Abstract
To provide a comprehensive and balanced perspective, the review also acknowledges well-established mainstream alternatives, including induction motors (IMs), which have demonstrated practical viability in commercial EV applications such as Tesla’s early production models. Variable flux motors (VFMs) have emerged as a transformative technology for new energy vehicle propulsion systems, addressing the fundamental trade-off between low-speed torque capability and high-speed efficiency that constrains conventional permanent magnet synchronous motors (PMSMs). This review systematically examines the structural design aspects of variable flux motors, encompassing hybrid permanent magnet topologies, magnetization state control mechanisms, flux regulation strategies, and electromagnetic optimization methodologies. Particular emphasis is placed on variable flux memory machines (VFMMs) employing low-coercive-force (LCF) magnets such as AlNiCo in combination with high-coercive-force (HCF) neodymium-iron-boron (NdFeB) magnets, as well as novel rotor shifting mechanisms and variable leakage flux designs. The paper synthesizes recent advances in series and parallel magnetic circuit configurations, swiveling magnetization techniques, and multi-objective design optimization frameworks. By analyzing comparative performance metrics across different VFM architectures and identifying persistent technical barriers including magnetization state control precision, demagnetization resistance, and manufacturing complexity, this review aims to provide a comprehensive reference for researchers and engineers engaged in next-generation wide-speed-range electric propulsion system development.
1 Fundamental principles and classification of variable flux motors
Variable flux motors represent a paradigm shift in electric machine design by enabling dynamic adjustment of the rotor magnetic flux linkage according to operating conditions, thereby eliminating the need for continuous field-weakening current injection at high speeds (BYD, 2025). The concept was originally developed by Ostovic and subsequently refined into what are now commonly termed variable flux memory machines (VFMMs) or memory motors, which can memorize the flux density level in rotor magnets through controlled magnetization state (MS) changes (Basnet, 2021). The fundamental operating principle relies on employing low-coercive-force (LCF) permanent magnets, typically aluminum-nickel-cobalt (AlNiCo) or ferrite, whose magnetization state can be altered by applying short-duration current pulses rather than requiring continuous demagnetizing current as in conventional PMSMs (Basnet, 2021). This capability allows VFMs to achieve a wide torque-speed envelope with significantly reduced energy losses compared to conventional field-weakening strategies, where copper losses in the d-axis current path can degrade high-speed efficiency by 10%–15% (Yu et al., 2024). The existing VFMMs can be broadly categorized into AC-magnetized types, which utilize stator armature windings to produce d-axis current pulses through vector control, and DC-magnetized types, which employ dedicated field windings for magnetization state manipulation (Basnet, 2021). AC-magnetized configurations offer structural simplicity similar to conventional PM machines with fewer circuit components, but impose higher requirements on armature winding and inverter ratings due to the substantial magnetizing current demands (Basnet, 2021). DC-magnetized variants provide more flexible magnetization control but introduce additional winding complexity and manufacturing cost. Beyond memory machine topologies, alternative variable flux approaches include mechanically adjustable rotor-stator axial displacement mechanisms, variable leakage flux designs with controllable magnetic barriers, and hybrid configurations combining electromagnetic and mechanical flux adjustment methods (Yu et al., 2024). Each approach presents distinct trade-offs between flux regulation range, torque density, control complexity, and manufacturing feasibility, necessitating careful topology selection based on specific NEV application requirements including urban driving cycles, highway cruising, and hill-climbing scenarios (Ding et al., 2026). Beyond the advanced variable-flux concepts discussed above, it is worth noting that the EV propulsion landscape also includes well-established mainstream alternatives. The Induction Motor (IM), for instance, features straightforward magnetic field control via field-oriented strategies, though it exhibits lower efficiency in high-torque regimes relative to PMBMs (Caricchi et al., 2009). Tesla notably adopted the IM as its sole propulsion drive in early production models, underscoring its proven commercial viability. Acknowledging such mainstream configurations alongside emerging VF technologies ensures a more complete reference for the research community. Subsequently, Tesla transitioned to an Internal Permanent Magnet Synchronous Reluctance Motor (IPMSynRM) for the rear axle, while retaining the IM as a secondary motor on the front axle (Chau et al., 2008). This dual-motor configuration allows the IM to boost driving torque when needed and to cover the high-speed range with higher efficiency, as deployed in Models S, X, 3, and Y. The two motors are tightly controlled and coordinated to maximize driving-cycle efficiency. A similar dual-motor concept has also been adopted in many General Motors’ EV models (Cheng et al., 2021). Additionally, the wound-field synchronous motor (WF-SynM) represents another mainstream alternative that has gained renewed attention in recent EV platforms. Unlike permanent magnet machines, the WF-SynM generates rotor flux through DC field winding excitation rather than relying on rare-earth magnets, offering complete flux controllability and immunity to demagnetization. This topology has been adopted in some recent BMW models, demonstrating its practical viability for premium electric vehicle applications (Ye et al., 2022). The primary advantage of the WF-SynM lies in its elimination of permanent magnet material, thereby mitigating supply chain risks and reducing raw material costs. However, the necessity of slip rings or brushless excitation systems to supply field current introduces additional mechanical complexity and maintenance considerations, while the continuous field winding excitation results in higher copper losses compared with PMBMs under certain operating conditions (Zhu and Howe, 2007).
