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Subtitle: From laminated silicon steel to laminated magnets — why high-speed, high-pole-frequency rotors must be "sliced open"
The trend toward higher speed and higher power density unites EV, aerospace, high-speed spindles, and robotics. But as frequency climbs, an old enemy surfaces: eddy currents. The solid silicon steel and solid NdFeB inside the rotor, under a high-frequency alternating field, become heat generators — eating efficiency and pushing the magnet toward demagnetization.
The solution almost every efficient high-frequency motor relies on is two words: lamination. This article explains why the laminated rotor (laminated silicon steel + laminated magnets) can hold down eddy-current heating and lift efficiency at high frequency.
The physics of eddy currents isn't complicated. A conductor in a changing magnetic field induces closed-loop currents (eddy currents), and those currents produce I²R losses and heat on the conductor's resistance. The trouble is that eddy severity rises fast with frequency — the higher the field alternation frequency, the larger the flux-change rate dB/dt, and the stronger the induced EMF and eddy currents.
More critical is the skin effect: eddy currents tend to concentrate near the conductor surface, and the skin depth δ is set by δ = √(2ρ / (ωμ)) (ρ resistivity, ω angular frequency, μ permeability). Higher frequency, shallower δ; current gets squeezed into a thin surface layer, and local current density and heating shoot up.
For the motor rotor, two field components amplify this effect: the fundamental frequency (speed × pole pairs), and PWM harmonics and slot harmonics — these high-frequency components make the rotor-surface dB/dt far exceed the fundamental. So a high-frequency motor's rotor eddy heating is often far worse than a fundamental-only estimate suggests.
Laminating the rotor and stator cores with silicon steel is among the most mature techniques in motor engineering. One sentence for the principle: slice the solid core into thin sheets parallel to the flux direction, insulate between sheets, and the eddy-current closed loops are cut off.
The eddy loop size scales with conductor thickness, and the loss rises steeply with thickness (in standard lamination theory, the per-sheet eddy loss P is proportional to the square of lamination thickness d²). Slicing the core from solid into 0.1–0.5 mm sheets suppresses eddy loss on a squared basis.
How to choose thickness? Follow the skin depth. In silicon steel, the skin depth varies with frequency: at 50/60 Hz power frequency δ is several millimeters, so 0.35–0.5 mm lamination suffices; but at kHz-range high frequency, δ drops to 0.15–0.3 mm, and lamination must thin to 0.1–0.2 mm to contain eddy currents. This is why high-speed, high-frequency motor laminations are far thinner than ordinary power-frequency motors.
Insulation and burr control. The inter-lamination insulation coating (inorganic/organic) must be complete — if sheets short to each other, lamination is pointless. Stamping burrs must be minimal, because a burr pierces the insulation and bridges adjacent sheets. These two are the process core of laminated cores.
Many know the core must be laminated, but overlook that the rotor's NdFeB magnet itself is also a conductor — sintered NdFeB resistivity is about 1.4×10⁻⁶ Ω·m, close to some metals. Under high-frequency, high-pole-frequency duty, the solid magnet also induces strong eddy currents and heats up.
For high-speed, multi-pole motors, the dB/dt at the magnet surface is extremely high; a solid magnet's eddy loss and temperature rise can directly threaten magnet safety — push past the knee, and the magnet silently demagnetizes, collapsing machine performance.
The fix is the same as the core: make the magnet into thin laminations, insulated between layers with 10–25 μm of epoxy, ceramic, or polyimide. Magnet eddy loss is acutely sensitive to thickness (per-plate loss scales roughly with the cube of thickness); replacing a 5 mm solid magnet with 0.35 mm laminations can cut eddy loss by more than 99%. Insulation slices the conductor into N pieces; the big loop becomes countless small loops, each losing only a tiny fraction of the original.
(For laminated-magnet materials, process, and applications, see FAIZEAL's earlier "Laminated Magnet" article.)
A truly high-frequency efficient rotor combines laminated silicon-steel core + laminated magnet — both heat sources have their eddy loops cut:
· Core lamination blocks silicon steel's eddy currents and iron loss.
· Magnet lamination blocks NdFeB's eddy currents and thermal-demagnetization risk.
· Together, the rotor can spin fast, not burn, and stay efficient at high frequency.
This is exactly why EV traction motors, aerospace motors, and high-speed compressor rotors universally use laminated rotors (especially interior permanent-magnet, IPM, rotors) — without lamination, eddy loss and heating at high speed simply cannot be contained.
Eddy loss is the most "frequency-hungry" item in motor losses, growing roughly with the square of frequency (f²). Lamination suppresses it, paying back in real efficiency:
· Direct loss reduction: Rotor eddy loss can drop an order of magnitude or more, eliminating the "invisible loss" beyond copper and iron loss.
· Protect the magnet: The magnet doesn't heat, so no temperature-rise demagnetization; the machine's magnetic-energy utilization doesn't collapse.
· Expand the high-efficiency zone: At high frequency (high speed, high pole frequency), the efficiency curve no longer collapses from eddy currents; the motor's efficient zone extends toward high speed.
· Lower torque ripple: Eddy currents distort the air-gap field and add torque ripple; after lamination the field is cleaner and operation smoother.
Quantitatively, laminated magnets alone cut magnet eddy loss by about 60–90% (depending on frequency and sheet thickness), and silicon-steel core lamination cuts the eddy component of iron loss by 70–90%. Combined, a high-frequency motor's total efficiency can rise by several percentage points — in high-power-density motors, those few points decide whether the motor makes it into the product.
Lamination has costs the engineer must weigh:
· Process complexity: Silicon steel needs stamping, stacking, controlled riveting/welding/bonding; laminated magnets add thin-sectioning and insulation onto brittle NdFeB — more process steps.
· Cost: Laminated parts are usually more expensive than solid ones; machining hours and material utilization both suffer.
· Insulation reliability: Once inter-lamination insulation fails, lamination effect goes to zero or worse.
· Residual iron loss: Lamination only kills the eddy component; hysteresis loss cannot be removed — that's a separate optimization dimension.
So use lamination "where it matters": for low-frequency power-frequency motors, 0.35 mm lamination is enough; for high-speed high-frequency motors, lamination must thin and magnets must be laminated too.
· EV traction motors: High speed (tens of thousands of rpm) + high power density make laminated rotors standard.
· Aerospace/aviation motors: Ultra-high power-to-weight and high speed make eddy control a life-or-death line.
· High-speed spindles / compressors / turbo-generators: Tens of thousands of rpm; lamination plus magnet lamination are both mandatory.
· Robotics / servos: High pole frequency and rich PWM harmonics; magnet lamination suppresses heating and demagnetization.
· Direct-drive wind / direct-drive motors: Though low speed, high pole count keeps frequency non-trivial; laminated cores remain fundamental.
The essence of the laminated rotor is applying "cut the conductor, cut the eddy loop" to both silicon steel and magnet. Under high-frequency response, eddy current is efficiency's number-one killer, and lamination is the only system-level means engineering has validated to drive eddy heating down to negligible.
For high-frequency motor laminated-magnet design, or sheet-thickness and insulation scheme evaluation, please feel free to reach out — FAIZEAL can provide technical support.
