A review of structural design for reduced-rare-earth permanent magnet motors in new energy vehicles

1 Rare-earth dependency and alternative motor topologies

The global electric vehicle industry has developed a profound dependence on rare-earth permanent magnets, with approximately 87% of EV motors worldwide utilizing NdFeB-based magnets containing 1–3 kg of rare-earth materials per motor in 2024 (Riba et al., 2016). This concentration of supply, with China dominating both mining and refining of rare-earth elements, has created significant geopolitical vulnerabilities that were starkly exposed by Chinese export restrictions in 2025, forcing automakers including Maruti Suzuki, Lucid Motors, Ford, and Suzuki Motor to delay or reduce production due to material shortages (Smith Stegen, 2015). The heavy rare-earth elements dysprosium and terbium, which are essential for preventing demagnetization at elevated temperatures, are particularly subject to supply disruption and price volatility. In response, the motor industry has intensified research into alternative topologies that reduce or eliminate rare-earth content while maintaining acceptable performance for automotive traction applications. The primary alternative topologies include synchronous reluctance motors (SynRMs) with no permanent magnets, switched reluctance motors (SRMs), externally excited synchronous motors (EESMs) that replace permanent magnets with rotor field windings, and ferrite-assisted synchronous reluctance motors (PMaSynRMs) that substitute low-cost ferrite magnets for rare-earth magnets, and induction motors (IMs), which require no permanent magnets and instead generate torque through rotor currents induced by the stator field (Riba et al., 2016; Zhu and Howe, 2007). Each topology presents distinct structural design considerations: SynRMs rely entirely on rotor saliency and flux barriers to generate reluctance torque, requiring sophisticated rotor geometry optimization to achieve competitive saliency ratios; SRMs offer simple, rugged construction with no rotor windings or magnets but suffer from high torque ripple and acoustic noise; EESMs provide dynamic flux control through electrically excited rotor windings but introduce slip rings or brushless excitation systems that add complexity and maintenance requirements; and PMaSynRMs represent a hybrid approach that combines reluctance torque with modest permanent magnet assistance from ferrite or other low-cost magnets to improve power factor and torque density; IMs offer proven reliability, mature manufacturing, and the lowest structural complexity among the traction alternatives, but rotor copper losses limit their continuous torque density and part-load efficiency (Zhu and Howe, 2007). The selection among these alternatives depends on the specific application requirements, with urban delivery vehicles and cost-sensitive segments favoring magnet-free designs while performance-oriented platforms may tolerate some rare-earth content in hybrid configurations. The accelerating industry shift toward sustainable motor technologies reflects both cost pressure from volatile rare-earth prices and policy-driven demand for supply chain security (Smith Stegen, 2015). The European Union’s Critical Raw Materials Act and similar policy initiatives in the United States and Japan are further accelerating investment in rare-earth-free motor research and domestic supply chain development.

Beyond topology substitution, structural and operational measures can reduce rare-earth consumption by relaxing the thermal and electromagnetic stresses imposed on the magnets. Improved cooling, including stator jacket and direct oil spray systems, maintains magnet temperatures within safe demagnetization margins and, by lowering peak operating temperature, can reduce or even eliminate the need for heavy rare-earth additives such as dysprosium, which are incorporated primarily to stabilize coercivity at elevated temperature (Wang et al., 2022). Machines with longer active lengths and shorter end-windings reduce end-turn copper losses and improve heat extraction (Gerada et al., 2014). Loss-minimization control strategies that apportion current between torque-producing and loss-generating components (Lee et al., 2009), together with winding and saliency designs that reduce electrical loading (Guo et al., 2023), further protect low-coercivity magnets. Rotor hotspot observers and flux-observer-based demagnetization detection derate or redirect flux when local temperatures or demagnetization risk rise, providing an additional protection layer without hardware changes (Cao and Wu, 2022). Hybrid configurations that retain a small amount of rare-earth magnet to shield bulk ferrite magnets, discussed in Section 5, offer a complementary route for performance-oriented applications. Finally, high-speed designs that deliver the same power at lower torque reduce the magnet volume and active material required for a given power rating (Gerada et al., 2014).

