AFPM Core Design Parameters
Screen core geometry with ID, OD, axial length, and pole count. Review the annular face area, envelope mass, and dimensional ratios, then use the evidence and validation limits below to plan the next engineering checks.
Published Sep 23, 2026 · Reviewed Oct 1, 2026
Core sizing conclusions for OEMs
Geometry screens and source-bounded design evidence. None of the cited examples defines a universal AFPM optimum.
Worked example: the prefilled geometry
For OD 200 mm, ID 120 mm, axial length 50 mm, and 10 poles, the calculator produces these reproducible geometric estimates.
- ID/OD ratio
- 0.600
- Core L/OD
- 0.250
- Gross annular face area
- 201.1 cm²
- Envelope volume
- 1,005.3 cm³
- Envelope mass at 7.4 g/cm³
- 7.44 kg
- Mechanical pole pitch
- 50.3 mm
These values follow the equations below. Area and mass are envelope estimates: subtract actual slots and features in CAD, and replace the assumed density with the selected material grade before using mass in a sourcing or structural decision.
Methodology and core envelope
Enter OD, ID, and L in millimeters (mm)
Ratio K_r = ID / OD
Core L/OD = Axial length / OD
Gross ring area = π × (OD² − ID²) / 4
Mean-radius pole pitch = 2π × ((OD + ID)/4) / Poles
Mass (kg) = volume (mm³) × density (g/cm³) × 10⁻⁶
These equations describe dimensions and an ideal annular envelope; they do not calculate electromagnetic torque or effective air-gap area. The cited 0.5–0.65 ID/OD range is from one 2008 high-speed axial-flux generator study optimizing voltage and efficiency. It is a study-specific comparison, not a general motor or torque-density target.
FocusSMC presents ID = 0.6 × OD and L/D = 1:4 in a vendor comparison with equal volume and air-gap area, and claims a 25% torque increase or 25% weight reduction. Its public page does not provide enough machine-specific test or model data to generalize those figures. JMAG/Höganäs documents a separate 10-pole, 12-slot SMC design example; that pairing is not a universal recommendation.
Assumptions & boundaries: Volume and mass use a solid annular envelope with an assumed SMC density of 7.4 g/cm³. Slots, teeth, magnet coverage, rotor geometry, shaft, bearings, and housing are excluded. Confirm density against the selected grade and use CAD/FEA for component mass, magnetic performance, thermal limits, and structural deflection.
Parameter interpretation and validation limits
Use the outputs to compare candidate envelopes. Confirm each one against the electromagnetic design, winding, material, thermal, and structural models before freezing dimensions.
Checks before freezing core dimensions
Turn the screening geometry into a design review using the actual duty point, material, assembly, and supplier process.
01Electromagnetic performance
Evaluate flux density, torque and voltage targets, losses, and ripple across the operating range with the selected winding and magnet geometry.
02Thermal limits
Use the duty cycle, measured or simulated losses, cooling path, insulation system, and winding hot spots to check continuous operation.
03Mechanical clearance
Check axial attraction, rotor and stator support stiffness, bearing compliance, runout, and the minimum air-gap clearance in the assembled system.
04Manufacturing and inspection
Review the selected material grade, press/tool access, winding access, critical datums, tolerances, and inspection plan with the manufacturing supplier.
Review the geometry against your operating target
Share the envelope, duty point, material grade, and topology to plan the next electromagnetic, thermal, and structural checks.
Discuss this core geometryFrequently asked questions
What is the most critical parameter in AFPM core design?
ID/OD is an important geometry variable, but there is no universal optimum across AFPM topologies and operating targets. The 0.5–0.65 band cited here comes from one high-speed generator study that optimized voltage and efficiency. Use it as a comparison point, then evaluate your own electromagnetic, winding, thermal, and mechanical constraints.
Why does the tool show an estimated mass that differs from my CAD?
This calculator assumes a solid annulus (ring) of material with a typical Soft Magnetic Composite (SMC) density of 7.4 g/cm³. It does not subtract the volume of the winding slots or account for specific material grades. Always use your CAD software for exact mass calculation.
How does the number of poles affect the core dimensions and SMC manufacturing?
Pole count affects electrical frequency and the available pole pitch, but the result depends on winding, speed, losses, ripple, and geometry. A JMAG/Höganäs study models one 10-pole, 12-slot SMC concept; it should not be treated as a default recommendation.
Can I design an AFPM with an ID/OD ratio outside 0.5 - 0.65?
Yes. Ratios outside 0.5–0.65 are not inherently invalid; that range is from one high-speed generator study. Check inner-bore packaging, winding space, active geometry, cooling, and the performance objective for the actual design.
What is the recommended Length-to-Diameter (L/D) aspect ratio?
There is no single ratio that guarantees acceptable stiffness. A vendor comparison presents 1:4, but rotor/stator support, axial magnetic force, clearances, and the full assembly determine deflection. Run a structural check for the selected geometry.
Does the gross annular face area equal the effective air-gap area?
No. It is the area of a continuous ring calculated from ID and OD. Slots, teeth, magnet coverage, leakage, pole arcs, and the opposing rotor geometry are not included, so use the electromagnetic model to determine effective active area.
Is the displayed pole pitch the same as slot pitch?
No. The calculator reports mechanical circumferential pole pitch at the mean radius, using the entered pole count. Slot pitch depends on slot count and geometry; neither value alone determines winding factor or torque ripple.
Can I use the estimated core mass in an RFQ or cost model?
Use it only as a rough envelope check. The estimate assumes a solid annulus and 7.4 g/cm³ density, so it excludes slots and detailed features and may not match the selected material grade. Use CAD mass and grade-specific data for quotation and cost decisions.
Continue your design validation
Evidence and references
Sources and claims reviewed on 2026-10-01. Numerical examples are scoped to the cited study, vendor comparison, or simulated design; confirm applicability to your specific topology.
- Kahourzade et al. — A Comprehensive Review of Axial-Flux Permanent-Magnet Machines
Peer-reviewed overview of AFPM construction, modelling, sizing, materials, and manufacturing. It is background context, not a source for a universal preferred ID/OD or L/D value.
- Sadeghierad et al. — Design Analysis of High-Speed Axial-Flux Generator (2008)
This high-speed generator study reports an ID/OD range of 0.5–0.65 for its voltage and efficiency objectives. It does not establish a general motor or torque-density optimum.
- FocusSMC — Axial Flux Machines geometry comparison
A vendor-published comparison presents L/D = 1:4 and ID = 0.6 × OD for equal-volume/equal-air-gap-area examples, claiming a 25% torque increase or 25% weight reduction. Treat it as a vendor example; the page does not provide enough machine-specific data to generalize the claim.
- JMAG / Höganäs — Double-Sided Axial Flux Machine SMC application example
A simulated double-sided axial-flux SMC concept uses 12 slots and 10 poles and selects the combination for its design objectives, including ripple. It is one example, not evidence that this combination is universally preferred or easier to press.
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