Magnets for Electric Motors

In a brushless PM machine (BLDC/PMSM), the stator windings create a rotating electromagnetic field and the rotor's permanent magnets lock onto it — the pairing described in Electromagnets vs. Permanent Magnets. The magnets supply the rotor field for free: no excitation current, no rotor copper losses, no slip. That free field is worth several efficiency points over an induction machine and is why PM motors dominate wherever energy or space is expensive — EVs, drones, HVAC, robotics, appliances.

• Torque scales with air-gap flux: the torque constant is proportional to the magnet flux linking the windings — more usable flux means more torque per amp, or the same torque from less copper and iron.
• Back-EMF scales the same way: the magnet sets the voltage-per-speed line, which is why grade and temperature drift show up directly in motor constants (the drift math is in the temperature guide).
• The magnet is also the vulnerable component: it lives at rotor temperature, adjacent to the stator's demagnetizing field, spinning under centrifugal load. Every following section is about keeping it alive there.

The architecture choice belongs to the motor designer; what it hands the magnet supplier is a format, a working point, and a demagnetization exposure — which is where magnet engineering takes over.

Motor magnets face the harshest combined stress in the permanent magnet world: the stator's opposing MMF at the exact moment rotor temperature has stripped away most of the magnet's coercivity (Hcj falls ~0.5–0.7 %/°C — the β coefficient from the temperature guide).

• The design case is a fault, not nominal operation: a short-circuit event, locked-rotor current, or maximum field-weakening at top speed — evaluated at maximum rotor temperature. If the magnet's working point crosses the knee during that event, part of every rotor in the field is permanently weakened (the classic Cause-2 failure in the demagnetization guide).
• The "hot corner": demag is local — the trailing edges and thin corners of magnets see the deepest opposing field. FEA demag analysis maps these regions; magnet chamfers and pocket geometry are shaped to protect them.
• The margin currency is Hcj: this is what temperature classes buy. The demag analysis outputs a minimum required coercivity at temperature; the class follows from it — not from a catalog habit.

• Start from rotor temperature, not ambient: continuous rotor temperatures of 100–180 °C are normal in dense machines — winding class and cooling define it. The class ladder (H → SH → UH → EH) and its availability limits are in the grades chart: hot motors design around N38–N45 because the top N-numbers don't exist in high classes.
• Grain-boundary diffusion changed the economics: GBD grades deliver SH/UH coercivity with a fraction of the heavy rare earth — ask for GBD availability on volume programs; it moves the price meaningfully (pricing guide).
• SmCo for the extremes: above ~200 °C rotor temperature, or where flux stability over temperature matters more than peak flux, Sm2Co17 takes over — aerospace actuators and downhole motors live there (material comparison).
• Ferrite still earns its place in cost-driven appliance and pump motors with generous rotor volume — with the cold-temperature demag check the material demands.

Sintered NdFeB is electrically conductive, and the stator's slot harmonics and PWM ripple induce eddy currents in the magnets — heating them precisely where heat is most dangerous. The countermeasure is segmentation: dividing each magnet into electrically isolated pieces so the eddy loops shrink.

• When it matters: high electrical frequency — high pole counts, high speeds, high PWM ripple. Low-speed, low-frequency machines often skip it.
• How it's done: magnets laminated from 2–10+ insulated slices (bonded with insulating adhesive), typically segmented in the axial direction; loss falls roughly with the square of segment count in the critical dimension.
• What it costs: more grinding, bonding, and handling per finished magnet — a real adder that should be traded against the thermal benefit, not specified by default.
• Bonded NdFeB sidesteps it: polymer-bonded material is orders of magnitude more resistive — one reason bonded rings serve high-frequency small motors despite lower flux (sintered vs. bonded).

• SPM rotors: adhesive positions the magnets; above modest tip speeds a containment sleeve carries the centrifugal load — carbon fiber (light, non-conductive) or Inconel/stainless (robust, but conductive: adds its own eddy losses). The adhesive-only rotor that "passed testing" is the classic field failure — the fail-safe rule from the bonding guide applies with interest.
• IPM rotors: the lamination pockets capture the magnets — retention becomes bridge-stress design in the laminations, and the magnets are typically slid in and secured with low-viscosity adhesive or resin injection.
• Thermal cycling is the endurance test: magnet, adhesive, laminations, and sleeve all expand differently; validate with temperature-cycled spin tests, not room-temperature spin alone.
• Assembly reality: rotors are commonly built with unmagnetized magnets and pulse-magnetized after assembly — safer handling, no debris attraction, no assembly forces (the practice runs through the manufacturing guide and safety guide).

• Spec the demag requirement, not just the grade: "Hcj ≥ X kOe at Y °C" (from your demag analysis) alongside the grade callout protects you when suppliers propose substitutions.
• Flux matching & binning: torque ripple and balance care about magnet-to-magnet consistency within a rotor. Options, in rising cost order: per-lot supply (one sintering lot per rotor batch), helmholtz-moment binning to windows, and 100% measured-and-serialized. Choose deliberately; don't pay for binning a design that doesn't need it.
• Acceptance testing: helmholtz moment limits per magnet, plus material certs per lot — the standard package from the testing guide. Automotive programs wrap it in PPAP (a service we run routinely, per the compliance guide).
• Delivery state: specify magnetized or unmagnetized, and if unmagnetized, who owns post-assembly magnetizing and its verification.
• Volume economics: motor programs hit every lever in the pricing guide — NdPr indexing on multi-year agreements, GBD grade negotiation, and tolerance right-sizing on pocket-fit blocks are the big three.