Figure 1 distills the structural landscape of reduced-rare-earth motor technology, highlighting the core supply chain vulnerability of heavy rare-earth dependence and the four alternative topologies that offer pathways to sustainability. SynRM and SRM achieve complete magnet elimination through reluctance torque and salient pole design, while EESM provides dynamic flux control via electrically excited rotor windings, and PMaSynRM represents the pragmatic hybrid approach combining ferrite assistance with optimized flux barriers. The central challenge of irreversible demagnetization under high temperature and heavy rare-earth loss demands integrated design strategies—reducing HRE content, boosting power density, and improving thermal performance—balanced against mitigation solutions including grain boundary modification, topology optimization, and thermal management. EESM advantages of adjustable flux and supply chain independence are validated by commercial adoption, while the material evolution from ferrite through FeCrCo to next-generation rare-earth-free magnets charts a clear progression toward complete supply chain decoupling. The development timeline positions material breakthroughs in the 2020s, scale-up and adoption in the 2030s, and mainstream deployment beyond 2040, aligning with policy drivers such as the EU Critical Raw Materials Act. Among these alternatives, the ferrite-assisted synchronous reluctance motor (PMaSynRM) has emerged as the most promising reduced-rare-earth topology for NEV traction applications, offering a pragmatic balance between the cost advantages of ferrite magnets and the performance requirements of automotive propulsion; Section 2 therefore examines this topology in detail, with particular attention to rotor flux barrier optimization, multi-objective algorithms, and demagnetization resistance under combined electrical, thermal, and mechanical loading.

2 Ferrite-assisted synchronous reluctance motor design

Building on the topology overview of Section 1, this section examines the electromagnetic and mechanical design of the PMaSynRM in greater technical depth. Ferrite magnets, composed primarily of iron oxide, are abundantly available, corrosion-resistant, and thermally stable, but their residual flux density and energy product are approximately one-tenth those of NdFeB magnets, necessitating significantly larger magnet volumes to achieve comparable air-gap flux (Zhang and Zhou, 2015). The structural design of PMaSynRM rotors involves embedding ferrite magnets within optimized flux barriers to simultaneously maximize reluctance torque through saliency ratio enhancement and supplement the torque with permanent magnet flux linkage. Multi-objective optimization algorithms, including differential evolution and genetic algorithms, have been extensively applied to determine optimal barrier geometries, magnet dimensions, and rotor pole configurations that maximize average torque while minimizing torque ripple and demagnetization risk (Nagarajan et al., 2017). The design process typically employs hierarchical methods that first optimize the saliency ratio through flux barrier dimension selection, then determine the optimal magnet placement and ratio to achieve the desired balance between torque density, efficiency, and power factor (Wu et al., 2018). A critical structural challenge in ferrite PMaSynRM design is the risk of irreversible demagnetization under peak torque conditions and low temperatures, where the coercivity of ferrite magnets is significantly reduced (Kang et al., 2003). The demagnetization field distribution analysis reveals that the outer diameter corners of ferrite magnets experience the highest demagnetizing field intensity during maximum current operation, and special attention must be paid to the magnetization process after rotor assembly to ensure complete magnetization of deeply embedded magnets (Obata et al., 2014). Design strategies to mitigate demagnetization include optimizing the magnet orientation within flux barriers, adding protective ribs or bridges to shield magnets from demagnetizing fields, and employing multi-phase configurations that reduce the peak current per phase (Islam et al., 2019). The mechanical integrity of the rotor is equally critical, as the centrifugal forces at high rotational speeds can cause deformation of the thin flux barrier bridges; optimized center posts and weight reduction notches have been demonstrated to enhance structural stability while maintaining electromagnetic performance (Islam et al., 2019). Experimental prototypes of five-phase ferrite PMaSynRMs have validated that comparable performance to rare-earth-based machines can be achieved with careful electromagnetic and mechanical co-design, though the overall torque density remains 15%–25% lower than equivalent NdFeB machines. The comparative evaluation of spoke-type interior permanent magnet motors using different magnet materials has confirmed that ferrite-based designs require significantly larger magnet volumes and enhanced cooling systems to prevent thermal demagnetization, but offer substantial cost advantages and supply chain security (Momen et al., 2019). The analysis and design of spoke-type ferrite permanent magnet motors with optimized rotor overhang and bridge geometry have shown that detailed rotor geometry optimization substantially improves torque quality and reduces harmonic distortion compared with conventional skewed rotor approaches (Kim et al., 2013).

