New Hybrid Flux Motor Design Promises Smoother, More Powerful EV Performance
A groundbreaking new motor design could help overcome longstanding limitations that have held back one of the most robust and cost-effective electric machine technologies from wider adoption in the electric vehicle (EV) market. Researchers from China University of Mining and Technology and Nanchang University have introduced an innovative axial-radial hybrid flux switched reluctance motor (ARFSRSRM) with a segmented rotor structure, specifically engineered to address two of the technology’s most persistent drawbacks: high torque ripple and low torque density.
The work, led by Dr. Wenju Yan and his colleagues Hongwei Yang, Jun Xin, Hao Chen, Fengyuan Yu from the School of Electrical Engineering at China University of Mining and Technology, along with Qing Wang from the College of Information Engineering at Nanchang University, presents a compelling solution that could reshape the future of EV propulsion systems. Their findings, published in the prestigious journal Power System Protection and Control, demonstrate a significant leap in performance for switched reluctance motors (SRMs), a type of electric machine known for its ruggedness, simplicity, and ability to operate efficiently across a wide range of speeds without the need for rare-earth permanent magnets.
The research, titled “Design and analysis of an axial and radial hybrid flux switched reluctance motor with identical poles for electric vehicles,” details a novel three-phase 12/14/12 motor architecture that fundamentally rethinks the magnetic circuit design of traditional SRMs. By seamlessly integrating both axial and radial magnetic flux paths within a single, compact unit and employing a segmented rotor, the team has created a motor that not only delivers more power but also operates with a remarkable degree of smoothness.
The significance of this development cannot be overstated. While permanent magnet synchronous motors (PMSMs) currently dominate the EV market due to their high power density and smooth operation, they come with significant drawbacks, including high material costs, supply chain vulnerabilities related to rare-earth elements, and complex thermal management requirements. SRMs, on the other hand, are built from simple stacks of laminated steel and copper windings, making them inherently more durable, easier to manufacture, and less susceptible to demagnetization at high temperatures. However, their reputation for noisy, jerky operation—a direct result of high torque ripple—has been a major barrier to consumer acceptance in passenger vehicles.
Dr. Yan and his team’s new design directly confronts this issue. The core innovation lies in the motor’s unique topology. The ARFSRSRM features a central segmented rotor that is shared by both an axial and a radial stator system. The axial component uses a dual-stator, single-rotor configuration, with stators positioned on either end of the rotor stack. The radial component features a conventional outer stator surrounding the same central rotor. This co-location of axial and radial magnetic circuits on a single rotor is what enables the motor’s enhanced performance.
The key to the design’s success is its ability to create a “short magnetic circuit” with a unique property: at the unaligned rotor position, the magnetic fluxes generated by the axial and radial stators are designed to oppose each other, effectively canceling out. This cancellation results in a significantly lower minimum inductance for the motor windings. The difference between the maximum inductance (at the aligned position) and this new, lower minimum inductance is a critical factor in determining the amount of torque an SRM can produce. A larger difference means a greater “magnetic co-energy,” which directly translates into higher output torque. This principle is the foundation of the ARFSRSRM’s increased torque density.
The researchers conducted a comprehensive analysis to validate their design. They first established the theoretical framework, deriving the power equations and defining the initial geometric parameters based on established design principles for both axial and radial flux machines. The final design targets a 1.5 kW output at 1000 rpm, a common requirement for auxiliary or smaller EV applications. With a stator outer diameter of 240 mm and a rotor outer diameter of 175 mm, the motor is designed to be compact and suitable for integration into modern vehicle platforms.
To optimize the motor’s performance, the team employed a sophisticated multi-objective optimization strategy using the Taguchi method, a robust statistical approach for experimental design. They identified four critical structural parameters that would have the most significant impact on the motor’s performance: the radial stator pole shoe length (Ht), the radial stator pole arc angle with shoe (βs1), the axial stator-to-rotor pole shoe gap width (Wsro), and the rotor slot width (Wrslot). Instead of exhaustively testing all 256 possible combinations of these four parameters at four different levels each, the Taguchi method allowed them to select a carefully constructed set of 16 experiments. This orthogonal test matrix approach dramatically reduced the computational burden of running full 3D finite element method (FEM) simulations while still providing statistically significant data.
The optimization process focused on three key performance indicators: average torque (Tav), torque smoothness coefficient (τ), and torque density (Tp). The torque smoothness coefficient, defined as the ratio of the average torque to the difference between the maximum and minimum torque, is a direct measure of torque ripple. A higher value indicates a smoother, less pulsating output. Torque density, measured in Newton-meters per cubic meter (N/m³), reflects how much torque the motor can produce relative to its volume, a crucial metric for space-constrained automotive applications.
By analyzing the results of the 16 FEM simulations, the researchers calculated a “weight matrix” for each performance objective, quantifying how much each parameter level influenced the outcome. They then combined these matrices using weighted factors (0.4 for average torque, 0.3 for smoothness, and 0.3 for density) to arrive at a single, comprehensive “total target matrix” that represented the best overall compromise. This systematic approach identified the optimal parameter combination as “1244,” corresponding to a radial stator pole shoe length of 1.5 mm, a pole arc angle of 16 degrees, an axial pole gap of 10.5 mm, and a rotor slot width of 17 mm.
