Axial Flux In-Wheel Motors:

Driving the Future of High-Efficiency Electric Vehicles

Introduction: Why Electric Drive Technology Matters

Electric drive technology forms the foundation of full vehicle electrification. In modern electric vehicles, the electric drive system acts as the main power source. Therefore, its performance directly affects efficiency, safety, and driving experience.

Among all electric drive solutions, the in-wheel electric drive system represents the most advanced form. By placing the motor directly inside the wheel, this system removes traditional mechanical components such as gearboxes and drive shafts. As a result, it improves transmission efficiency and simplifies vehicle structure.

However, this design also introduces strict challenges. Limited internal space forces engineers to create motors that deliver high power density, compact dimensions, and short axial length at the same time. For this reason, axial flux motors have become a major research focus for both companies and universities worldwide.

Why Axial Flux Motors Suit In-Wheel Drive Systems

Compared with conventional radial motors, axial flux motors offer a flatter and thinner structure. More importantly, they deliver higher torque density and better heat dissipation.

Because of these advantages, engineers increasingly choose axial motors for in-wheel drive applications. In addition, the short magnetic path and large effective air-gap area further enhance electromagnetic performance.

Overall, axial flux motors match the strict size and performance requirements of in-wheel systems far better than traditional motor types.

Classification of Axial Flux Motor Structures

Engineers typically classify axial flux motors based on the number and arrangement of stators and rotors. Common configurations include:

• Single-stator single-rotor

• Double-stator single-rotor

• Single-stator double-rotor

• Multi-disc structures

At present, the electric drive industry mainly adopts double-stator single-rotor and single-stator double-rotor designs. These structures balance performance, manufacturability, and reliability.

Given the operating conditions of in-wheel motors, research efforts mainly focus on four key goals:

• High power density

• High efficiency

• Wide speed range

• High reliability

1. High Efficiency and High Power Density

Reducing Losses While Increasing Output

To increase efficiency and power density, engineers focus on several strategies. First, they reduce copper and iron losses. Next, they lower overall motor mass. At the same time, they increase pole pair numbers and operating speed. Finally, they improve cooling performance.

Each improvement contributes directly to higher torque density and better system efficiency.

Dual-Stator Single-Rotor Axial Motors (BEST Technology)

The dual-stator single-rotor disc motor, often associated with BEST technology, uses two stator cores positioned on both sides of a central rotor. Permanent magnets close the magnetic circuit through both stators.

This structure effectively increases the electrical loading of the motor. As a result, it significantly boosts power density without enlarging the motor size.

Because both stators participate in torque production, this design suits in-wheel applications that demand high torque in a compact space.

Single-Stator Dual-Rotor Axial Motors (TORUS Structure)

In the TORUS structure, the stator sits in the middle while two rotors rotate on either side. The stator windings interact with permanent magnets from both directions, which doubles the effective torque contribution.

Engineers typically arrange the rotor magnetic poles in NS or NN configurations. Notably, the NS arrangement allows the motor to operate without a traditional stator yoke.

As a result, the motor becomes lighter and more efficient.

Yokeless Axial Motors (YASA Concept)

The YASA motor removes the stator yoke entirely. This decision brings several benefits.

First, it increases the winding slot fill factor. Second, it reduces iron losses and stator core mass. Third, it shortens winding end lengths.

Together, these improvements raise torque density while also improving efficiency.

Advanced Magnetic Circuit Innovations

Researchers continue to push axial motor performance even further. For example, a research team led by Wang Xiaoyuan at Tianjin University proposed a parallel magnetic circuit axial flux permanent magnet motor.

This motor uses a YASA-type stator but introduces an innovative rotor structure. The design increases reluctance torque while improving air-gap flux density using focused and Halbach magnet arrangements.

Compared with a conventional YASA motor under the same conditions, this design:

• Increases torque density by 9%

• Reduces permanent magnet usage by 38%

• Lowers rotor losses

Clearly, advanced magnetic circuit design plays a key role in future motor development.

Halbach Magnet Arrays for Higher Efficiency

To further improve efficiency, engineers introduce the Halbach magnetization concept, originally developed for particle accelerators.