3 Externally excited synchronous motors and magnet-free designs

Externally excited synchronous motors (EESMs) represent a fundamentally different approach to rare-earth elimination by replacing permanent magnets entirely with electrically excited rotor windings, enabling dynamic flux control that is impossible with fixed magnets. The rotor structure of an EESM incorporates field windings energized by an external DC power source, traditionally through slip rings and brushes but increasingly through brushless excitation systems that employ rotating rectifiers or inductive power transfer to eliminate maintenance-prone mechanical contacts (Di Gioia et al., 2018; Müller et al., 2023). This architecture provides inherent rare-earth-free operation while offering the significant advantage of controllable rotor flux, which allows the motor to achieve wide constant-power speed ranges without the efficiency penalties associated with field-weakening current injection in permanent magnet machines. The stator design of EESMs closely resembles that of conventional synchronous motors, with distributed or concentrated windings in laminated steel cores, but the rotor structure is substantially more complex, requiring field winding slots, pole shoes, and excitation systems that increase overall machine length and manufacturing cost (Stancu et al., 2018). The control architecture for EESMs employs maximum torque per ampere (MTPA), flux weakening (FW), and maximum torque per voltage (MTPV) strategies similar to IPM machines, but with the additional degree of freedom provided by the controllable rotor excitation current, which complicates the cross-coupling between rotor and stator circuits and requires sophisticated control algorithms to optimize performance across the operating envelope (Chu et al., 2014). Series production of EESMs in electric vehicles has been pioneered by major European OEMs, which typically pair an externally excited synchronous machine with an induction machine in dual-motor platforms to achieve complete rare-earth elimination across the powertrain (Di Gioia et al., 2018; Stancu et al., 2018). Renault and Nissan have also adopted EESM technology in various EV models, while suppliers including ZF, BorgWarner, and Valeo have developed production-ready EESM variants. The I2SM (In-Rotor Inductive-Excited Synchronous Motor) concept developed by ZF represents a particularly innovative structural approach, transmitting excitation energy to the rotor via an inductive coupling within the rotor shaft, and achieving power and torque densities comparable to rare-earth PM machines while eliminating magnets entirely; inductive excitation concepts of this type have been validated through design optimization and measurements on EV-scale prototypes (Müller et al., 2023). Hofer Powertrain has developed brushless rotor excitation systems optimized for high-speed e-motors that reduce friction, wear, and mechanical failure risks while providing better thermal stability than conventional permanent magnet synchronous motors. However, EESMs generally exhibit slightly lower efficiency on standardized drive cycles due to the continuous excitation losses in the rotor windings, and the additional power electronics for excitation control increase system complexity and cost. Switched reluctance motors offer another magnet-free alternative with extremely simple rotor construction consisting solely of laminated steel salient poles, but their high torque ripple, acoustic noise, and complex control requirements have limited their adoption in passenger vehicle applications despite their inherent ruggedness and fault tolerance (Kiyota and Chiba, 2012).

Induction motors (IMs) constitute the most widely deployed magnet-free traction topology, benefiting from mature manufacturing infrastructure and proven reliability across millions of vehicles; however, rotor copper losses limit their continuous torque density, and their part-load efficiency is generally lower than that of synchronous alternatives, which is why several OEMs combine IMs with permanent magnet or externally excited machines in dual-motor platforms (Pellegrino et al., 2012).

Flux switching machines (FSMs) represent a hybrid magnet-free topologies, summarizing their rare-earth content, performance attributes, and commercialization status.ogy where both armature and field windings are located on the stator, with the rotor containing neither windings nor magnets, offering simple and robust construction suitable for high-speed operation, though they require continuous supply to both windings throughout operation (Cheng et al., 2011). Table 1 consolidates the comparative landscape of reduced-rare-earth motor topologies, summarizing their rare-earth content, performance attributes, and commercialization status. In summary, the magnet-free design space comprises SynRMs, SRMs, IMs, flux switching machines, and EESMs; among these, EESMs were selected for focused treatment in this section because they uniquely combine complete rare-earth elimination with externally controllable rotor flux, a capability that directly addresses the constant-power speed range requirements of automotive traction and that has already been validated in series production by multiple manufacturers.

TABLE 1. Comparative overview of reduced-rare-earth motor topologies for electric vehicle traction applications.

