Key Points for Axial Flux Motor Rotor Selection Under Different Operating Conditions – Hub Motors, Robot Joints, Drone Propulsion

Introduction: Paradigm Shift from "Cylinder" to "Disc"

When people think of electric motors, most envision a long cylinder where the stator encloses the rotor and the magnetic field propagates radially. However, a motor that defies this conventional shape is driving a new technological revolution – the axial flux motor. It compresses the stator and rotor into an almost flat disc, as compact as a sandwich cookie.

The core of this flattening revolution lies in a fundamental change in magnetic path direction. In a traditional radial flux motor, the magnetic field radiates outward from the axis; in an axial flux motor, the magnetic field runs parallel to the axis, with stator and rotor facing each other in a disc arrangement. This shift brings astonishing performance advantages: with the same material usage, the torque of an axial flux motor is proportional to the cube of the rotor diameter (while for a traditional radial motor it is only the square of the diameter), achieving a torque density increase of 2–3 times and efficiency exceeding 96%. At the same time, its axial length is only 1/3 to 1/2 that of a conventional motor, with volume reduced by more than 50% and weight reduced by approximately 40%–50% under the same power.

The key to achieving such high power density and torque density in axial flux motors lies in the ingenious design of the rotor structure. Different application scenarios impose distinct performance requirements, and the selection of rotor magnetic circuit structure, permanent magnet material, and topology often directly determines whether the motor can fully realize its advantages. This article starts with three typical application scenarios – hub motors, robot joints, and drone propulsion – and systematically analyzes the core points of rotor selection.

1. Hub Motors: Trade-off Between Torque Density and Wide Speed Range

Hub motors are installed inside the wheel rim, where space is extremely limited – this is the primary design constraint. They must simultaneously provide high torque density (for starting and climbing), a wide speed range (from low-speed crawling to high-speed cruising), and good heat dissipation capability.

In terms of rotor structure selection, hub motors commonly use surface-mounted and spoke (interior) types, each with different design priorities. Surface-mounted permanent magnets are directly attached to the surface of the rotor core, offering a simple structure, high air-gap flux density, and suitability for applications pursuing ultimate power density. However, high-speed rotation of a large-diameter rotor generates enormous centrifugal force, requiring a retaining sleeve to secure the surface-mounted magnets. This demands high-strength non-magnetic materials, and the sleeve itself increases the air gap, thereby reducing output.

Spoke-type (interior) permanent magnets are embedded inside the rotor. Through flux concentration, they significantly improve torque density and flux-weakening speed-extension capability. For example, the STAF-PMSM spoke-type hub motor designed by Jiangsu University uses a dual-rotor structure to increase the air-gap excitation area, achieving flux concentration excitation. It delivers a maximum torque of 280 N·m and maximum power of 15 kW, making it suitable for distributed in-wheel drive new energy vehicles. Moreover, the interior structure effectively protects the permanent magnets from direct exposure to high temperature and mechanical impact, overcoming the risk of magnet detachment that surface-mounted types face at high speeds.

Thermal management is another core challenge for hub motors. Under high-power operation, electromagnetic losses are concentrated and cooling conditions are poor. This requires accurate thermal modeling based on loss analysis to achieve effective cooling. Currently, the dual-stator single-rotor axial flux motor (AFIR) improves power density by increasing electrical loading with two stators, while the yoke-less axial flux motor (YASA) eliminates the stator yoke to reduce iron loss, lowering thermal load while improving efficiency and torque density.

Overall, rotor selection for hub motors must balance torque density, speed-extension capability, and reliability. For low-speed high-torque requirements, surface-mounted or spoke-type structures are preferred, but if a wide speed range is needed, the spoke-type is more suitable because of its flux concentration and flux-weakening ability.

2. Robot Joints: Dual Requirements of Low Inertia and Precision Control

Robot joints demand distinctly different characteristics from hub motors. In large joints such as hips, waists, and legs, high torque output and extreme lightweighting are the core requirements – compared to traditional radial motors, axial flux motors in these scenarios can reduce space occupation by 30%–60% and weight by more than 30%, with some designs reaching 60%–70%. In small joints such as wrists and fingers, precision and low inertia become higher priorities.

