market.

• Motor Efficiency at Partial Loads:

The efficiency of permanent-magnet motors can decrease at partial loads, which is common in many real-world applications. Improving efficiency across a wide range of operating conditions is crucial for maximizing energy savings.

• Size and Weight Constraints:

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In some applications, especially in industries where weight and size are critical factors (e.g., aviation and automotive), finding the right balance between power density, weight, and size remains a challenge.

• Manufacturing and Integration Complexity:

Fabricating and integrating permanent magnets into motor designs can be complex. Ensuring consistent quality, especially for mass production, and addressing manufacturing challenges are areas of focus.

• Durability and Long-Term Reliability:

Long-term reliability and durability are critical factors, especially in industrial and automotive applications. Researchers and engineers are continually working on improving the robustness of permanent-magnet-based motors to ensure a longer lifespan.

• Dynamic Performance:

Achieving optimal dynamic performance, such as high torque density and fast response times, is an ongoing area of research to meet the demands of various applications.

• Cognitive Implications:

As electric motors become more integrated into autonomous systems and artificial intelligence applications, there may be a need for motors that can adapt to changing conditions in real time. This involves developing control algorithms that optimize motor performance based on varying inputs.

In this paper, we will focus on what has been done up until now and which future technologies will help make electrification more viable, like new production technologies, recycling methods, and motor designs.

5. Conclusions

The global economy is undergoing a transformative shift towards green electrification, a change driven by the urgent need to combat global warming. Numerous countries are rapidly moving away from internal combustion engines, setting ambitious targets for the adoption of electric and hybrid vehicles. For instance, Europe aims to achieve zero CO2 emissions from new cars and vans by , aligning with its 'Fit for 55' initiative [214]. In the United States, the goal is for half of all vehicles sold by to be electric or hybrid, sparking nearly $85 billion in investment into the electric vehicle (EV) industry over and [215]. China, leading the charge, mandates full electrification of new buses and urban logistics vehicles by and aims for all new passenger cars to be electric by [216]. This has propelled China to become a global leader in EV production and sales, with 6.8 million EVs sold in alone, dwarfing the U.S. sales of 800,000 EVs in the same period [217].

This electrification surge is not just a trend but a revolution, with carmakers releasing new electric models monthly and some planning a complete transition to electric drivetrains within a few years. This boom in production heightens the demand for raw materials, batteries, and electric motors, particularly magnets. Research is predominantly focused on developing more efficient batteries, improving by 10% annually, to address consumers' range anxiety. However, this emphasis on batteries has led to a relative neglect of electric motor innovation. The design of permanent magnet (PM) electric motors, for instance, has seen modest changes over the past 50 years.

The rare earth element (REE) crisis has highlighted the vulnerabilities in the supply chain for permanent magnets, essential components in electric motors. These crises have spurred research and development in both permanent magnet materials and motor designs. Globally, efforts to diversify the supply of REEs are gaining momentum, with 146 advanced-stage REE projects, including new mines and existing operations, underway worldwide [218]. The opening of a new REE mine in Wyoming, the first in the U.S. in 70 years, is a significant development, signaling efforts to reduce reliance on foreign REE sources, predominantly from China [219].

However, achieving independence in the REE sector requires more than just raw material extraction. The construction of production facilities outside China, such as the $10 billion investment at the Mountain Pass mine, aims to establish a complete supply chain for magnet production [220]. Similar initiatives are underway in Europe, although they face ecological and economic challenges, including resistance from local communities.

Recycling is emerging as a crucial component of a sustainable REE supply chain. Projects like Europe's Susmagpro have demonstrated the viability of large-scale recycling of permanent magnets. Yet the challenge of sourcing end-of-life magnets persists. To create a true circular economy for permanent magnets, there is a need for increased awareness among users and manufacturers, accompanied by legal requirements for recycling, labeling obligations, and recycling quotas.

In terms of material innovation, efforts to reduce the heavy and light REE content in PM magnets have been successful, but there is still no viable alternative to Nd-Fe-B magnets for high-power applications. Additive manufacturing offers new possibilities in magnet and motor design, but its current limitations in magnetic properties and mass production present obstacles to widespread adoption.

The future of green electrification is not limited to electricity alone. Innovations in alternative sustainable energy solutions, like Audi's e-fuel [221], which produces e-diesel from wind energy, offer promising avenues. However, the success of such alternatives depends on the development of highly efficient PM generators for wind farms.

The shift toward green electrification is an unparalleled revolution, reshaping our world's economic landscape and addressing the critical challenge of climate change. At the heart of this transformation is the need to optimize the performance and sustainability of permanent magnets, which are critical components in electric motors and generators. The pursuit of this goal involves a deep dive into the nanoscale intricacies of magnet materials, particularly the exploration of the microstructure-coercivity relationship in Nd-Fe-B magnets, which remains the gold standard for high-power applications.