Figure 1 provides a consolidated visual framework of variable flux motor technology, spanning from AC/DC magnetization classification and LCF magnet characteristics through series/parallel/hybrid circuit trade-offs to MS control strategies and commercialization roadmap. The hybrid magnetic circuit emerges as the most balanced topology for electric vehicle applications, combining wide flux regulation with robust demagnetization resistance, while swiveling magnetization and W-shaped rotor innovations offer pathways to transcend inherent topological limitations. The control domain demands adaptive strategies—3D NN hysteresis modeling, closed-loop observers, and ANN-based MTPA—to address nonlinear voltage limits and magnetization-state-dependent trajectories that defy conventional PMSM control paradigms. Looking ahead, near-term commercialization is anchored by BYD’s integrated adjustment patents, mid-term disruption by rare-earth-free iron nitride magnets, and long-term convergence toward corner module integration, though challenges in MS precision, thermal demagnetization, and manufacturing scalability must be resolved to enable mainstream adoption. The following section examines hybrid PM topologies in greater depth, focusing on the geometric optimization of iron bridges and barrier configurations that govern the torque density–flux regulation trade-off.
FIGURE 1
2 Hybrid permanent magnet topologies and magnetic circuit configurations
The hybrid permanent magnet topology, combining high-coercive-force (HCF) NdFeB magnets with low-coercive-force (LCF) AlNiCo or ferrite magnets, has become the dominant structural approach for achieving practical variable flux capability in NEV traction motors (Sarlioglu et al., 2017). These hybrid configurations can be classified into series magnetic circuit, parallel magnetic circuit, and hybrid magnetic circuit arrangements, each offering distinct performance characteristics (Wei et al., 2024). In the series magnetic circuit configuration, the constant flux from HCF magnets always assists the variable flux from LCF magnets, thereby stabilizing the operating point of the variable permanent magnets and preventing unintentional demagnetization under loaded conditions (Basnet, 2021). The series topology offers the highest maximum torque among the three configurations and provides excellent on-load demagnetization resistance, though it exhibits the smallest flux regulation range and requires higher magnetizing currents to alter the magnetization state (Jayarajan et al., 2019). The parallel magnetic circuit configuration, conversely, provides the largest flux regulation range, enabling superior high-speed efficiency by effectively canceling a significant portion of the air-gap flux when the LCF magnets are reversely magnetized (Basnet, 2021). However, this topology suffers from the lowest maximum torque and presents significant challenges regarding unintentional demagnetization of LCF magnets due to the strong demagnetizing field from adjacent HCF magnets (Jayarajan et al., 2019). The hybrid magnetic circuit configuration represents a balanced compromise, combining the wide flux regulation capability of parallel arrangements with the demagnetization resistance advantages of series configurations, making it particularly suitable for electric vehicle applications requiring both high starting torque and extended constant-power speed ranges (Jayarajan et al., 2019). Recent research has introduced novel spoke-type variable flux motors with swiveling magnetization, where the magnetic pole direction of LCF magnets can be physically rotated rather than magnetically reversed, achieving the wide flux regulation range of parallel types while maintaining the demagnetization