Torque-to-inertia ratio is a key design parameter for robot joint motors. Research shows that the torque of an axial flux motor is proportional to the cube of the rotor diameter, meaning that extremely high low-speed torque output can be achieved in the compact space of a flattened joint, while the thin disc structure can be embedded directly into the joint and simplifies heat dissipation.

For rotor selection, robot joints give priority to surface-mounted structures or Halbach arrays. The surface-mounted structure, with its low rotor loss and low moment of inertia, enables faster dynamic response – acceleration response time can be reduced from 15 ms to 5–8 ms, which is crucial for robot motions requiring rapid start/stop and precise positioning. A Halbach array, through a specific magnetization direction pattern, enhances the magnetic field on one side while nearly canceling it on the other, allowing elimination of the rotor core and further reducing rotor inertia and losses.

Magnetic circuit design and permanent magnet material selection also require precise control. Axial flux motors use an annular magnet layout, which shortens the magnetic path length and increases torque density compared to the radial layout of traditional radial flux motors. Also, because robot joints often include reducers or even quasi-direct drive (QDD) schemes, higher coercivity and thermal stability are required. When cost permits, high-coercivity grades with heavy rare earths such as dysprosium and terbium can effectively prevent demagnetization from reverse magnetic fields during operation.

For miniature joints in the 16–18 mm range, PCB-type axial flux motors are showing unique advantages. Using etching instead of traditional copper windings, they offer high manufacturing consistency, low iron loss, and extreme lightweighting.

3. Drone Propulsion: Extreme Challenges of Power Density and Resistance to Thermal Demagnetization

Drone propulsion systems face a fundamental contradiction: every extra gram of weight reduces flight time, and every degree of temperature rise reduces power. Data shows that for an axial flux motor with a thrust-to-weight ratio exceeding 25:1, reducing mass by 1 kg can increase range by about 10 km. Therefore, lightweighting and high power density are the primary design criteria for drone propulsion motors.

In terms of power density, axial flux motors show overwhelming advantages in drone propulsion. Their volumetric power density can reach 14.9 kW/kg, far exceeding that of traditional radial motors. Measured power densities range from 5.8 to 21 kW/kg, with torque densities of 15 to 25 Nm/kg. The latest "Yufeng" T-series axial flux propulsion system achieves a continuous power density of 10 Nm/kg and a peak torque density of 20 Nm/kg, making it well suited for direct-drive propulsion in advanced aircraft such as manned eVTOL and compound-wing drones.

Beyond power density, drone propulsion motors also face the risk of demagnetization in high-temperature environments. During flight, motors operate at high power for extended periods, causing rapid temperature rise in windings and permanent magnets. If missions are conducted in summer heat or desert areas, the combination of ambient temperature and self-heating creates severe demagnetization challenges for the permanent magnets.

Permanent magnet material selection directly affects the high temperature reliability of drone motors. Among common permanent magnet materials, neodymium-iron-boron (NdFeB) offers the highest magnetic performance, but standard grades (N series) have a maximum operating temperature of only 80–100°C, and irreversible magnetic loss may occur above 200°C. High-coercivity NdFeB grades (SH, UH, EH, AH series) can operate up to 150–240°C, but their high-temperature stability is still inferior to samarium-cobalt (SmCo). SmCo magnets can operate stably above 300°C, with a Curie temperature exceeding 720°C, and their magnetic properties vary only 1/4–1/3 as much as NdFeB with temperature. The disadvantages are slightly lower magnetic energy product and higher cost. For consumer drones, high-performance NdFeB is sufficient for most needs; but for industrial drones and manned eVTOL under high-temperature, high-power conditions, SmCo – despite its cost – is a necessary choice for reliability.

4. Quick Overview of Rotor Types: Comparison of Surface-Mounted vs. Interior

Based on the above analysis, the main rotor structure types for axial flux motors are summarized in the following table:

5. SDM: A Key Driver for Surface-Mounted Axial Flux Motor Rotors

After analyzing the key rotor selection points for the three major scenarios, we arrive at a core element – the engineering capability for high-performance permanent magnets and surface-mounted rotor structures. This is precisely where SDM's technical advantages lie.

