Flow of Axial Flux Motor Winding: Process Chain, Winding Method Comparison, Tension Control, and the Stator Core Interface

Axial flux motor winding is not radial flux winding rotated ninety degrees. The stator slot geometry in an axial flux machine is asymmetric, nonlinear, and often extremely narrow. Published production analysis describes the consequence directly: the wire movement path is complex, which easily leads to violent fluctuations in wire tension, and traditional winding equipment faces difficulty in path planning, wire drawing, or damage to the insulation layer caused by those fluctuations. That is the bottleneck that separates a bench-wound prototype from a production-ready stator.

The published process chain for axial flux winding is specific enough to audit. IEEE research develops and tests a sequence that starts with selecting configurations and creating variants with strand diameters between 0.15 mm and 0.5 mm, compares two winding methods — air coils and core coils — using specifically designed tools, uses bondable wires to secure coil geometry, evaluates oven heating versus resistive heating for fixation, compares round versus rectangular litz wire, assesses coil geometry with 3D scans, and performs breakdown voltage tests to evaluate insulation damage incurred during winding. The study concludes that a core winding process is optimal for producing litz wire windings, providing better and more consistent results. That conclusion is the starting point for a procurement specification, not a footnote.

This guide walks through the flow of axial flux motor winding stage by stage, with the published data at each stage: the winding method comparison, the strand-diameter effect, the geometry-fixation routes, the tension-control requirement, the copper fill factor target, and the stator core parameters that set the physical boundary the winding must work within. It closes with a five-step audit for procurement engineers who need to qualify a winding process rather than a winding diagram.

What Is the Flow of Axial Flux Motor Winding? The Definition and Why It Differs from Radial Winding

The flow of axial flux motor winding is the sequence of operations that places enameled or litz wire into the stator slots of an axial flux machine in a defined pattern, secures the coil geometry, and verifies that the insulation survived the process. It is not a sub-category of radial winding. Published production analysis explains the structural difference: compared with radial motors, the axial flux stator groove usually presents asymmetric and nonlinear special-shaped characteristics, and the winding space is extremely narrow. In pursuit of high performance, manufacturers often require extremely high coil fill rates, which means enameled wires must be placed in near-perfect precision arrangement in a limited space.

That geometry creates a specific failure chain. The complex wire path leads to violent tension fluctuations, which cause the enameled wire to be thinned or the insulating film to be damaged, which creates a turn-to-turn short risk, which may not appear until the motor has operated under thermal load. Published fault analysis of axial flux machines confirms the consequence: an interturn short circuit removes a turn from the stator winding, induces an emf in the shorted turn, and the resulting fault current is limited only by the self-impedance of the fault — which essentially determines the transition time between turn and groundwall insulation failure. The winding flow is therefore not a handling sequence; it is the process that determines whether the insulation barrier survives long enough to reach the field.

Winding Method Classification: Air Coil vs Core Coil, Litz vs Solid, Round vs Rectangular

Air coil versus core coil

Sources: IEEE litz wire winding production study for axial flux machines (air coils vs core coils; core winding process optimal, better and more consistent results); RWTH Aachen REFORMAT project (direct winding onto stator core eliminates subsequent air-coil installation).

The method choice has a direct procurement consequence. A supplier who winds air coils and transfers them is adding a handling step that the published comparison identifies as less consistent for litz wire. A supplier who winds directly onto the core has adopted the published optimum but must demonstrate that the winding tool can reach the axial flux slot geometry without damaging the core’s coating. The RWTH Aachen REFORMAT project pursues the core-coil route specifically because it eliminates the subsequent installation of an air coil and enables the use of profiled stranded wires, which can increase motor efficiency depending on the application.

Litz wire versus solid wire, round versus rectangular

Published IEEE research evaluates litz wire for axial flux because litz windings reduce AC losses in traction motors, but notes that similar investigations for axial flux machines were lacking. The study creates variants with strand diameters between 0.15 mm and 0.5 mm and concludes that strand diameters significantly impact coil geometry and robustness. It also assesses whether forming round litz wire into rectangular shapes is necessary given the increase in cost, and concludes that for production methods including pressing steps, the forming process of round litz wires could be eliminated — although potential decreases in dimensional accuracy and increased stress on winding insulation need to be addressed. That is a direct trade between cost, geometry accuracy, and insulation stress that belongs in the winding specification.

Published Parameters by Stage: Strand Diameter, Tension, Wire Diameter, Speed, Fill Factor, and Breakdown Voltage

Sources: IEEE litz wire winding production study for axial flux machines (strand diameters 0.15–0.5 mm, bondable wire, oven vs resistive heating, 3D scan, breakdown voltage); published axial flux winding machine specification (wire φ0.50–1.5 mm, speed 0–500 RPM, stack 10–50 mm, stator OD φ150–450 mm); production analysis (dynamic constant-tension closed-loop, milliNewton feedback); published PCB and flat-copper fill data (60–75% coreless, 93%→96% with flat copper).

