New High-Saliency Modular Rotor Boosts EV Motor Performance
A groundbreaking advancement in electric vehicle (EV) motor technology has emerged from Nanjing University of Science and Technology, where a team of researchers has unveiled a novel permanent magnet synchronous motor (PMSM) design poised to redefine performance benchmarks for next-generation electric drivetrains. Led by Dr. Weiwei Geng, the research team has developed a high-saliency, modular flux-concentrating rotor that significantly outperforms the widely used V-shaped rotor design in critical areas such as torque density, power output, and operational efficiency across a wide speed range. This innovation, detailed in the prestigious Proceedings of the CSEE, represents a significant leap forward in the quest for more powerful, efficient, and compact electric motors, addressing core challenges that have long limited the performance of EVs.
The heart of this new technology lies in its unique rotor architecture, a sophisticated fusion of two established but previously separate concepts: the spoke-type rotor and the Halbach permanent magnet (PM) array. Spoke-type rotors are known for their ability to concentrate magnetic flux, thereby generating higher torque and power densities. However, they have historically suffered from a critical flaw—significant magnetic flux leakage within the rotor’s inner circumference. This leakage not only wastes valuable magnetic energy but also reduces overall motor efficiency and can lead to unwanted heating. On the other hand, V-shaped rotors, which have become the de facto standard in the EV industry (used in vehicles from Tesla, Honda, and Toyota), offer a good balance of performance and manufacturability, with a high saliency ratio that enables strong reluctance torque and excellent field-weakening capabilities for high-speed operation. Yet, their design incorporates structural ribs to maintain mechanical integrity, which inadvertently create additional paths for magnetic flux to leak, diminishing the effective use of the expensive rare-earth magnets.
The research team, spearheaded by Jing Wang, Dongxu Liu, and Caiquan Wu, ingeniously combined the best attributes of both while eliminating their primary weaknesses. By integrating the Halbach array principle—which naturally directs magnetic flux outward into the air gap and away from the rotor’s interior—with the spoke-type topology, they created a structure that inherently minimizes internal flux leakage. The pivotal innovation was the complete removal of the traditional iron ribs that have plagued V-shaped designs. Without these ribs, the path for parasitic flux is severed, allowing more of the magnetic energy from the SmCo magnets to be productively channeled across the air gap to the stator. This results in a substantially higher air-gap flux density, a fundamental parameter that directly correlates with the motor’s ability to produce torque. The team’s finite element analysis confirmed this, showing a 12.5% increase in the fundamental wave amplitude of the air-gap flux density compared to the V-shaped rotor, rising from 1.12 Tesla to a robust 1.26 Tesla.
This enhanced flux concentration is not achieved at the expense of the motor’s other critical performance metric: the saliency ratio. The saliency ratio, defined as the ratio of the q-axis inductance to the d-axis inductance (Lq/Ld), is a cornerstone of modern IPMSM design. A high saliency ratio allows the motor to generate significant reluctance torque in addition to the torque produced by the permanent magnets themselves. This dual-torque mechanism is what gives IPMSMs their high efficiency and power density. The new modular rotor design achieves a maximum saliency ratio of 2.01, surpassing the 1.95 ratio of the V-shaped rotor. This seemingly small numerical difference translates into a substantial real-world advantage. It means the motor can produce more torque for a given amount of current, or conversely, it can produce the same torque with less current, thereby reducing resistive (copper) losses in the windings and improving overall system efficiency. This was validated in the study’s torque characteristics analysis, where the new design demonstrated a higher maximum reluctance torque, a key contributor to its superior performance.
The performance gains are most evident when examining the motor’s output capabilities. Under peak current conditions, the new design achieved a maximum power output of 61.1 kW, a notable improvement over the V-shaped rotor’s 60.5 kW. However, the most striking advantage is revealed in the high-speed, field-weakening region. At the maximum test speed of 6,000 revolutions per minute (r/min), the new motor maintained a power output of 75.3 kW, a remarkable 20% higher than the 62.7 kW delivered by the V-shaped rotor. This superior field-weakening capability is a direct consequence of the high saliency ratio and the optimized magnetic circuit. The current vector diagram analysis showed that the new rotor’s “characteristic current” places its voltage limit ellipse in a more favorable position relative to the current limit circle. This larger overlapping area allows the motor to sustain higher torque at high speeds before hitting its voltage ceiling, enabling a wider constant-power speed range. The research quantified this as a theoretical speed regulation ratio of 2.8:1, compared to 2.5:1 for the V-shaped rotor. This extended range is crucial for EVs, as it allows a single motor to efficiently cover a broader spectrum of driving conditions, from low-speed city driving to high-speed highway cruising, potentially reducing the need for complex multi-speed transmissions.