resistance of series types (Lee et al., 2022). This swiveling approach significantly reduces the magnetizing impulse current required for flux state transitions, improving practical feasibility for automotive inverter systems (Dorrell et al., 2010). Additionally, variable flux hybrid permanent magnet synchronous machines (VFHPMSMs) incorporating W-shaped rotor arrangements with AlNiCo magnets at the pole center and V-shaped NdFeB magnets at the sides have demonstrated good flux-adjusting ability and efficiency through finite element analysis and experimental validation (Sato and Todaka, 2018). The iron bridge and rotor lamination region between constant and variable permanent magnets play a critical role in determining torque density and flux regulation capabilities, requiring careful geometric optimization to balance these competing objectives (Wei et al., 2024). The tangential magnetization method has been proposed to overcome armature demagnetizing effects in AlNiCo-based VF-PMSMs, where curved barriers prevent q-axis flux from passing through the magnet while allowing uniform d-axis flux distribution (Basnet, 2021). Table 1 consolidates the comparative insights from Sections 1 and 2, revealing that no single VFM architecture dominates across all performance metrics. The series configuration maximizes torque density and demagnetization resistance but sacrifices flux regulation range, while the parallel topology achieves the widest speed envelope at the cost of elevated demagnetization risk and lower peak torque (Srinivasan et al., 2026). The hybrid magnetic circuit emerges as the pragmatic compromise for general EV traction, though the swiveling magnetization and built-in axial adjustment mechanisms offer alternative pathways to decouple flux regulation from magnet material limitations. Notably, the VLF-FIM topology achieves competitive flux adjustability with the lowest control complexity and highest manufacturing maturity, positioning it as a near-term candidate for cost-sensitive platforms where hybrid PM configurations remain economically prohibitive (Li et al., 2014). The following section examines variable leakage flux and mechanical flux adjustment approaches that circumvent magnetization state manipulation entirely, offering complementary strategies for achieving wide-speed-range operation (Woolmer and McCulloch, 2007).
TABLE 1
| Architecture | Magnetic circuit | Flux regulation mechanism | Max torque | Flux range | Demag resistance | Control complexity | Manufacturing maturity | Best application |
|---|---|---|---|---|---|---|---|---|
| Series VFM | HCF assists LCF in series | Current pulse via d-axis | Highest | Smallest | Excellent | Moderate | Moderate | High-torque urban EV |
| Parallel VFM | HCF//LCF in parallel | Reverse magnetization of LCF | Lowest | Widest | Poor (high risk) | High | Moderate | Extended highway cruising |
| Hybrid VFM | Combined series-parallel | Selective MS manipulation | Moderate | Wide | Good | High | Moderate | General EV traction |
| Swiveling VFM | Rotatable LCF poles | Physical rotation (not reversal) | Moderate | Wide | Excellent | Moderate | Low | High-performance EV |
| VLF-FIM | Magnetic barriers + bridges | Armature current control (Ld > Lq) | Moderate | Moderate | Good | Low | High | Cost-sensitive platforms |
| Axial variable-gap | Fixed PMs | Variable air-gap | Mechanical axial displacement | Moderate | Moderate | Excellent | Low (mechanical) | Low |
| Built-in axial Adj | Internal rotor slider | Precision axial shift (2 mm/step) | Moderate | Wide (50% flux reduction) | Good | Moderate | Low | Prototype/demo stage |
Comparative overview of variable flux motor architectures across magnetic circuit, flux regulation, and application domains.
