Arc Segment

Why an arc segment instead of a complete ring

A stator or a rotor of any real diameter cannot always take a one-piece ring, so the magnet is pressed as a segment and the segments are assembled. An arc segment is specified by inner and outer radius, arc angle or chord width, wall thickness and length, and by whether the part is fixed in a stator or retained on a rotor. Each of those is a drawing question, not a catalogue one.

What the drawing has to fix

• Geometry — inner and outer radius, arc angle or chord width, wall thickness, length.
• Mounting — fixed in a stator, or retained on a rotor.
• Assembly — a segment separated from its neighbours needs its positioning and retention checked in the assembly, not only on the single piece.
• Orientation — for anisotropic ferrite, the magnetic direction has to be compatible with the orientation established while the part is pressed.
• Magnetization — the radial pole count, and the datum the poles are dimensioned from.

The last two are the ones most often left off a drawing. Orientation and magnetization are separate steps: orientation is established while the part is formed, magnetization is applied afterwards.

Ferrite arc segments are wet pressed

On the wet-pressed route the ferrite is compacted as a slurry at 30–40 % water content and that water is drained during pressing. It is the water that lets the particles align as the part is formed, which is why the route is used where the arc geometry and the flux requirement both matter. Dry pressing dries the same slurry below 10 % moisture before compaction: it is the economical route for simple geometries and cost-sensitive repeat production, and a part that needs more flux per unit volume than that alignment gives is reviewed against wet pressed ferrite. Arc segments sit on the wet side of that line.

Magnetization is radial — the pole count is what varies

Tile segments are magnetized radially: the flux crosses the wall thickness of the arc, so the pole face is the inner or the outer surface. Radial two-pole, four-pole and six-pole magnetization are all produced on this geometry. Which one applies follows from the circuit; the pole count, the pitch and the angular position of the poles are set by the magnetizing fixture and confirmed on the drawing. The pressed orientation has to serve that same radial direction, which is why an anisotropic ferrite arc is pressed and magnetized as one commitment rather than two separate ones.

Tolerances follow the part, not the shape

There is no single tolerance band for “arc segment”. What a segment can hold depends on the actual dimensions of the part: the inner and outer radius, the wall thickness, the arc angle, the length. A thin-walled segment on a large radius and a thick segment on a small one do not hold the same band, so the tolerance is agreed against the drawing’s dimensions rather than quoted as a figure for the shape. That is also why the drawing has to carry real dimensions — without them there is nothing to hold to.

Surface and grade

Tile ferrite segments are normally supplied uncoated. Where an assembly calls for a coating, it is stated on the drawing and confirmed per project.

The grade is not fixed by the geometry either. It follows the performance the circuit needs: the hard ferrite grade table is the range to choose from, and a high-performance grade is selected against the customer’s requirement rather than assumed because the segment is small or the air gap is tight.

Magnetized or unmagnetized delivery

Neither state is the default in itself. Segments can be delivered magnetized or unmagnetized, and which way a project goes follows the customer’s assembly — whether the magnet is bonded, clamped or potted into a stack before the field is applied, or whether it goes straight into the circuit in the state it arrives. The choice is usually settled by handling on the customer’s line rather than by the magnet: an unmagnetized segment can be assembled and then magnetized with the assembly in place, while a magnetized one has to be handled as a magnet from the moment it is picked up.

What does not change either way is the drawing. The magnetization direction, the radial pole count and the datum the poles are dimensioned from are fixed before shipment regardless of whether the field is applied, because the magnetizing fixture is built to that drawing. A drawing that marks the delivery state — magnetized, or supplied unmagnetized — removes the one question that is otherwise asked at dispatch. The sketch on this page states it in the title block, which is the clearest place for it.

Material systems and routes

This geometry is worked here in the routes listed further down this page: wet-pressed ferrite, and the sintered and compression-molded NdFeB routes. Each route page states what that route is confirmed for. A route that is not listed for a shape says the route is not confirmed for it — not that the geometry cannot be made.

What to send for an arc-segment review

• The drawing, or a sample: inner and outer radius, arc angle or chord width, wall thickness, length.
• Whether the part is fixed in a stator or retained on a rotor, and how it is held.
• N and S on the sketch, the radial pole count — two, four or six — and the datum the poles are dimensioned from.
• The grade requirement, or the flux the circuit needs, if it is already fixed.
• Whether the assembly needs a coating.
• Whether the segments are to arrive magnetized or unmagnetized.

Send it through the technical review form; datasheets and route diagrams are in the download library, and documented arc-segment projects are collected under case studies.