Efficiency is another area where the new design excels. While the higher flux density does lead to a slight increase in core (iron) losses, this is more than offset by the gains in power output and reduced copper losses from the more efficient torque production. The study’s efficiency map analysis showed that the new motor achieves a peak efficiency of 97.5%, compared to 97.0% for the V-shaped rotor. This 0.5% difference may seem marginal, but in the context of an EV’s overall energy budget, it represents a significant improvement in range and energy consumption over the vehicle’s lifetime. The efficiency advantage is maintained across a wide operating range, with the new motor showing higher efficiency in both rated and field-weakening operating conditions. For instance, at a high-speed, high-power point of 5,500 r/min, the new motor operated at 96.01% efficiency, while the V-shaped rotor’s efficiency was 95.21%. This consistent efficiency edge is a testament to the holistic optimization of the design.
A critical aspect of any high-performance motor, especially one designed for the demanding environment of an electric vehicle, is its mechanical integrity at high rotational speeds. The removal of the structural ribs, while beneficial for magnetic performance, raised a significant concern about the rotor’s ability to withstand the immense centrifugal forces at 6,000 r/min. To address this, the research team implemented a robust structural solution: a 0.5 mm-thick carbon fiber composite sheath wrapped around the rotor’s outer circumference. Carbon fiber is renowned for its exceptional strength-to-weight ratio, making it an ideal material for high-speed rotor protection. The finite element stress analysis was unequivocal. Without the carbon fiber sheath, the rotor’s outer iron segments would experience a maximum displacement of 0.22 mm and a maximum stress of 255 MPa, levels that could lead to catastrophic failure. With the sheath in place, the maximum displacement was reduced to a negligible 0.05 mm, and the maximum stress in the iron was contained to around 140 MPa, well within the safe operating limits of the material. This successful validation of the rotor’s structural integrity is a crucial step in transitioning the design from a laboratory concept to a viable, real-world engineering solution.
The practical viability of the design was further confirmed through the construction and rigorous testing of a 30 kW prototype motor with a 16-pole/72-slot configuration. The experimental results were in excellent agreement with the finite element simulations, lending strong credibility to the theoretical analysis. The measured no-load line-to-line back-electromotive force (back-EMF) at 3,000 r/min was 170.5 V, just 3.7% lower than the simulated value, a difference well within acceptable margins and attributable to factors like temperature-dependent magnet remanence. Efficiency tests across various operating points showed a close match between simulation and experiment, with the maximum experimental efficiency reaching 96.1% at rated conditions. These successful experiments not only validate the design’s electromagnetic performance but also demonstrate its manufacturability and robustness under real operating conditions.
Beyond raw performance, the research also delved into the critical area of noise, vibration, and harshness (NVH), a paramount concern for consumer acceptance of EVs. While the new design exhibited a slightly higher cogging torque—a common source of noise at low speeds—its overall NVH performance was found to be favorable. The analysis of radial electromagnetic forces, the primary source of magnetic noise, showed that while both motors had significant force components at lower frequencies, the new modular rotor design exhibited significantly lower force amplitudes in the high-frequency range. This is a crucial advantage, as high-frequency noise is often more perceptible and annoying to human ears. The measured noise levels under rated conditions were comparable, with the new motor’s maximum noise staying below 60 dB, indicating that the design’s acoustic performance is well within acceptable limits for automotive applications.
The implications of this research for the EV industry are profound. By achieving a 11.7% increase in both torque and power density compared to the incumbent V-shaped rotor, the new design offers a clear path to more compact and lighter motors. This can lead to significant benefits, including increased vehicle range, improved handling due to lower unsprung mass, and greater design flexibility for automakers. The larger rotor inner diameter of the new design (157 mm vs. 140 mm) is another significant advantage, as it provides more space for integrating components like sensors, cooling systems, or even a secondary machine, enhancing the overall integration and compactness of the electric drive system. Furthermore, the successful use of a carbon fiber sheath to ensure structural integrity opens the door for even higher-speed designs in the future, pushing the boundaries of EV performance.
In conclusion, the work by Wang, Geng, Liu, Wu, Lei, Qiang, and Jian from Nanjing University of Science and Technology presents a comprehensive and highly successful redesign of the IPMSM rotor. By thoughtfully merging the principles of the spoke-type and Halbach designs and eliminating the detrimental ribs, they have created a modular, high-saliency flux-concentrating rotor that sets a new standard for EV motor performance. Its superior torque and power density, wider speed range, higher efficiency, and validated structural integrity make it a compelling candidate for the next generation of electric vehicles. This research not only advances the state of the art in motor design but also provides a practical, experimentally verified solution that could soon find its way into the drivetrains of future EVs, driving the industry toward greater efficiency and performance. Jing Wang, Weiwei Geng, Dongxu Liu, Caiquan Wu, Lei Li, Qiang Li, Jian Guo, Nanjing University of Science and Technology, Proceedings of the CSEE, DOI: 10.13334/j.0258-8013.pcsee.222707