By applying Halbach arrays to axial flux motors, designers can eliminate the rotor core. This change reduces core losses and rotor inertia.

As a result, the motor operates more efficiently, especially at high speed.

Vernier Axial Flux Hub Motors

Another promising solution combines axial flux motors with vernier motor principles. By increasing the equivalent pole number, this design significantly enhances torque density.

Consequently, vernier axial flux hub motors deliver strong torque output while maintaining a compact structure.

Managing Torque Ripple in Stator Permanent Magnet Motors

Stator permanent magnet motors offer excellent power and torque density. However, they also tend to produce large torque ripple.

To address this issue, NEST technology introduced a dual-rotor axial flux-switching motor. Engineers carefully optimized pole width and stator slot height.

After optimization, torque ripple dropped by nearly 80%, while maintaining high output performance.

2. Achieving a Wide Speed Range

The Field Weakening Challenge

Both BEST technology and conventional radial motors rely on permanent magnet excitation. Because permanent magnets generate a fixed magnetic field, adjusting flux at high speed becomes difficult.

Therefore, improving field weakening capability remains a major challenge for axial motors.

Structural Solutions for Speed Expansion

To overcome this limitation, researchers focus on two motor types:

• Axial flux permanent magnet memory motors

• Hybrid excitation axial flux-switching motors

These designs allow partial control of magnetic flux, which extends the operating speed range.

Practical Example: Dual-Stator Disc Hub Motor

BEST technology developed a disc-type hub motor with a dual-stator single-rotor structure. This motor operates at a bus voltage of 300 V and reaches a maximum speed of 1184 r/min.

It delivers a peak torque of 274 N·m, making it suitable for demanding in-wheel applications.

The motor uses fractional-slot concentrated windings, which improve torque density and reduce torque ripple.

Series–Parallel Winding Switching

Engineers also apply series–parallel winding switching to expand the speed range.

• At low speed, windings connect in series. This increases back EMF and boosts torque.

• At high speed, windings switch to parallel. This reduces back EMF and enhances field weakening capability.

As a result, the motor maintains stable performance across a wide speed range.

3. High Reliability for In-Wheel Motors

Why Reliability Matters Most

As the core power source of electric vehicles, hub motor reliability directly affects driving safety. Any failure inside the wheel can compromise vehicle control.

Therefore, engineers must design axial motors with strong fault tolerance.

Dual-Stator Six-Phase Axial Motors

One advanced solution uses a dual-stator six-phase axial flux permanent magnet motor. Engineers divide the windings into two independent three-phase groups.

Each group installs in a separate stator with a 30-degree spatial offset. Because the windings remain physically isolated, the system maintains operation even during partial faults.

Compared with traditional six-phase motors, this design:

• Improves fault tolerance

• Reduces eddy current losses by 25% in normal operation

• Reduces losses by up to 70% during fault conditions

Dual-Rotor Axial Switched Flux Motors

Another innovative design uses a dual-rotor axial switched flux permanent magnet motor.

In this structure:

• Each phase uses centrally located stator windings

• Permanent magnets adopt a spoke-type arrangement

• Two rotors operate in a misaligned configuration

This design improves fault tolerance while also reducing torque ripple. As a result, it meets the strict reliability demands of in-wheel drive systems.

Future Trends in Axial Flux In-Wheel Motors

Looking ahead, axial flux motor development will continue to focus on:

• Higher power density

• Better efficiency

• Wider speed range

• Stronger reliability

At the same time, advances in materials, cooling systems, and magnetic design will further improve performance.

In particular, lightweight stator cores, high-slot-fill windings, and advanced magnet arrangements will define next-generation in-wheel motors.

Conclusion: Axial Flux Motors Power the Next Generation of EVs

Axial flux motors offer a unique combination of compact structure, high torque density, and excellent efficiency. Because of these advantages, they play a critical role in modern in-wheel electric drive systems.

Through continuous innovation in magnetic circuits, winding design, and motor topology, engineers now push axial motors closer to their theoretical performance limits.

Overall, axial flux in-wheel motors represent a key technology for the future of electric vehicles. As electrification accelerates worldwide, these motors will continue to drive progress across the industry.