MOTOR-15 — Radial vs. Axial Flux Machines

FACT — While virtually all mass-market production electric vehicles utilize cylindrical Radial Flux Machines (RFM), a paradigm shift is underway in high-performance powertrain engineering. Driven by Ferrari (SF90 Stradale), McLaren (Artura), Koenigsegg (Gemera), and Mercedes-Benz’s acquisition of British innovator YASA, disc-shaped Axial Flux Machines (AFM) are redefining peak volumetric and gravimetric torque density.

In this lesson, we explore the two geometric paradigms of electric machine design, analyze the physics powering the axial flux revolution, and examine the mechanical hurdles of disc motor manufacturing:

1. Geometric Paradigms: Radial vs. Axial Field Alignments
2. Axial Flux Topologies (Torus, YASA, Double-Stator)
3. Why Axial Flux Yields 2–3x Higher Torque Density
4. Mechanical Challenges: Kilonewton-Scale Axial Attraction Forces
5. Soft Magnetic Composites (SMC) & 3D Flux Paths
6. Supercars, In-Wheel Drives, and Hybrid Transmission Packaging

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1. Geometric Orientations: Flux and Current Vectors

In any electrical machine, torque originates from the Lorentz force interaction ($\vec{F} = \vec{J} \times \vec{B}$) between the magnetic flux density ($\vec{B}$) and current-carrying armature conductors ($\vec{J}$).

RADIAL FLUX MACHINE (RFM):
  - Magnetic Flux (B): Flows radially (Center outward across cylinder)
  - Phase Current (J): Flows axially (Parallel along shaft length)
  - Lorentz Force (F): Acts tangentially (Circumference of cylindrical air gap)
  - Air Gap Geometry: Cylindrical tube / sleeve.

AXIAL FLUX MACHINE (AFM):
  - Magnetic Flux (B): Flows axially (Parallel to shaft axis)
  - Phase Current (J): Flows radially (Spokes moving from inner to outer radius)
  - Lorentz Force (F): Acts tangentially (Perpendicular disc radius)
  - Air Gap Geometry: Planar circular disc surface.

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2. Axial Flux Topologies

Axial flux machines are categorized by the relative arrangement of stator and rotor discs:

Axial Flux Topologies:
  a) Single Stator - Double Rotor (Torus / YASA):
       [ ROTOR DISC 1 ]  (Permanent Magnets)
       ----------------  (Air Gap 1)
       [ STATOR DISC  ]  (Segmented Tooth Coils, No Yoke)
       ----------------  (Air Gap 2)
       [ ROTOR DISC 2 ]  (Permanent Magnets)

  b) Double Stator - Single Rotor:
       [ STATOR DISC 1 ] (Armature Windings)
       ----------------  (Air Gap 1)
       [ ROTOR DISC   ]  (Central Magnet Disc)
       ----------------  (Air Gap 2)
       [ STATOR DISC 2 ] (Armature Windings)

YASA (Yokeless and Segmented Armature) Topology:

Pioneered by Dr. Tim Woolmer at Oxford University, the YASA topology eliminates the stator back-iron (yoke) completely:

• The stator consists of discrete, isolated pole teeth clamped in a lightweight structural carrier.
• Eliminating the back-iron slashes stator steel mass by $\approx 50\%$. Magnetic flux travels straight through from a magnet on Rotor Disc 1, across the tooth, and into the opposing magnet on Rotor Disc 2 in a clean axial line.
• Copper end-turns are virtually eliminated; every millimeter of copper actively contributes to torque production in the air gap.

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3. Why Axial Flux Delivers 2–3x Higher Torque Density

Electromagnetic torque equals magnetic shear stress ($\sigma$) multiplied by the air-gap surface area and moment arm radius.

Mathematical Derivation:

• Radial Flux Machine (RFM): Torque scales with cylindrical radius squared ($R^2$) and axial stack length ($L$): $T_{RFM} = 2\pi \sigma \cdot R^2 \cdot L$ Lengthening the machine increases stack length, but the moment arm radius remains fixed.
• Axial Flux Machine (AFM): Torque integrates across the disc from inner radius ($R_i$) to outer radius ($R_o$): $T_{AFM} = \frac{\pi}{4} \sigma \cdot B_{avg} A_{rms} (R_o^2 - R_i^2)(R_o + R_i)$

Optimal torque density occurs when the ratio of inner-to-outer radius is configured as $k = \frac{R_i}{R_o} \approx \frac{1}{\sqrt{3}} \approx 0.577$.

