Magnetic Circuit Design for PM Motors: Ring Construction Before Magnet Grade

In November 2025 we had three rotor ring constructions measured on identical geometry: Ø36.6 mm OD, Ø29.8 mm ID, 17 mm high, 10 poles, scanned at the OD surface on a multipole analyzer. Radial-oriented ring, N45SH: 415.7 mT peak, trapezoidal. Segmented ring, arc segments bonded to a hub and built a grade up in N50SH: 521.4 mT peak, square, with saddle peaks at the pole edges.

One-piece sintered Halbach ring, back in N45SH: 629.0 mT peak, near-sinusoidal. The Halbach ring produced 51% more peak flux than the radial ring, and both were N45SH, so none of that difference came from the material. It also produced 21% more than the segmented ring, which had been built in the higher N50SH grade.

On this geometry the orientation pattern mattered more to peak flux than the N45SH to N50SH grade step. The waveform matters as much as the peak. Trapezoidal is fine on six-step and a problem on a sinusoidal drive. The square profile from the segmented ring carries saddle peaks at the pole edges, which is where the bond lines between arc segments fall.

Of the three, only the Halbach ring produced a near-sine back-EMF without skewed magnetization or pole shaping, which is the behaviour field-oriented drives in robotics applications are designed around. Its peak spread from pole to pole across the N poles was 8.6 mT, about 1.4%, the tightest of the three.

Radial-oriented rings are still the default construction. The supply base is the broadest and the tooling is the simplest, and skewed multipole magnetization can be ordered into the part itself, which reduces cogging without any change to the stator. On programs where unit cost dominates and the drive can work with a trapezoidal back-EMF, a radial ring will do the job well.

Segmented rings, built from arc segments bonded onto a hub, give the highest peak flux per dollar of the three. Arc segments are the cheapest manufactured form for the remanence, and they are then assembled. What you get back is waveform harmonics and bonded joints. Every bond line needs mechanical and thermal qualification, and the measured saddle peaks sit at those joints.

Segment count also affects rotor loss, which we cover separately in the article on segmentation and eddy current loss. One-piece sintered Halbach rings earn their price on a narrower set of programs. Waveform quality, flux per unit of magnet volume, or the behaviour of the bore has to matter more than piece cost before the construction is worth specifying.

There are no bonded segments to qualify, and because the array self-shields, the bore is nearly field free. That is what allows an encoder, a resolver, a control board or a bearing to sit inside the ring without a separate shield or a change to the layout. Diameter constrains the choice at the small end.

Below Ø20 mm OD the part moves to a multipole-oriented ring process, and pole count becomes diameter limited: around Ø6 mm the practical limit is 8 poles. Check that against the electrical design before the rotor layout is fixed.

With the construction settled, grade comes down to holding the operating point: remanence sets the ceiling, and intrinsic coercivity and temperature class decide how much of it survives at operating temperature and at the worst-case demagnetizing operating point. Two grade windows apply here. Radial-oriented rings sit in a narrower band than the block and arc catalogue, N30 to N48, with temperature classes available up to EH in that construction.

The wider sintered NdFeB grades catalogue runs N30 to N58 across three series: F-Series with no heavy rare earth content and no MOFCOM export licence requirement, N-Series conventional, and D-Series produced by grain boundary diffusion, with temperature classes reaching AH at 230°C. Grain boundary diffusion is how thermal margin gets added without giving up remanence.

In one verified factory case, an N45H was upgraded to N45UH using a 0.35% Tb surface coating. Intrinsic coercivity rose 50%, from 17.67 to 26.55 kOe, with no loss of Br, and the part used about 75% less dysprosium than the conventional alloying route. On EV traction motors and continuous-duty actuators, that is the route we take when the operating temperature has moved past what the original grade supports.

The demagnetization side of the question is treated in detail in our article on thermal demagnetization and temperature limits.

The manufacturing envelope belongs in the design review. Radial-oriented rings run a standard envelope of OD 20 to 75 mm, wall 2 to 7 mm, and height 5 to 50 mm, held to ±0.05 mm, with straight or skewed multipole magnetization. One-piece sintered Halbach rings are radially thin and net-shape, with no bonded segments, and critical Halbach features are controlled to ±0.02 mm.

Those two figures come from two different processes. Carrying the ±0.02 mm number onto a radial ring drawing, or the reverse, is one of the more common ways a quotation package goes out wrong. The supply base is the other practical constraint. One-piece sintered Halbach rings in series production are a capability few plants worldwide hold.

A plant in our production network runs them in series supply to robot joint-motor programs, which is a different proposition from a prototype shop that can produce a handful. If the ring is part of a larger rotor or sensing sub-assembly, specify the magnet assemblies requirements too: concentricity to the shaft, bond line and retention, magnetization fixturing, and measurement after assembly rather than before.

For sensing positions, deviation angle can be held under ±3° with 100% post-magnetization inspection. Quality documentation runs to ISO 9001:2015 and IATF 16949:2016 through the certified plant in our production network, with PPAP Level 3 packages available for automotive-grade programs.

With the seven points below we can come back on construction, grade and temperature class together instead of one item at a time. Our engineering response is within 1 business day, and pricing within 2 business days.