3 Variable leakage flux and mechanical flux adjustment mechanisms
Beyond magnetization state manipulation in hybrid PM machines, alternative variable flux approaches have been developed based on controllable leakage flux paths and mechanical rotor displacement mechanisms. Variable leakage flux flux-intensifying motors (VLF-FIMs), which combine the flux-adjustability of variable leakage flux designs with the flux-intensifying characteristics of synchronous reluctance machines, represent an innovative topology that achieves flux variability through strategically designed magnetic barriers and bridges in the rotor structure, rather than through magnet material property manipulation (Liu et al., 2021). In these designs, a leakage flux path is established on the q-axis while magnetic barriers reduce Lq and magnetic bridges increase Ld, creating the characteristic Ld greater than Lq property that enables flux intensification and weakening through armature current control. This approach inherits advantages from both variable leakage flux machines and flux-intensifying motors, offering a simpler manufacturing process compared to hybrid PM configurations while maintaining reasonable flux adjustment capability (Fan et al., 2014). The rotor topology evolution from conventional interior PM motors to VLF-FIMs involves careful design of magnetic barrier widths and bridge dimensions to optimize the trade-off between variable flux property and torque density. Axial flux variable gap motors represent another mechanical approach to flux adjustment, where the rotor-stator air-gap length is dynamically varied to alter the effective magnetic flux linkage (Oh et al., 2002). This mechanically adjustable field-weakening strategy enables the motor torque-speed characteristics to better match vehicle load requirements across different driving conditions, though the energy consumed by the mechanical adjustment mechanism must be carefully balanced against efficiency gains to ensure net energy savings (Oh et al., 2002). More recently, a novel variable flux motor with a built-in axially adjustable rotor mechanism has been developed, featuring a distributed torque peripheral rotor slider assembly that allows precise axial displacement of the rotor relative to the stator (Yang et al., 2024). This mechanism enables dynamic control of air-gap magnetic flux by shifting the rotor axially, with prototype demonstrations showing that air-gap flux can be reduced by approximately 50% at maximum extension, extending top speed by up to 240% while maintaining an average drive cycle efficiency of 92% compared to 87% for conventional PMSMs (Yang et al., 2024). The built-in mechanism occupies internal rotor space that would otherwise remain unused, preserving motor compactness while adding flux controllability. The crank handle connected to the internal mechanism allows users to configure the rotor axial position, with each rotation producing a precise 2 mm linear shift that reduces air-gap magnetic flux by a predictable percentage (Yang et al., 2024). However, mechanical flux adjustment approaches introduce additional moving parts, wear considerations, and control complexity that must be weighed against the performance benefits for specific automotive applications.
4 Magnetization state control and electromagnetic design optimization
Precise magnetization state (MS) control represents a critical technical challenge for variable flux motors, as the operating trajectory in the dq-current plane becomes nonlinear and dependent on the instantaneous magnetization level. Unlike conventional PMSMs where the voltage limit forms a perfect ellipse in the dq-plane, VF-PMSMs exhibit semi-elliptical voltage limit curves due to the nonlinear relationship between magnet flux linkage and d-axis current (Basnet, 2021). The maximum torque per ampere (MTPA, i.e., the operating point yielding the highest torque for a given stator current magnitude) trajectory changes with magnetization state, requiring adaptive control schemes that account for the current MS level during real-time operation (Basnet, 2021). Various control strategies have been proposed including three-dimensional neural-network hysteresis models of AlNiCo magnets for rapid MS variation law estimation, closed-loop MS estimation methods using flux observers with voltage disturbance state filters, and artificial neural network-based MTPA control schemes that simplify control effort by reducing inductance nonlinearity effects under MS transitions (Basnet, 2021). The magnetization state is typically defined as the percentage of actual magnet flux linkage relative to the maximum achievable flux linkage, with high MS selected for high-torque requirements and low MS for high-speed operation (Basnet, 2021). Analytical magnetic models for variable-flux interior permanent magnet synchronous machines have been developed using subdomain methods, solving partial differential equations for magnetic vector potential in polar coordinates with appropriate boundary conditions between rotor yoke, PM and field winding regions, rotor barriers, air-gap, and stator winding domains (Wang et al., 2021). These analytical models enable rapid parametric studies and design optimization without computationally expensive finite element analysis, though they require simplifications such as neglecting axial end effects and assuming radial slot geometries. Multi-objective optimization frameworks for variable flux motor design typically target simultaneous maximization of torque density, flux regulation range, and efficiency while minimizing torque ripple, demagnetization risk, and magnetizing current requirements (Yu et al., 2024). The design variables include PM dimensions and arrangements, magnetic barrier geometries, rotor pole-shoe profiles, and stator slot configurations, subject to constraints on outer diameter, axial length, and maximum current density imposed by the NEV platform (Yu et al., 2024). Finite element methods are extensively employed to evaluate the electromagnetic performance at different magnetization states, including flux linkage characteristics, back-EMF waveforms, torque-speed envelopes, and loss distributions under various operating conditions (Wei et al., 2024). The investigation and regulation of high-efficiency region boundaries in variable magnetic flux permanent magnet motors has revealed that purposeful design of variable flux leakage topology can effectively broaden the high-efficiency operating region, providing a promising research path for wide-speed-range high-efficiency motor design (Yu et al., 2024). Experimental validation of prototype VF-PMSMs has confirmed the variable magnetization state property, torque-speed capability, and loss reduction capability of series magnet configurations, demonstrating the practical viability of this technology for automotive traction applications (Xia et al., 2004).
