New Method Cuts Torque Ripple in EV Motors
A groundbreaking approach to reducing torque ripple in interior permanent magnet synchronous motors (IPMSMs) has been introduced by researchers at Tianjin University, offering a promising solution to one of the most persistent challenges in electric vehicle (EV) motor design. The study, led by Wang Lixin, a doctoral candidate, and co-authored by Professor Wang Xiaoyuan, Gao Peng, and Liu Shuangshuang from the School of Electrical Engineering and Information at Tianjin University, presents a novel analytical framework that redefines how rotor structures are optimized for smoother motor performance.
Torque ripple—the fluctuation in output torque during motor operation—has long been a critical concern in the development of high-performance EVs. While IPMSMs are widely favored for their high power density, efficiency, and strong field-weakening capability, their tendency to produce significant torque ripple can lead to undesirable vibrations and noise, negatively impacting ride comfort and overall driving experience. Traditional methods to mitigate this issue, such as stator skewing or rotor segmentation, often come at the cost of reduced torque output or increased manufacturing complexity and expense. As the global EV market continues to grow, the demand for cost-effective, high-efficiency solutions that do not compromise performance has never been greater.
The research team’s innovation lies in a conceptual shift: treating the magnetic barriers within the rotor—commonly known as flux barriers or ge ci qiao—as “virtual slots.” These structural features, designed to prevent demagnetization and shape the magnetic field, are typically viewed as passive components. However, Wang and his colleagues demonstrate that these barriers actively influence the air-gap relative permeance, a key factor in determining electromagnetic behavior. By abstracting the flux barriers as virtual slots, the team was able to develop a comprehensive analytical model that captures the interaction between rotor geometry and magnetic field harmonics with unprecedented clarity.
This model allows for a precise understanding of how the angular position of these virtual slots affects torque ripple. Unlike previous studies that focused primarily on shaping the rotor magnetomotive force (MMF) waveform or minimizing total harmonic distortion in air-gap flux density, this work emphasizes the role of air-gap relative permeance modulation caused by rotor structure. The insight is significant: while earlier optimization techniques often targeted the rotor’s magnetic field directly, this new method addresses the way the rotor structure modulates the magnetic field through the air gap, a subtler but equally impactful mechanism.
The core of the methodology involves identifying the dominant harmonic components responsible for torque ripple. For an 8-pole, 48-slot motor—a common configuration in EV applications—the researchers pinpointed the 12th harmonic of the air-gap relative permeance induced by the virtual slots as the primary contributor to torque ripple. This harmonic interacts with stator tooth harmonics and the fundamental rotor MMF to generate a 12-time electrical frequency torque oscillation, which dominates the overall ripple spectrum. By strategically positioning the virtual slots, the amplitude of this 12th harmonic can be minimized, effectively suppressing the largest component of torque ripple.
The analytical model was rigorously validated using finite element analysis (FEA), a standard tool in motor design that simulates electromagnetic fields with high spatial resolution. The team constructed detailed FEA models of both single-layer and double-layer magnet rotors, systematically varying the angular positions of the virtual slots and measuring the resulting torque characteristics. The simulations confirmed the predictions of the analytical model: when the virtual slots were placed at specific angles—such as 22.5° and 37.5° for single-layer designs—torque ripple dropped dramatically. In one case, ripple was reduced from over 40% to less than 10%, a remarkable improvement that underscores the model’s predictive power.
Building on these findings, the researchers applied their method to a real-world case study: a 30-kW IPMSM designed for automotive use. The original motor, with virtual slot positions at 14° and 42°, exhibited a torque ripple of 25.5% at rated conditions. Applying the optimization principles derived from their model, the team redesigned the rotor by adjusting the slot positions to 22° and 54°, while maintaining the same magnet volume and overall structural integrity. The redesigned motor showed a torque ripple of just 6.8%, a reduction of nearly 75%. Crucially, this improvement was achieved without sacrificing average torque, efficiency, or field-weakening capability—key performance metrics for EVs.
To further validate their approach, the team compared their virtual slot optimization against a conventional method: rotor skewing through axial segmentation. In this technique, the rotor is divided into multiple segments, each slightly twisted relative to the next, to cancel out spatial harmonics. While effective, this method typically reduces average torque due to the phase shift between segments. In the test case, the segmented rotor reduced torque ripple to 12.3%, but at the cost of a 5% drop in average torque. In contrast, the virtual slot optimization achieved a lower ripple (6.8%) with no measurable loss in output, highlighting its superior balance of performance and efficiency.
The study also examined the motor’s behavior across a wide operating range, including constant-torque and field-weakening regions at both rated and maximum current levels. The results were consistently positive: torque ripple was significantly suppressed under all conditions, with the most dramatic improvements seen in the high-current, high-speed field-weakening zone—precisely where EVs operate during aggressive acceleration or highway driving. This broad-spectrum effectiveness suggests that the method is not just a niche fix but a robust enhancement applicable to real-world driving cycles.