SDM is a national high-tech enterprise focused on magnets and magnetic solutions, with 16 years of experience in professional magnet production. The company has a strategic cooperation with China Aluminum, the largest rare earth mining enterprise in China, ensuring a stable and secure supply of rare earth raw materials. At the same time, SDM conducts in-depth collaborative research with the Chinese Academy of Sciences and works with customers on finite element analysis (FEA), providing accurate simulation support from the very beginning of magnetic circuit design, thereby shortening development cycles and reducing trial-and-error costs.

In the field of surface mounted axial flux motor rotors, SDM offers systematic manufacturing and design advantages:

First, a complete production system with high-level certifications. The company holds IATF 16949 (automotive quality management system), has maintained a zero-defect (0 PPM) record as a Tier-2 supplier to General Motors since 2010, and also possesses ISO 9001, ISO 14001, ISO 45001, carbon footprint and BSCI certifications. Its products comply with RoHS, REACH, and SGS testing requirements. This means that every batch of permanent magnets undergoes rigorous quality control, from raw material traceability to finished product shipment.

Second, mature integrated process technology for surface-mounted rotor structures. In an axial flux motor, a surface-mounted permanent magnet rotor disc must simultaneously solve three major engineering difficulties: high-strength fixing of magnets, stability under high-speed operation, and manufacturability/assembly. SDM provides various magnetic material options, including high-coercivity NdFeB grades and SmCo series. It uses a combination of low-loss, high-strength polymer press plates/fixing frames, rotor back-irons, and carbon fiber retaining sleeves to ensure reliable magnet positioning under high-speed operation, while minimizing rotor eddy current losses. This solution has proven its comprehensive advantages of low rotor loss, high structural strength, and good assembly processability.

Third, a top-tier technical team supporting high-end customization. The technical team, built by experts in magnetic materials from the Chinese Academy of Sciences, includes 2 PhDs, 5 master's degree holders, 8 senior engineers, and more than 80 engineering and technical personnel. The company has established a municipal R&D center and a post-doctoral workstation. Thus, SDM can not only produce conventional magnets but also provide full-process technical solutions for actual magnetic circuit requirements under different working conditions (hub motors, robot joints, drone propulsion), including magnet grade selection (ultra-high coercivity NdFeB N/M/UH grades, SmCo5 / Sm-Co-₇ series), demagnetization temperature margin calculation, and finite element simulation.

Fourth, industry-university-research collaboration and broad product portfolio. SDM maintains cooperative relationships with the Ningbo Institute of Materials Technology and Engineering (CAS) and Southwest Jiaotong University, continuously tracking advances in magnetic materials. Its product range covers micro-motor stators and rotors, maglev motors, sensors, resolvers, optical isolators, permanent magnet and soft magnetic components, providing one-stop magnetic material support for motor designs across different industries.

Conclusion

With its flattened structure and transformative power density, the axial flux motor is redefining the power architecture of electric vehicles, humanoid robots, and low-altitude aircraft. In this technology race centered on "torque density" and "lightweighting", the rotor structure design and the quality of permanent magnet materials set the lower limit, while the surface-mounted structure – with its simple design, fast dynamic response, and high torque density – occupies an irreplaceable position in robot joints, low-speed high-torque hub drives, and other applications demanding high efficiency and low inertia.

From precise optimization of the magnetic circuit topology to high-temperature stability design of permanent magnet materials, only by mastering the complete chain of core material technology and rotor manufacturing processes can a true moat be established in the fierce market competition. SDM, with its credentials as a national high-tech enterprise, 16 years of accumulated experience in permanent magnets, technical support from a CAS-built expert team, and a systematic quality management system, provides a solid foundation for the high reliability and high performance of surface-mounted axial flux motor rotors. Whether it is the wide speed-range challenge of hub motors, the low-inertia precision control demands of robot joints, or the extreme requirements for power density and demagnetization resistance in drone propulsion, SDM offers full-process engineering solutions from materials to simulation – precisely the indispensable driving force that moves axial flux motors from laboratory to large-scale application.