5 Commercial development, material innovations, and future perspectives
The commercial development of variable flux motors has accelerated significantly with recent patent filings and strategic licensing agreements from major automotive and technology companies (Emadi, 2014). Chinese automaker BYD has filed a comprehensive series of patents for an advanced variable-flux permanent magnet synchronous motor (VF-PMSM) that integrates mechanical and magnetic adjustment elements within the rotor, including axially and radially movable components that alter effective flux linkage to shift the motor operating point (Electric Motor Engineering, 2025). This architecture enables controlled flux weakening without relying exclusively on current-based field-weakening strategies in the inverter, limiting copper and iron losses at elevated speeds and enhancing thermal stability under continuous high-load operation (Electric Motor Engineering, 2025). The technology is engineered to maintain high efficiency over a broader constant-power speed range, delivering lower specific energy consumption during sustained high-speed driving without increasing battery capacity or cooling system complexity (He et al., 2023). Niron Magnetics has licensed a comprehensive portfolio of foundational variable flux motor patents, combining breakthrough motor design technologies with rare-earth-free iron nitride magnets to enable unprecedented efficiency improvements across automotive traction and other high-value applications (AudioXpress, 2025). Unlike conventional electric motors, VFMs eliminate the fundamental trade-off between low-speed and high-speed efficiency, enabling benefits ranging from greater vehicle range to smaller battery pack sizes (Jiang and Jahns, 2015). Iron nitride magnets present particular promise for VFM applications because they offer magnetic properties better suited to variable flux operation than conventional rare-earth magnets, which are optimized for fixed high-coercivity rather than controllable magnetization states (AudioXpress, 2025). The development of high-temperature LCF magnet grades and advanced magnetization state estimation algorithms will further enhance VFM practicality for automotive environments. Future research directions include integrated corner module concepts combining variable flux propulsion with braking and steering functions, digital twin frameworks for predictive maintenance of magnetization state stability, and advanced manufacturing techniques such as additive manufacturing for complex rotor barrier geometries (Rahman, 2004). The convergence of improved magnetic materials, sophisticated control algorithms, and optimized electromagnetic designs positions variable flux motors as a compelling alternative to conventional PMSMs for next-generation NEV platforms requiring extended speed ranges, improved highway efficiency, and reduced rare-earth material dependence (Yu et al., 2024). Standardization of VFM interfaces and scalable manufacturing processes will be essential for transitioning this technology from specialized applications to mainstream automotive platforms, while continued research into magnetization state stability under thermal cycling and vibration will address the fundamental material constraints that currently limit widespread deployment. The potential for VFMs to reduce or eliminate rare-earth material usage aligns with growing supply chain security concerns and sustainability objectives, further motivating continued investment in this technology across the global automotive industry.
Statements
Author contributions
YL: Conceptualization, Data curation, Writing – original draft. HZ: Investigation, Methodology, Writing – original draft. JT: Formal Analysis, Funding acquisition, Writing – review and editing.
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The author(s) declared that financial support was received for this work and/or its publication. We gratefully acknowledge financial support from the New Energy Vehicle Intelligent Network Technology Shandong Province Higher Education Institutions Future Industry Engineering Research Centre and Shandong Province Higher Education Institutions Marine Vessel Special Motor Key Technology Development and Component Manufacturing University-Enterprise Collaborative Innovation Center.
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Summary
Keywords
AlNiCo magnet, flux regulation, hybrid permanent magnet, magnetization state control, new energy vehicle, variable flux memory machine, variable flux motor, wide speed range
Citation
Liu Y, Zhang H and Tan J (2026) A review of structural design for variable flux motors in new energy vehicles. Front. Mech. Eng. 12:1949338. doi: 10.3389/fmech.2026.1949338
Received
27 July 2026
Revised
11 August 2026
Accepted
11 August 2026
Published
20 August 2026
Volume
12 - 2026
Edited by
Adel Razek, UMR8507 Laboratoire Génie électrique et électronique de Paris (GeePs), France
Reviewed by
Nady Boules, NB Motors, L.L.C., United States
Updates
Copyright
© 2026 Liu, Zhang and Tan.
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*Correspondence: Jianwei Tan, tanjianwei@bit.edu.cn
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.