Arc Segment Magnets: How to Specify Them

1. What an arc segment magnet is

An arc segment (or tile) magnet is a curved piece shaped to follow a rotor or stator, so it sits close to the air gap in a round assembly. It is used in brushless DC, servo and traction motors, where it gives a uniform flux across the air gap, smoother torque and lower cogging [3].

2. The six dimensions and the magnetization direction

One supplier summarises an arc as six items; leave one out and the factory has to guess [1]:

• Outer radius (OR): the convex face towards the air gap.
• Inner radius (IR): the concave face, which is the bonding surface in a surface-mounted rotor.
• Arc angle: the angle the segment covers, in degrees.
• Axial length.
• Wall thickness: OR minus IR. The same source says breakage and grinding yield get worse quickly below about 2 mm on a large radius [1].
• Magnetization direction: parallel or radial, with the north face marked.

3. Parallel or radial magnetization

In parallel magnetization the field lines run parallel, as if a rectangular block were bent into an arc. The fixture is simpler, and the flux peaks at the pole centre, which gives a slightly trapezoidal waveform. In radial magnetization the flux points along the radius, normal to the air gap, which gives a more uniform air-gap flux and a more sinusoidal back-EMF, but it needs a special radial fixture [1]. Which one you need follows from the motor design and the drive: six-step BLDC drives tolerate the trapezoidal form, while sinusoidal FOC/PMSM drives gain from the smoother one [1]. Tell the supplier which you need; do not leave it to them.

4. Tolerances

As an example of what a supplier publishes, one source lists standard tolerances of ±0.10 mm on outer radius, inner radius and wall thickness, ±0.5° on arc angle and ±0.15 mm on axial length, with tighter “precision” values available, and says flux per pole is typically held within ±5% [1]. These are one supplier's figures, not a standard and not our commitment. Tolerances differ from factory to factory and from part to part, so state the tolerance you need on the drawing and ask the factory to confirm it. If pole-to-pole flux balance matters, ask for matched sets [1].

5. Grade and temperature

The grade must be chosen together with the maximum operating temperature [1]. Motor magnets usually need a high-coercivity suffix (SH, UH and above); see our selection guide for how suffix and temperature fit together. Many of these grades rely on dysprosium or terbium, so they can be subject to a Chinese export licence [4]; read our licence guide before you fix a delivery date.

6. Eddy currents and segmenting

Sintered NdFeB conducts electricity, so rotor magnets carry induced currents that heat them from the inside; heat lowers coercivity and raises the risk of demagnetisation [2]. One way to reduce the loss is to cut a magnet into electrically insulated pieces. In theory the loss falls with the square of the number of pieces; in practice the gain is smaller, for example about 30–40% with two pieces and 12–20% with four, and the practical optimum is usually two to six segments [2]. The pieces must be insulated from each other; a conductive adhesive or sleeve can short them and cancel the benefit [2]. Whether to segment is a motor-design decision, so settle it with your design engineer before you order.

7. What to send us for a quote

• Drawing or the six dimensions above, with tolerances.
• Magnetization: parallel or radial, north face marked.
• Grade and maximum operating temperature.
• Coating (for example Ni-Cu-Ni or epoxy) and the environment.
• Quantity per order and per year, and the delivery address.
• Whether you need segmented or matched sets, and what inspection and test reports you require.

Frequently asked questions

Which dimensions do I need for an arc segment magnet?

Six items: outer radius, inner radius, arc angle, axial length, wall thickness (the outer minus the inner radius) and the magnetisation direction, with the north face marked. Leave one out and the factory has to guess.

Parallel or radial magnetisation for a motor?

Parallel magnetisation needs a simpler fixture and gives a slightly trapezoidal waveform. Radial magnetisation gives a more uniform air-gap flux and a more sinusoidal back-EMF, but needs a special radial fixture. Six-step BLDC drives tolerate the trapezoidal form; sinusoidal FOC/PMSM drives gain from the smoother one. Tell the supplier which you need.

Does cutting a magnet into segments reduce eddy-current loss?

Yes, if the pieces are electrically insulated from each other. In theory the loss falls with the square of the number of pieces; in practice the gain is smaller, and the practical optimum is usually two to six segments. A conductive adhesive or sleeve can short the pieces and cancel the benefit, so settle it with your design engineer before you order.

References

1. [1]  Radial Magnets, Inc. (n.d.). Arc segment magnets. https://radialmagnet.com/arc-segment-magnets/
2. [2]  Radial Magnets, Inc. (n.d.). Magnet eddy current losses. https://radialmagnet.com/magnet-eddy-currents-losses/
3. [3]  HSMAGNET (n.d.). Motor magnets: arc segment magnets and rotor magnets explained. https://hsmagnet.com/?p=14493
4. [4]  MainRich Magnets (2026). Rare earth magnet export compliance in 2026. https://mainrichmagnets.com/rare-earth-magnet-export-compliance

Important notice: This guide is general technical information based on the cited public sources. Figures from other suppliers are examples, not a specification, an offer or a commitment by Rami Technics. Confirm every design decision against the datasheet of the exact grade and shape and test a sample under real conditions.