Magnetic Loss Testing in High-Frequency Axial Flux Motors

Executive Summary

• The High-Frequency Challenge: Modern AFPM motors operate between 800 Hz and 2,000 Hz, causing massive iron losses in traditional laminated steel cores due to $f^2$ eddy current scaling.
• The Crossover Point: At frequencies above 400-600 Hz, Soft Magnetic Composites (SMC) surpass laminated steel in thermodynamic efficiency.
• Validation Requirement: OEM procurement teams must demand batch-specific B-H curves, W/kg loss data, and density reports (>7.3 g/cm³) at their exact operating frequency to validate SMC stator suppliers.

As OEMs push for higher power densities in electric vehicles, drones, and robotics, motor designers are adopting Axial Flux Permanent Magnet (AFPM) topologies and driving them at increasingly high electrical frequencies. It is not uncommon to see modern axial flux machines operating at 800 Hz, 1,000 Hz, or even up to 2,000 Hz in aviation applications.

While high-frequency operation allows the motor to shrink in size while maintaining power output, it introduces a severe physical penalty: massive iron losses in the stator core. This technical review examines how high-frequency magnetic losses scale, and why evaluating Soft Magnetic Composites (SMC) via rigorous loss testing is an absolute necessity for procurement and engineering teams.

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The Iron Loss Equation

Total iron loss (core loss) in a magnetic material is generally modeled using the Steinmetz equation, broken down into three physical phenomena:

1. Hysteresis Loss ($P_h$): Energy lost due to the continuous realignment of magnetic domains within the material as the magnetic field alternates. This scales linearly with frequency ($f$).
2. Eddy Current Loss ($P_e$): Energy lost as heat caused by circular electrical currents induced in the core material by the changing magnetic field. This scales with the square of the frequency ($f^2$).
3. Anomalous Loss ($P_a$): Related to magnetic domain wall dynamics, scaling roughly with $f^1.5$.

Because eddy current losses scale geometrically ($f^2$), they quickly become the dominant source of heat, inefficiency, and thermal throttling in high-speed, high-pole-count axial flux motors.

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Laminated Steel vs. SMC at High Frequencies

The Limitation of Laminations

To mitigate eddy currents, traditional stators are built by stacking thin sheets of electrical steel separated by an insulating coating. At standard industrial frequencies (50/60 Hz), 0.35 mm or 0.50 mm laminations are highly efficient.

However, as frequencies approach 1,000 Hz, designers must use ultra-thin laminations—0.20 mm or even 0.10 mm—to keep eddy currents from generating catastrophic heat.

• The Problem: Stamping and stacking 0.10 mm steel is notoriously difficult, expensive, and fragile. The stamping process also induces mechanical stress that damages the grain structure at the cut edges, severely degrading magnetic permeability in small stators.

The SMC Advantage

Soft Magnetic Composites (SMC) bypass the lamination problem entirely. SMC consists of high-purity iron powder particles, where each individual particle is coated with an inorganic microscopic layer of electrical insulation before being compacted under 800 MPa into a solid 3D shape.

Instead of insulating macroscopic sheets of steel, SMC insulates at the microscopic particle level. This restricts eddy currents to the diameter of a single iron particle (typically 50 to 150 microns).

Consequently, the eddy current loss component in SMC is incredibly low and remains linear-like even well beyond 1,000 Hz.

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How We Validate Magnetic Loss in Production

At AxialFluxCore, we don't just rely on theoretical material data sheets; we conduct rigorous, batch-level magnetic loss validation to ensure our SMC stators meet OEM specifications before shipping.

1. Toroid Testing (IEC 60404-6 Standard)

The baseline for SMC magnetic validation is the Toroid test. We press SMC powder into a standard ring specimen alongside the production batch. We wrap the ring with primary and secondary copper windings and apply a high-frequency AC field.

By measuring the B-H curve and integrating the area under the hysteresis loop, we can precisely calculate the total core loss (measured in W/kg).

• Typical Factory Target: For a high-density SMC grade at 1,000 Hz and 1.0 Tesla, we target core losses between 45 W/kg to 65 W/kg, dramatically outperforming standard 0.35mm silicon steel at the same frequency.

2. Density and Permeability Correlation

Because SMC properties are highly dependent on the compaction process, a poorly compacted part with low density will exhibit higher hysteresis loss and lower relative permeability ($\mu_r$).

• Factory Benchmark: We strictly monitor green density (aiming for >7.4 g/cm³) using Archimedes' principle to ensure the relative permeability ($\mu_r$) consistently hits the 400 to 600 range required by high-performance AFPM designs.

3. Insulation Integrity Testing (Resistivity)

If the pressing pressure is too high, or the tooling design is flawed, the microscopic insulation layer on the powder particles can rupture, causing "particle-to-particle" electrical shorting. This instantly spikes the eddy current losses. We perform localized electrical resistivity checks (targeting >1,000 μΩ·m) to verify that the insulation matrix remains fully intact post-compaction and post-curing.

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Designing for the "Crossover Point"

At very low frequencies (e.g., 50 Hz), laminated steel typically exhibits lower overall losses than SMC because steel has a lower base hysteresis loss. However, as frequency increases, the $f^2$ scaling of eddy currents in steel causes its total loss curve to skyrocket.

The "crossover point"—the frequency at which SMC becomes more efficient than laminated steel—typically occurs between 400 Hz and 600 Hz. For most modern AFPM applications (eVTOL, humanoid robotics, drone propulsion), the operating frequency sits comfortably above 800 Hz, making SMC the clear thermodynamic and efficiency winner.

Validation Checklist for Buyers

If your engineering team is grappling with thermal management issues in a high-frequency axial flux motor, your stator material is likely the bottleneck. When sourcing SMC cores, ask your supplier for:

1. Batch-specific B-H curves and W/kg loss data at your exact operating frequency.
2. Verified density reports (>7.3 g/cm³) across the thickest section of the stator.
3. Proof of resistivity to ensure the insulation layer survived compaction.

Frequently Asked Questions

Q: Can we use high-density SMC for frequencies above 1,500 Hz? A: Yes. While 1,500 Hz introduces significant thermal challenges for the copper windings, the SMC core itself maintains a highly linear eddy current response well beyond 2,000 Hz, provided the compaction density exceeds 7.4 g/cm³.

Q: Do you provide B-H curves specific to our operating temperature? A: Absolutely. We conduct toroid testing at room temperature and elevated operational temperatures (e.g., 150°C) to ensure the magnetic permeability meets your thermal constraints before mass production.

Reach out to our sales engineering team to discuss your frequency targets, and we will provide detailed SMC iron loss curves and sample validation data to support your next prototype phase.