Axial-Flux Permanent Magnet Motors

Axial-flux motors are emerging as one of the most important electric motor technologies for high-performance EVs. Compared with conventional radial-flux motors, they can deliver very high torque and power from a much thinner package. This makes them especially attractive for sports cars and performance EVs, where space, weight, thermal stability, and repeated acceleration matter as much as peak output.

An axial-flux permanent-magnet motor, often shortened to AFPM motor, uses the same basic electromagnetic principle as a conventional Permanent Magnet Synchronous Motor (PMSM), but arranges the magnetic circuit differently. Instead of building the motor as a long cylinder, an axial-flux motor is shaped more like a thin disc.

A simple way to picture the difference is this: a radial-flux motor works like a cylinder, while an axial-flux motor works like a pair of magnetic discs facing each other.

How It Works

In a conventional Permanent Magnet Synchronous Motor (PMSM), the main magnetic flux path runs perpendicular to the axis of rotation. In an axial-flux motor, the main flux path runs parallel to the axis. The result is a motor shaped like a thin disc rather than a long cylinder.

Rotor and stator layout: The core components of an AFPM motor are flat discs. The most common layout in high-performance automotive use is the H-configuration, also called the yokeless-and-segmented-armature (YASA) topology. Two rotor discs carrying permanent magnets sit on either side of a central stator disc, sandwiching it from the left and right.

Magnetic circuit: The permanent magnets on the two rotor discs face the stator from both sides at the same time. Magnetic flux passes directly from one rotor, through the stator winding, and into the opposing rotor. This creates a short, low-loss magnetic path. There is no conventional back iron behind the magnets, which is why this topology is called "yokeless".

Stator winding: The stator consists of individually wound segments arranged in a ring around the axis. Each segment is a separate coil. This simplifies winding, cooling, and assembly compared with a continuous radial stator, but it also requires precise manufacturing and control of tolerances.

Commutation and control: Like any synchronous motor, an AFPM motor requires electronic commutation. A high-performance inverter feeds three-phase alternating current to the stator coils. Position feedback from a resolver or encoder allows field-oriented control to keep the stator field aligned with the rotor magnets.

Torque production: Because both rotor faces contribute torque simultaneously and the magnets sit close to the outer diameter of the disc, the effective lever arm is large for a given motor mass. This is the main reason AFPM motors can achieve very high torque density.

Why It Matters in EVs

For EV manufacturers, the main attraction of axial-flux motors is not only high peak power. The thin motor shape can make the entire drivetrain easier to package, especially in performance cars with multiple motors. It also allows engineers to place more of the active magnetic material farther from the axis of rotation, increasing torque without making the motor longer.

For drivers, the benefit is felt as strong launch performance, fast torque response, and better repeatability during hard driving. The technology is therefore most likely to appear first in expensive performance EVs before becoming common in mainstream models.

Axial-flux motors are especially useful when a manufacturer wants to combine high output with compact packaging. This is why the technology is attractive for performance EVs, electric sports cars, hybrid hypercars, and applications where drivetrain length is limited.

Axial-Flux vs Radial-Flux Motors

Advantages

High torque and power density: An AFPM motor can deliver more torque per kilogram and per litre than a radial-flux PMSM of comparable output. The short flux path and large active diameter let the motor produce strong torque from a very compact package. In the Mercedes-AMG GT 4-Door Coupé, the axial-flux motors are only a few centimetres wide, yet each motor can deliver several hundred kilowatts depending on the application.

High continuous power potential: With the right direct-cooling design, an AFPM motor can sustain a high fraction of its peak power for longer than many compact radial-flux machines. This makes the technology well suited to applications that demand repeated full-load acceleration, such as track driving, where a conventional motor may need to reduce output as temperatures rise.

Compact packaging: The thin disc form factor frees up space along the drive axle. This is useful for drivetrains where two motors share a common axle, or where the motor must fit between the battery pack and the wheels without lengthening the wheelbase.

