AFPM Core Flux Path Design
Quick AFPM pole-flux and core-section screening.
Published 24 Sep 2026 · Reviewed 10 Oct 2026
Understanding Axial Flux Topologies
The magnetic return path depends on rotor pole pairing and stator architecture. In the simplified NN arrangement below, the stator carries a circumferential return flux through a yoke. Opposite poles across a yokeless stator tooth create a more direct axial tooth path. Actual machines also have rotor return paths, leakage, and fringing that this sketch does not model.
Axial Flux Path Topologies (NN vs NS)
NS (Yokeless) Arrangement
Opposite poles face across a segmented stator tooth. The tooth carries the main axial flux; the rotor yokes complete the magnetic circuit. YASA is one specific yokeless, segmented armature architecture, not a synonym for every NS arrangement.
- • No continuous stator yoke: This arrangement removes that stator section; total machine mass still depends on rotor yokes and support structure.
- • Packaging tradeoff: Removing the stator yoke can reduce stator mass and axial length, while the segmented teeth and rotor yokes still need structural and thermal support.
- • SMC option: SMC can support three-dimensional flux routes and net-shape teeth; grade-specific losses and compaction limits still need review.
- • Tradeoff: Rotor yoke sizing, tooth retention, cooling, and assembly become part of the same design decision.
NN (Yoked) Arrangement
Like poles face across the stator in this simplified NN arrangement. Flux returns circumferentially through the stator yoke toward an adjacent pole.
- • Stator yoke sizing: In a symmetric model, each circumferential return path carries about half the pole flux; confirm local peaks in FEA.
- • Material decision: Laminations can be a good fit when the flux follows the sheet plane and the geometry can be built without excessive joints or leakage.
- • Structure: A continuous yoke can simplify stator support, but the final load path depends on the housing and assembly.
What the first-pass sizing can tell you
For AFPM design engineers and OEM sourcing teams, use the result to compare a proposed core section with an initial flux target. It is not a topology optimizer or a release-ready magnetic design. AFPM sizing studies treat the inner-to-outer diameter ratio as one design variable among pole count, air-gap loading, winding, losses, and thermal constraints; the preferred ratio changes with those constraints. See the general sizing work by Mahmoudi et al. (2013) and a 2026 YASA split-ratio study that includes iron-loss and thermal constraints.
- Choose the split ratio with the whole machine: Di/Do defines annular active area, but the calculator does not optimize winding room, torque, cooling, rotor structure, or efficiency. Do not treat a geometry-only optimum or any single ratio as universal.
- Use material-specific flux limits: Bt and By are design inputs, not universal saturation values. Check the selected grade's B-H curve at the intended temperature and the manufacturer's loss data at the working flux density and frequency.
- Read the yoke result as a section estimate: it assumes each pole's flux divides equally into two circumferential paths and the yoke spans the full radial length. Local geometry, joints, and leakage can raise peak flux density.
- Match the material to the path and process: SMC enables compact three-dimensional flux routes; laminations can be attractive when flux stays in the sheet plane and the component can be assembled cleanly. Neither choice guarantees lower losses without a grade-specific comparison.
Screening method and assumptions
Flux and tooth section
Pole face area = annular area ÷ (2 × pole pairs). Estimated pole flux = mean effective Bg × pole face area. Required net tooth area = pole flux ÷ Bt.
Stator yoke section
For NN only, estimated net yoke thickness = half the pole flux ÷ (By × radial span). Convert net magnetic section to gross dimensions using the actual stacking or packing factor.
These equations assume uniform mean flux over a full annular pole face and ignore pole arc, leakage, fringing, harmonics, armature reaction, joints, and local three-dimensional saturation. Use a magnetic circuit model for early iteration, then validate the selected geometry with material curves and 3D FEA.
| Design condition | Likely starting point | Decision check |
|---|---|---|
| Flux mostly follows a sheet plane | Laminated electrical steel | Confirm flux direction, joint losses, and stack assembly. |
| Flux bends in three dimensions or needs net shape | Soft magnetic composite (SMC) | Check density, strength, coating, and grade-specific loss. |
| Loss performance is a key selection driver | Compare candidate grades | Compare W/kg at the same B, frequency, waveform, and temperature. |
| Core geometry or saturation is uncertain | Prototype both routes where practical | Validate with measured curves, FEA, and a thermal test. |
Example material data: one SMC grade
The figures below are one supplier's Somaloy® 700 1P data at 800 MPa compaction pressure, not generic values for all SMC. They show why a frequency-only crossover rule is unreliable: loss also changes with flux density and the exact material grade.
| Somaloy 700 1P data point | Value | Test context |
|---|---|---|
| Relative permeability, maximum | 540 | Supplier material data |
| B at H = 1,000 A/m | 1.56 T | IEC 60404-4 |
| Core loss at 1.0 T, 1,000 Hz | 126 W/kg | Ring sample; IEC 60404-6:2003 |
Sources and limits
- Höganäs, Somaloy® 1P material data — specific magnetic properties and core-loss measurements used in the example table above.
- Mahmoudi et al., AFPM sizing and prototyping (2013) — general-purpose sizing equations and design variables.
- Zhu et al., YASA split-ratio optimization under thermal constraints (published 19 May 2026) — shows the optimum shifts when iron loss and thermal limits are included.
- Höganäs, coated powders for electromagnetic applications — supplier overview of SMC and three-dimensional flux capability.
Sources checked 10 October 2026. Supplier figures apply only to the named grade and stated test conditions. Compare candidate materials at the same flux density, frequency, waveform, and temperature; use FEA and testing for release decisions.
Prepared by AxialFluxCore Engineering.
Need a manufacturability review?
Share the topology, material grade, and section assumptions with engineering before you release a core drawing.
Related AFPM core design guides
Frequently Asked Questions
Is there one optimal Di/Do ratio for an AFPM core?
No. Di/Do affects active area and the available radial space, but a useful value depends on topology, winding layout, torque, losses, cooling, and package limits. The calculator does not prescribe a preferred ratio.
What should I use for the tooth and yoke flux-density limits?
Use a design limit informed by the selected grade's B-H curve, temperature, waveform, and loss target. The calculator does not choose a material limit; the entered values are assumptions to compare against the supplier's data and FEA results.
Can laminated steel work in a yokeless stator?
It can, depending on flux direction, lamination orientation, segmentation, insulation, and assembly. Compare the buildable laminated design with SMC using measured loss data and a review of mechanical retention and joints.
Does SMC always have lower loss above a fixed frequency?
No general crossover frequency applies to every SMC and steel grade. Core loss depends on flux density, frequency, waveform, temperature, processing, and sample geometry. Compare supplier curves under matching conditions; the Somaloy example above is for one grade and test setup.
Is the calculator output ready for a production drawing?
No. It is a uniform-flux section estimate. Confirm pole arc, leakage, tooth geometry, winding, local saturation, material loss, temperature, and structural limits with a magnetic model, 3D FEA, and prototype testing before releasing tooling or drawings.
What does available pole area mean in the result?
It is the full annular area between the entered diameters divided by the total pole count (2 × pole pairs). This approximation assumes each pole pitch spans its share of that annulus; actual pole arc and winding geometry can reduce the active area.
Validate Your Core Design
Transitioning from theoretical flux path calculation to a manufacturable stator requires deep materials expertise. Contact our engineering team for SMC pressing feasibility or custom laminated core prototyping.