NdFeB Magnets for Axial Flux Motors: Geometry, Grade, and Procurement Differences vs Radial Flux

In a radial flux IPM motor, magnets are embedded in the rotor as rectangular blocks or arcs, with magnetization directed radially (outward toward the stator). In an axial flux motor, magnets sit on the flat face of a disc rotor, with magnetization directed axially (parallel to the shaft). The active magnet surface is the large flat face, not the thin edge. This fundamentally changes the magnet shape: instead of long, narrow arc segments, AFPM motors use wide, flat discs or sectors. A typical AFPM sector magnet for an EV motor might be 40-60 mm in radial span, 30-50 mm in circumferential arc, and 3-6 mm thick. The thin-and-wide geometry means the magnetization direction (through the thickness) is the shortest dimension, which is the opposite of most radial flux magnets where magnetization runs along the longer dimension.

Three magnet geometries dominate AFPM designs. Full-disc magnets are circular discs covering the entire pole area. They are the simplest to manufacture but create large eddy-current loops and are only used in low-speed or low-frequency applications. Sector magnets (pie-slice or trapezoidal shapes) are the standard for high-performance AFPM motors. Each pole uses one sector, with the taper angle set by the pole count (e.g., 30 degrees for a 12-pole motor). The tapered edges require wire EDM or profile grinding, which is why sectors cost more than rectangles. Ring segments are used in larger AFPM generators (wind turbines, aerospace) where the rotor diameter exceeds 300 mm. Each pole is assembled from 2-4 ring segments rather than a single sector to manage eddy losses and manufacturing constraints.

• •Full disc: simplest, highest eddy loss, low-speed only
• •Sector (trapezoidal): standard for EV and robotics AFPM, wire EDM or profile ground
• •Ring segment: used in large-diameter generators, assembled from multiple pieces per pole

In radial flux motors, the dominant time-varying field at the magnet is axial (along the shaft), and eddy currents flow in circumferential loops. Axial segmentation (slicing perpendicular to the shaft) breaks these loops. In AFPM motors, the dominant time-varying field at the magnet is from slot harmonics that vary radially and circumferentially across the disc face. The induced eddy currents flow in radial and tangential directions across the flat face of the magnet. Circumferential segmentation (dividing a disc or sector into narrower circumferential slices) is therefore more effective than through-thickness slicing. A sector magnet divided into 3-4 circumferential strips reduces eddy losses by 60-80%, comparable to the effect of 4-6 axial slices in a radial flux motor. Through-thickness segmentation (laminating the magnet in the axial direction) provides additional but diminishing benefit because the magnet is already thin (3-6 mm) in the axial direction.

AFPM motors are uniquely sensitive to magnet flatness because the air gap is set by the distance between two parallel disc surfaces rather than a cylindrical bore. Typical AFPM air gaps are 0.5-1.0 mm, roughly half the 1.0-2.0 mm common in radial flux motors of similar power rating. A 0.05 mm variation in magnet thickness changes the effective air gap by 5-10%, which directly modulates torque, cogging, and flux distribution. For this reason, AFPM magnet drawings typically specify flatness of 0.02-0.03 mm across the disc face and parallelism of 0.02-0.03 mm between the two flat faces. These are achievable with double-side lapping or precision surface grinding with vacuum fixturing, but they add 20-40% to machining cost compared to the +/-0.05 mm standard tolerance on radial flux blocks.

The AFPM topology does not change the magnet's thermal demagnetization behavior. An N42SH magnet in an axial flux motor has the same Hcj, the same temperature coefficient, and the same irreversible demagnetization knee as in a radial flux motor. What changes is the thermal path: AFPM motors often have the magnets closer to the stator winding (smaller air gap), which can increase magnet heating from stator copper loss radiation. Additionally, the disc rotor geometry can trap heat because the magnet sits between the rotor back-iron and the air gap with limited thermal mass. Liquid cooling in AFPM motors typically flows through channels in the stator housing, not through the rotor, so rotor-side magnets rely on air gap convection and conduction through the back-iron disc. Expect magnet temperatures 10-20 C higher than in a comparable radial flux motor with equivalent stator cooling. Compensate by going one thermal grade higher or adding 20 C margin to your grade selection.

AFPM magnets are more expensive per piece than radial flux magnets for three reasons. First, the sector geometry requires wire EDM or profile grinding rather than simple surface grinding, adding 30-60% to machining cost. Second, the tighter flatness and parallelism tolerances require precision fixturing and often double-side lapping. Third, the thin cross-section (3-6 mm) makes the magnets more fragile during handling and coating, increasing scrap rates by 5-10%. At the RFQ stage, specify the exact sector geometry with inner radius, outer radius, and included angle rather than providing only a sketch. Include the flatness and parallelism requirements explicitly. State whether the final dimension is pre-coating or post-coating (coating adds 15-25 microns per side for epoxy, which matters in a 0.5 mm air gap). Expect lead times 1-2 weeks longer than equivalent rectangular magnets due to the additional machining setup.

• •Sector geometry adds 30-60% machining cost vs rectangular blocks
• •Flatness tolerance of 0.02-0.03 mm adds 20-40% grinding cost
• •Thin cross-section increases scrap rate 5-10%
• •Lead times 1-2 weeks longer than standard geometries
• •Always specify post-coating dimensions for tight air gap applications