4 Demagnetization resistance and thermal stability design considerations

The most critical structural design challenge for reduced-rare-earth motors, particularly those employing ferrite or other low-coercivity magnets, is ensuring adequate demagnetization resistance under the severe operating conditions of automotive traction applications. Ferrite magnets exhibit coercivity values approximately one-fifth those of NdFeB magnets, making them far more susceptible to irreversible demagnetization from the combined effects of high demagnetizing fields at peak torque and elevated temperatures during continuous operation (Luk et al., 2020; Kang et al., 2003). The demagnetization risk is most severe at the magnet corners and outer edges where the demagnetizing field is concentrated, and at low temperatures where the knee of the demagnetization curve shifts to higher field intensities. The investigation of demagnetization effects on C-shape anisotropic ferrite permanent magnets has revealed that the middle portion of the magnet experiences the most severe demagnetization, causing the surface magnetic flux density distribution waveform to collapse from the center and become U-shaped, which may induce vibration during motor operation and negatively impact motor stability (Chien et al., 2024). Structural design strategies to enhance demagnetization resistance include careful magnet placement within flux barriers to minimize exposure to direct armature reaction fields, the use of flux-concentrating spoke-type arrangements that increase the effective magnet operating point, and the incorporation of magnetic shunts or bypass paths that redirect demagnetizing flux away from vulnerable magnet regions (Obata et al., 2014). The thermal design of the motor directly impacts demagnetization margins, as improved cooling systems can maintain magnet temperatures within safer operating regions; direct oil cooling has become particularly important for ferrite-based machines, as the reduced thermal margin compared to rare-earth magnets demands more aggressive heat removal to prevent temperature-induced coercivity degradation (Wang et al., 2022; Boglietti et al., 2009). The mechanical design of the rotor must also accommodate the larger magnet volumes required for ferrite machines, which increases rotor inertia and centrifugal loading on the flux barrier bridges. Topology optimization techniques have been applied to simultaneously maximize electromagnetic performance and mechanical robustness, generating organic rotor geometries that distribute stress more uniformly while maintaining high saliency ratios (Nagarajan et al., 2017). Finite element analysis is essential for predicting demagnetization behavior under combined electrical, thermal, and mechanical loading, with multi-physics models accounting for temperature-dependent magnetic properties and stress-induced anisotropy in laminated steel cores (Guo et al., 2025). The comparative study of synchronous reluctance and ferrite-assisted synchronous reluctance motors for traction applications has demonstrated that while PMaSynRMs achieve superior torque capability and lower torque ripple, both topologies encounter challenges in maintaining constant power at speeds above 6,000 rpm due to the high current density and lower energy product of ferrite magnets, which are susceptible to irreversible demagnetization under field-weakening operation (Barcaro et al., 2012). The design of ferrite spoke-type permanent magnet synchronous motors requires careful analysis of the magnetization yoke to ensure complete magnetization of deeply embedded magnets while avoiding demagnetization of surrounding magnets during the magnetization process (Kim et al., 2013).

5 Advanced materials, hybrid configurations, and future perspectives

The development of advanced magnetic materials and hybrid motor configurations represents the frontier of reduced-rare-earth motor research, offering pathways to bridge the performance gap between conventional rare-earth PM machines and alternative topologies. Iron nitride (FeN) magnets based on the α″-Fe16N2 phase offer magnetic properties intermediate between ferrite and NdFeB while being composed entirely of abundant iron and nitrogen, eliminating rare-earth dependence entirely (Wang, 2020). These materials are particularly well-suited for variable flux motor applications where the controllable magnetization state aligns with their intrinsic magnetic characteristics (Zhao et al., 2023). Hybrid permanent magnet configurations that combine small quantities of rare-earth magnets with larger volumes of ferrite magnets have been proposed to leverage the high energy product of NdFeB for critical flux paths while using ferrite for bulk magnetomotive force, reducing overall rare-earth content by 60%–80% while maintaining acceptable performance (Wu et al., 2018). The structural design of such hybrid rotors requires careful magnetic circuit analysis to ensure that the rare-earth magnets protect the ferrite magnets from demagnetization through their stronger coercivity, effectively creating a magnetic “shield” that extends the safe operating envelope of the low-cost magnets. Grain boundary diffusion techniques and dual-alloy NdFeB magnets that reduce heavy rare-earth content by 30%–50% while maintaining high-temperature performance represent near-term material innovations that complement structural design strategies. The low-cost high-performance ferrite permanent magnet machines in EV applications have been comprehensively reviewed, highlighting that while ferrite motors offer significant cost advantages and supply chain security, the challenges of low magnet energy, irreversible demagnetization, and inferior mechanical strength remain constraints that must be addressed through innovative electromagnetic and structural design (Luk et al., 2020). The future trajectory of reduced-rare-earth motor development points toward increasingly integrated design approaches that combine advanced materials, optimized electromagnetic structures, and sophisticated thermal management within multi-objective optimization frameworks. Standardized design platforms and scalable manufacturing processes will be essential for transitioning these technologies from research prototypes to mass-market NEV platforms, while continued research into fully magnet-free high-performance machines will address the ultimate goal of complete rare-earth elimination in automotive propulsion systems. The convergence of supply chain security concerns, environmental sustainability objectives, and advancing computational design capabilities ensures that reduced-rare-earth motor technology will remain a central focus of electric vehicle engineering for the foreseeable future.

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Footnotes

Edited by: Adel Razek, Laboratoire Génie électrique et électronique de Paris (GeePs), France

Reviewed by: Nady Boules, NB Motors, L.L.C., United States