The results of this optimization were striking. Compared to the initial design, the optimized ARFSRSRM showed a 23.4% increase in average torque, a 19.54% improvement in the torque smoothness coefficient, and a substantial 32.7% boost in torque density. These gains are not merely theoretical; they represent a tangible improvement in real-world motor performance. A motor with higher torque density can be made smaller and lighter for the same power output, improving vehicle efficiency and range. A motor with a higher smoothness coefficient will produce less vibration and noise, leading to a more refined and comfortable driving experience.
To verify the accuracy of their model and the validity of their claims, the research team conducted a rigorous validation process. They referenced a previously published prototype of a similar 12/10/12 ARFSRSRM motor, which had been built and tested in a laboratory setting. The comparison between the simulated magnetic linkage characteristics and the actual measured data from the physical prototype showed a close match, confirming the fidelity of their 3D FEM model. Furthermore, dynamic simulations of the new 12/14/12 design under both current chopping control (CCC) and angle position control (APC) modes produced results that were consistent with the expected behavior of a well-functioning SRM, further bolstering confidence in the simulation framework.
The true test of the ARFSRSRM’s superiority came from a direct comparison with a conventional radial flux segmented rotor switched reluctance motor (RFSRSRM) of identical outer dimensions and key structural parameters. This side-by-side analysis highlighted the advantages of the hybrid design. Static FEM simulations revealed that the ARFSRSRM achieved a significantly higher ratio of maximum to minimum inductance (2.58 compared to 1.94 for the RFSRSRM). This larger inductance swing is the direct cause of the increased torque production.
The static torque waveforms told an even more compelling story. When the researchers simulated the torque output of the axial-only, radial-only, and combined axial-radial configurations, they found that the total torque produced by the hybrid motor was greater than the simple sum of the torques from the individual axial and radial components. This super-additive effect proves that the two magnetic circuits are not just operating in parallel but are interacting synergistically to produce a more powerful output, a key finding that underscores the effectiveness of the integrated design.
Dynamic simulations under realistic operating conditions further confirmed the motor’s capabilities. When simulated under a 96 V DC supply, the optimized ARFSRSRM delivered an average torque of 14.39 Nm at 1000 rpm in CCC mode, with a torque smoothness coefficient of 1.13. In APC mode at 1500 rpm, it produced 9.54 Nm of average torque with a smoothness coefficient of 0.83. In contrast, the conventional RFSRSRM, when simulated under similar conditions, produced lower average torque (10 Nm and 9 Nm respectively) and exhibited higher torque ripple, as evidenced by a larger difference between its maximum and minimum torque values. The torque density of the ARFSRSRM was also markedly higher, reaching 5.9 x 10³ N/m³ in CCC mode compared to 3.92 x 10³ N/m³ for the RFSRSRM.
Beyond raw power and smoothness, the researchers also evaluated the motor’s dynamic response. Simulations of a “variable speed” scenario, where the motor’s speed was ramped up from 700 to 900 rpm under a constant 10 Nm load, and a “variable load” scenario, where the load was increased from 8 to 12 Nm at a constant 1000 rpm, showed that the ARFSRSRM could maintain stable operation with its speed settling within 0.15 seconds in both cases. This rapid response time is essential for the precise control required in modern EV drivetrains, particularly for features like regenerative braking and traction control.
The implications of this research are far-reaching. By successfully mitigating the two primary weaknesses of SRMs—torque ripple and low power density—Dr. Yan and his colleagues have brought this inherently robust and cost-effective technology much closer to being a viable mainstream option for EVs. The elimination of rare-earth magnets not only reduces material costs but also makes the supply chain for these motors more secure and less environmentally damaging. The motor’s simple, robust construction also suggests a potential for higher reliability and lower maintenance costs over the vehicle’s lifetime.
The design also incorporates several practical features that enhance its manufacturability and performance. The use of a segmented rotor held in place by an epoxy resin disk reduces windage losses, improving efficiency at high speeds. The dual axial stator structure provides inherent magnetic force balance, which minimizes vibration and extends bearing life. The “identical poles” concept, where adjacent stator poles have the same winding configuration, simplifies the manufacturing and assembly process.
While the current research is based on a 1.5 kW prototype, the design principles are scalable. The modular nature of the segmented rotor and the flexibility of the hybrid flux topology suggest that this architecture could be adapted for a wide range of power levels, from small urban EVs to larger passenger cars and even commercial vehicles. The success of this project also highlights the power of combining advanced electromagnetic modeling with systematic optimization techniques to push the boundaries of what is possible in electric machine design.
In conclusion, the work of Wenju Yan, Hongwei Yang, Jun Xin, Hao Chen, Fengyuan Yu, and Qing Wang represents a significant advancement in the field of electric propulsion. Their axial-radial hybrid flux switched reluctance motor is not just an incremental improvement but a fundamental rethinking of how magnetic energy can be harnessed in an electric machine. By delivering a smoother, more powerful, and more compact SRM, they have opened a promising new pathway for the development of next-generation electric vehicles that are not only more efficient and sustainable but also more affordable for a broader range of consumers.
Wenju Yan, Hongwei Yang, Jun Xin, Hao Chen, Fengyuan Yu, Qing Wang, China University of Mining and Technology, Nanchang University, Power System Protection and Control, DOI: 10.19783/j.cnki.pspc.246010