Lower rotor inertia: A disc-shaped rotor can have lower rotational inertia than a long cylindrical rotor. The motor can respond quickly to torque commands, contributing to sharp throttle response and precise power delivery.

Potentially high efficiency: The short magnetic flux path can reduce some losses, and the compact winding layout can help reduce copper use. However, real-world efficiency depends on the complete motor design, inverter, cooling system, and operating point. Axial-flux motors are not automatically more efficient in every driving situation.

Limitations

Rare earth materials: Like most high-performance permanent-magnet motors, an AFPM motor uses rare earth magnets such as neodymium iron boron (NdFeB) or samarium cobalt (SmCo). These magnets are expensive, and the supply chain is concentrated in a limited number of countries.

Mechanical complexity: The twin-rotor H-configuration applies large axial forces to the stator from both sides. The motor housing has to absorb these forces without allowing the air gap between rotor and stator to change. This raises the bar for bearing design, housing stiffness, assembly accuracy, and manufacturing tolerances.

Heat path through the stator disc: A radial-flux motor can cool its stator through a water jacket around the outside of the housing. In an AFPM motor, the stator is sandwiched between two rotors, so cooling has to reach the stator coils directly. High-performance designs typically use direct oil cooling sprayed onto or through the segmented coils, which adds plumbing, pumps, sealing requirements, and thermal-management complexity.

Cost and scalability: AFPM motors are still less mature in mass production than conventional radial-flux motors. The segmented stator, precise air-gap control, magnet placement, and direct-cooling system can increase cost, especially before production volumes rise.

New manufacturing processes: Stamping rotor discs, aligning the magnets, winding the segmented stator, and controlling the very small air gaps require production techniques that have not traditionally been part of mainstream electric motor manufacturing. Mercedes-Benz has reported that around 65 of the roughly 100 production steps used to build its axial-flux motors at the Berlin-Marienfelde plant are new, with many described as world firsts.

Drag when free-rolling: Like all permanent-magnet motors, an AFPM motor can produce drag when the rotor is spinning without power, because the magnets continue to induce voltage in the stator coils. Some EVs reduce this loss by disconnecting the motor mechanically when it is not needed, especially on a secondary axle.

Use in Production Electric Vehicles

The Mercedes-AMG GT 4-Door Coupé is one of the first series-production battery-electric cars to use axial-flux motors as its main traction motors. The car uses three axial-flux motors developed by YASA: one on the front axle and two on the rear axle, packaged into a rear High-Performance Electric Drive Unit and a front drive unit. In the GT 63 variant, peak system output is up to 860 kW.

YASA is a British electric motor specialist acquired by Mercedes-Benz in July 2021. Its axial-flux technology has been developed for future Mercedes-AMG performance EVs, where high power density, compact packaging, and repeatable performance are central design goals.

Other high-performance manufacturers have also shown interest in axial-flux designs. Ferrari has signposted axial-flux motor technology for its Elettrica programme, while Koenigsegg uses the compact Quark axial-flux motor in the Gemera. These applications show where the technology currently makes the most sense: expensive, performance-focused vehicles where compact size and high output justify the added complexity.

Summary

Axial-flux motors offer a different way to package an electric traction motor. By arranging the magnetic flux along the axis of rotation rather than across it, they can deliver very high torque and power from a thin, disc-shaped package.

The technology is not a universal replacement for radial-flux motors. It brings challenges in cooling, manufacturing, cost, air-gap control, and rare earth magnet use. However, for high-performance EVs, the advantages are significant. Axial-flux motors allow engineers to package more power into less space, improve torque response, and support repeated high-load driving.

For this reason, axial-flux motors are likely to become an important technology in the next generation of electric performance cars before gradually spreading into broader EV segments as production scales and costs fall.

Learn more

Watch this Munro Live video for a deeper look at axial-flux motor design and manufacturing.