Advanced research in material science is crucial for enhancing the energy efficiency and durability of these magnets. This includes investigating new alloy compositions, refining grain boundary engineering techniques, and exploring novel sintering processes to improve the thermal stability and corrosion resistance of magnets. Additionally, the development of alternative magnet materials that reduce or eliminate the reliance on rare earth elements is a key area of focus. Such materials need to match or surpass the performance characteristics of current REE-based magnets, particularly in terms of magnetic strength and temperature resilience.

Artificial intelligence (AI) and machine learning are emerging as pivotal tools in this domain [222]. By analyzing vast datasets encompassing material properties, manufacturing processes, and performance metrics, AI algorithms can uncover patterns and insights that elude traditional research methods. This approach can significantly accelerate the discovery of new magnet materials and the optimization of magnet designs, paving the way for more efficient and environmentally friendly electric motors and generators.

Furthermore, additive manufacturing (AM) technologies, such as Powder Bed Fusion-Laser Beam (PBF-LB), offer exciting opportunities for creating magnets with complex geometries and integrated cooling systems. These innovations could revolutionize motor designs, enabling more compact, efficient, and thermally stable electric motors. However, challenges in achieving the desired magnetic properties and scalability of AM-produced magnets must be addressed to realize their full potential in large-scale applications.

The journey towards green electrification is not just a matter of replacing fossil fuels with electric power. It is a scientific quest to push the boundaries of material science, physics, and engineering to develop sustainable, high-performance technologies that will drive the future of transportation, energy generation, and beyond. As we advance in this endeavor, the role of magnets becomes increasingly central, underscoring the need for continuous innovation and collaboration across disciplines to achieve a truly electrified and sustainable future.

Nomenclature

ACalternating currentAF-PMSMaxial flux permanent magnet synchronous machineAFMaxial flux motorAIartificial intelligenceAMadditive manufacturingBAAMbig-area additive manufacturingBLDCbrushless direct currentCEAMConcerted European Action on MagnetsCVTContinuously variable transmissionDCdirect currentDdiscoveriesEMIelectromagnetic interferenceEOLend-of-lifeEVelectric vehicleGBPgrain boundary phaseGBDPgrain-boundary diffusion processGBRGrain boundary restructuring H a Anisotropy fieldHDDhard disk driveHDDRhydrogenation disproportionation desorption recombinationHEVhybrid electric vehicleHEThunstable electric turbineHREheavy rare earthHREEheavy rare earth elementHPMSHydrogen processing of magnet scrapHci/HcJintrinsic coercivityIPMinterior permanent magnetIPMSMinterior permanent-magnet synchronous motorKKelvinL-PBFLaser powder bed fusionMEXmaterial extrusionMIMMetal injection moldingMRImagnetic resonance imagingμ0MSsaturation magnetizationPMSMpermanent-magnet synchronous motorPCBprinted circuit boardPMpermanent magnetsRErare earthRE-TMrare earth-transition metalsREErare earth elementsRFMradial flux motorRF-PMSMradial flux permanent magnet synchronous machineRPMrevolutions per minuteSCIMsquirrel cage induction machineSLMselective laser meltingSMCsoft magnetic compositeSPMsurface permanent magnetSPMSspark-plasma sinteringSRMswitched reluctance motorSynRMSynchronous Reluctance MotorTTeslaTEtriggering eventsTEMtransmission electron microscopyTS-RFPMTrapezoidal stator radial flux permanent magnetWEEEwaste from electrical and electronic equipmentWEEwaste electronic equipment(BH)maxmaximum energy product

Funding Statement

The authors would like to thank the Ministry of Higher Education, Science, and Technology of the Republic of Slovenia (P2-) and the European Commission for funding this work. SUSMAGPRO has received funding from the European Union's Horizon research and innovation program under grant agreement No. . MaXycle has received funding from the ERA-MIN2 research and innovation program on raw materials to foster a circular economy. Inspires has received funding from EIT RawMaterials, proposal number .

Author Contributions

Conceptualization, B.P. and B.S.; investigation, B.P. and B.S.; resources, B.P. and B.S.; data curation, B.P. and B.S.; writing'original draft preparation, B.P. and B.S.; writing'review and editing, P.J., T.T., S.K., K.Ž. and S.Š.; visualization, B.P. and B.S.; funding acquisition, B.P., B.S., S.K. and S.Š. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

Author Boris Saje was employed by the company Kolektor d.o.o. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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