Key AFM Engineering Advantages:

1. Longer Moment Arm: All electromagnetic torque is generated at the maximum outer perimeter of the disc. Because torque is force multiplied by radius ($F \cdot r$), leverage is fundamentally optimized.
2. Exceptional Torque Density: While conventional automotive radial machines deliver $10 - 15\text{ Nm/kg}$, YASA axial flux machines routinely exceed $30 - 40\text{ Nm/kg}$!
3. Ultra-Slim Pancake Profile: Axial length is compressed down to $70 - 100\text{ mm}$. The entire machine packages like a brake disc between engine and transmission or inside wheel hubs.
4. Direct Thermal Heat Sinking: The open stator perimeter allows direct liquid cooling plates to contact stator winding faces with minimal thermal resistance.

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4. Mechanical Challenge: Kilonewton-Scale Axial Magnetic Pull

The greatest hurdle in axial flux machine manufacturing is the massive axial magnetic attraction force ($F_{axial}$) acting across the planar air gap.

Strong NdFeB magnets facing the high-permeability stator teeth across an air gap of merely $\approx 1\text{ mm}$ exert tremendous normal attraction governed by the Maxwell stress tensor:

$F_{axial} = \frac{B_g^2 \cdot A_{pole}}{2\mu_0}$

WARNING — In a typical $200\text{ kW}$ automotive axial flux motor, this axial clamping force reaches $15 - 30\text{ kN}$ ($1.5 - 3.0\text{ metric tons}$ of continuous pull)!

Any dynamic shaft deflection or bearing endplay can cause the rotating disc to physically contact the stator at $10,000\text{ rpm}$, resulting in catastrophic destruction. Precision angular contact or tapered roller bearings and ultra-rigid composite carriers are mandatory.

Herein lies the brilliance of the Single-Stator / Double-Rotor (YASA) architecture: Stator teeth are pulled equally in opposite directions by Rotor 1 and Rotor 2. The net magnetic attraction on the stator cancels out to zero, drastically relieving steady-state axial loads on motor bearings!

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5. Soft Magnetic Composites (SMC)

Standard radial machines laminate flat silicon steel sheets axially to handle 2D planar magnetic flux.

In axial flux machines, flux turns corners in complex 3D paths. Stamping and bending lamination sheets in 3D geometries is unviable and induces destructive eddy current loops. To overcome this, engineers utilize Soft Magnetic Composites (SMC):

• Microscopic pure iron powder grains individually coated in an insulating polymer barrier, compacted under extreme pressure ($800\text{ MPa}$) in precision dies.
• Provides isotropic 3D magnetic permeability in all spatial directions.
• Confines eddy currents at the micro-grain boundary, keeping iron core losses low even at high electrical switching frequencies.

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6. Automotive Powertrain Applications

The ultra-compact packaging of axial flux motors unlocks novel integration possibilities:

1. Supercars and Hypercars:
   • Ferrari SF90 Stradale: Dual axial flux motors on the front axle and one sandwiched between the V8 twin-turbo engine and transmission.
   • McLaren Artura: Axial flux motor integrated directly inside the 8-speed dual-clutch transmission bellhousing.
   • Koenigsegg Gemera / Dark Matter: Raxial flux e-motor producing $800\text{ Nm}$ in a $39\text{ kg}$ package.
2. Mercedes-AMG Dedicated EV Platform (AMG.EA): Mercedes acquired YASA to make axial flux motors the core propulsion standard for all future high-performance AMG EVs.
3. In-Wheel Direct Drives: Thin disc geometry fits entirely inside a 19-inch to 21-inch wheel rim, eliminating driveshafts, differentials, and reduction gearboxes.

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7. Summary

• Radial flux machines (RFM) produce torque along a cylindrical air gap via radial flux and axial current.
• Axial flux machines (AFM) produce torque across a planar disc via axial flux and radial current.
• Applying Lorentz force at the maximum disc radius unlocks 2–3x higher torque density ($>30\text{ Nm/kg}$) with an ultra-thin pancake form factor.
• YASA topology eliminates stator yoke iron, slashing machine mass while canceling opposing rotor magnetic pull forces.
• 3D magnetic flux paths mandate the use of Soft Magnetic Composites (SMC); AFM represents the ultimate high-torque solution for supercars, hybrid transmission integration, and next-generation dedicated performance EV platforms.

Next Lesson

• MOTOR-16 — Clarke and Park Transformations: The mathematical bedrock of Field-Oriented Control (FOC); transforming 3-phase AC time-domain currents into orthogonal rotating d-q coordinates.