To confirm the practical viability of their design, the researchers fabricated physical prototypes and conducted extensive electromagnetic testing. The test setup included a dynamometer, high-precision torque sensor, and data acquisition system, allowing for accurate measurement of back-electromotive force (back-EMF), torque, and ripple under load. The experimental results closely matched the simulation predictions, with back-EMF waveforms and average torque values aligning within a few percent. While the measured torque ripple was slightly higher than simulated—due in part to unmodeled PWM-induced current harmonics from the controller—the trend was unmistakable: the optimized rotor delivered a significantly smoother torque output.
The implications of this research extend beyond the immediate improvement in motor smoothness. By providing a clear, physics-based design principle, the virtual slot concept empowers engineers to make informed decisions early in the design process, reducing reliance on time-consuming trial-and-error optimization. This is particularly valuable in the fast-paced automotive industry, where development cycles are tight and cost pressures are high. Moreover, because the method does not require additional materials or complex manufacturing steps, it can be readily adopted by motor manufacturers without significant retooling or investment.
The work also contributes to a deeper understanding of the fundamental physics governing IPMSM performance. By highlighting the role of rotor-induced permeance harmonics, it challenges the conventional wisdom that torque ripple is primarily a function of stator slotting or rotor MMF shape. Instead, it reveals that the rotor’s structural geometry—often treated as a secondary concern—plays a direct and quantifiable role in electromagnetic behavior. This insight could inspire new avenues of research, such as the co-optimization of stator and rotor slotting patterns or the development of adaptive rotor designs that dynamically adjust virtual slot positions through smart materials.
From a sustainability perspective, the reduction in torque ripple translates to lower mechanical stress on drivetrain components, potentially extending the lifespan of the motor and transmission. Smoother operation also reduces acoustic noise, contributing to a quieter cabin environment—a key selling point for premium EVs. Furthermore, by maintaining high efficiency across the operating range, the optimized motor helps maximize vehicle range, a critical factor for consumer adoption.
The research has been published in the Transactions of China Electrotechnical Society, a leading peer-reviewed journal in the field of electrical engineering. The paper, titled “Torque Ripple Reduction Analysis of Interior Permanent Magnet Synchronous Motor for Electric Vehicle,” provides a comprehensive account of the methodology, simulations, and experimental validation. It represents a significant step forward in the quest for high-performance, low-noise electric motors, offering a practical, scalable solution that aligns with the industry’s need for innovation without compromise.
As the automotive world transitions toward electrification, the importance of refining every aspect of the powertrain cannot be overstated. While much attention has focused on battery technology and charging infrastructure, the motor itself remains the heart of the EV. Improvements in motor design, such as the one presented by Wang Lixin and his team, may not make headlines, but they are essential to delivering the quiet, smooth, and responsive driving experience that consumers expect. This work exemplifies how fundamental engineering research, grounded in rigorous analysis and real-world testing, can yield tangible benefits for the next generation of electric vehicles.
In an era where artificial intelligence and machine learning are often touted as the keys to innovation, this study stands as a testament to the enduring value of classical engineering principles. By returning to first principles—Maxwell’s equations, Lorentz force law, and Fourier analysis—the researchers have uncovered a powerful new tool for motor design. Their work is a reminder that sometimes, the most impactful breakthroughs come not from complex algorithms, but from a fresh perspective on well-understood physical phenomena.
The success of this project also highlights the importance of collaboration between academia and industry. While the research was conducted in a university setting, its focus on a practical, manufacturable solution ensures its relevance to real-world applications. The fact that the optimized design required no exotic materials or processes makes it immediately applicable to existing production lines. This bridge between theory and practice is essential for translating scientific discoveries into technological progress.
Looking ahead, the virtual slot concept could be extended to other types of electric machines, such as switched reluctance motors or synchronous reluctance motors, where torque ripple is also a significant challenge. The underlying principle—that structural features can be treated as modulators of magnetic permeance—may prove to be a universal tool in the designer’s toolkit. As motor designers continue to push the boundaries of performance, efficiency, and compactness, methods like this will be crucial for achieving the next level of refinement.
In conclusion, the work of Wang Lixin, Wang Xiaoyuan, Gao Peng, and Liu Shuangshuang represents a significant advancement in the field of electric motor design. By reimagining the rotor’s flux barriers as virtual slots and developing a precise analytical model to guide their placement, they have provided a powerful new method for reducing torque ripple in IPMSMs. Validated through simulation and experiment, the approach delivers substantial performance improvements without compromising other key metrics. As the automotive industry accelerates toward a fully electric future, innovations like this will play a vital role in shaping the driving experience of tomorrow.
Wang Lixin, Wang Xiaoyuan, Gao Peng, Liu Shuangshuang, School of Electrical Engineering and Information, Tianjin University. Torque Ripple Reduction Analysis of Interior Permanent Magnet Synchronous Motor for Electric Vehicle. Transactions of China Electrotechnical Society. DOI: 10.19595/j.cnki.1000-6753.tces.231345