Tension control: the parameter that decides whether the insulation survives

Published production analysis describes the mechanism and the solution. The special stator groove shape makes the wire movement path complex, which easily leads to violent tension fluctuations. The production answer is a highly responsive ultra-low-inertia servo pay-off combined with a milliNewton-level closed-loop feedback system that monitors and accurately controls tension fluctuation across the whole winding process in real time. That level of precision prevents the enameled wire from being thinned or the insulating film from being damaged, ensuring that the electromagnetic performance of each turn is highly consistent. A multi-axis linkage path-planning algorithm then controls the landing point of the enameled wire in the special-shaped slot, producing a neat, shingle-like arrangement that raises the effective fill area.

Fill factor: the copper-density outcome

Published data places copper fill rate in coreless axial flux designs at 60–75%. For conventional wound stators, rectangular flat copper coils raise slot fill factor, lower copper loss, and improve heat dissipation, enabling low temperature rise and operational efficiency from 93% to 96%. Distributed winding is described as unbeneficial for axial flux because the larger diameter produces very long end windings that increase copper loss, which is why concentrated winding is the standard. The fill factor and the winding topology together determine how much of the available slot area carries current and how much heat that current generates.

Application Profiles: How the Winding Flow Changes for Drone, UAV, and eVTOL Programmes

The pattern is that the binding parameter tracks the mission’s dominant winding failure mode. FPV and high-speed programmes bind on strand diameter and tension control because fine wire and high electrical frequency amplify AC loss and insulation stress. Heavy-lift and logistics programmes bind on fill factor and copper loss because continuous torque depends on copper volume. eVTOL and agricultural programmes bind on geometry consistency and insulation integrity because the cost of a field failure is high and the duty cycle is long. Prototype programmes can accept air coils for iteration speed, but the transfer step and geometry inspection must be documented.

A 5-Step Axial Flux Winding Process Audit for Procurement Engineers

1. Require the winding method to be named, and compare against the published optimum. Ask whether the supplier uses air coils, core coils, or a hybrid. Published IEEE research concludes that core winding is optimal for litz wire axial flux stators, providing better and more consistent results. A supplier using an air-coil process for a litz-wire production stator has not adopted the published optimum, and the quotation should reflect that the transfer step and its geometry risk are still in the process.
2. Specify the strand diameter and the wire construction, and require the supplier to state the impact. Published data evaluates strand diameters between 0.15 mm and 0.5 mm and concludes that strand diameter significantly impacts coil geometry and robustness. Name the strand diameter, the number of strands, and whether the wire is round or rectangular. Ask how the supplier compensates for the cost-versus-accuracy trade if round litz is used without forming into rectangular shapes.
3. Audit the tension-control system, not just the tension setting. Ask for the feedback resolution (published production analysis specifies milliNewton-level closed-loop feedback), the response time, and how the system handles the asymmetric, narrow axial flux slot geometry. Ask for the path-planning method that controls the landing point of the wire in the special-shaped slot. A supplier who quotes a tension range but not a feedback resolution has not specified the parameter that prevents enamel damage.
4. Require the geometry-fixation route and the post-winding verification to be stated. Ask whether the supplier uses bondable wire, oven heating, or resistive heating to secure coil geometry, and whether the finished coil is assessed by 3D scan. Ask for the breakdown voltage test that evaluates insulation damage incurred during winding. A supplier who performs neither the 3D scan nor the breakdown voltage test is shipping a coil whose geometry and insulation are assumed rather than verified.
5. Specify the stator core parameters that set the winding boundary, and run a trial wind before committing. Name the core construction route (SMC density or lamination grade), core loss at the operating frequency, coating dielectric strength, and flatness and parallelism of the air-gap faces. Reenives publishes radial flux stator cores at 0.20 mm silicon steel, core loss ≤9.5 W/kg at 1.0 T / 400 Hz per IEC 60404-2, B50 1.65–1.72 T, stacking factor ≥97%, concentricity ≤0.02 mm, burr <0.03 mm, and C5 epoxy coating at >1400 V per ASTM D149 — the same value-plus-standard-plus-location discipline applies to axial flux, with flatness and parallelism added and the material route named explicitly. Then send production cores and production wire for a trial wind, and inspect the fill factor, the coil geometry, and the breakdown voltage result before volume commitment.

FAQ: Process Stages, Air vs Core Coil, Tension Control, Wire Diameters, Geometry Fixation, Fill Factor, and Core Interface

Summary: The Winding Flow Is the Yield

The flow of axial flux motor winding is a four-stage chain — configuration selection, winding method execution, coil geometry fixation, and post-winding verification — and each stage carries a published control point. Published IEEE research concludes that core winding is optimal for litz wire axial flux stators, providing better and more consistent results, and that strand diameters between 0.15 mm and 0.5 mm significantly impact coil geometry and robustness. Published production analysis places the tension-control requirement at milliNewton-level closed-loop feedback, because the asymmetric, narrow axial flux slot geometry causes violent tension fluctuations that thin the enamel or damage the insulating film. And published fill data places copper fill at 60–75% in coreless designs, with flat copper raising efficiency from 93% to 96%.

The procurement takeaway is that the winding flow is the yield. A supplier who can name the winding method (core coil), the strand diameter and its geometry impact, the tension-control feedback resolution, the geometry-fixation route, the 3D scan assessment, and the breakdown voltage test is describing a production process. A supplier who quotes only a turn count and a wire gauge is describing a design and leaving the insulation, the geometry, and the fill factor to the winder’s default. For a drone programme where a turn-to-turn short becomes a field failure, that distinction is the whole procurement decision.