OEM Customized Magnetic External Rotor

FAIZEAL

OEM Custom Cup Rotor for Outer-Rotor BLDC

Segmented NdFeB Arc Magnets on Cup Inner Wall — Closed Magnetic Circuit, High Torque Density

Robot Joint  |  Gimbal  |  UAV Propulsion  |  E-Bike Hub  |  Direct-Drive Torque Motor  |  Spindle

  CUSTOM CUP ROTOR FOR OUTER-ROTOR BLDC

  Segmented NdFeB Arc Magnets on Cup Inner Wall — Closed Magnetic Circuit, High Torque Density

What This Rotor Is — and What It Is Built to Do

The cup rotor shown on the cover is an outer-rotor (inside-out) brushless DC motor rotor. The permanent magnet ring is built as discrete arc segments mounted on the inner cylindrical wall of a machined metal cup — the cup spins around a stationary stator that sits in the center bore. The magnets are segmented, individually magnetized, and bonded to the cup ID with a structural adhesive. The cup body itself forms a closed magnetic return path: flux leaves the magnet ring on the working (inner) face, crosses the air gap into the stator teeth, returns through the stator back-iron, and closes back through the cup wall. There is no separate sleeve and no exposed magnet edge.

The architecture is the textbook solution for low-profile, high-torque-density, high-inertia applications: robot joints, gimbals, UAV propulsion, e-bike hub motors, direct-drive torque motors, and high-speed spindle drives. The cup form factor — magnets on the inner wall, shaft through the center — gives a short axial stack with a large active diameter, which is exactly the geometry that maximizes torque per unit volume for a given inertia budget.

This is a custom-engineered build, not a catalog part. Every dimension — pole count, magnet grade, cup material and wall thickness, bore diameter and tolerance, cup depth, balance grade — is sized to your stator, your drive, your duty cycle, and your service environment. We make one rotor at a time, to one set of customer drawings.

The Engineering Argument — Why a Cup Rotor for Your Motor

An outer-rotor cup motor puts the magnet ring on the inside of a rotating cup, with the stator fixed in the center. Compared to an inner-rotor (conventional) motor, the cup rotor gives four engineering advantages that matter to your design:

• Large active diameter, short axial stack: the magnets sit at the largest possible radius. For a given torque, torque scales with radius squared (T ~ B . I . r^2), so a cup rotor delivers more torque per unit axial length than a conventional inner-rotor motor of the same OD. This is why cup rotors dominate pancake / flat motor applications

• High rotor inertia, naturally matched: the mass of the cup is at a large radius, so the rotor inertia is high. For flywheel, reaction-wheel, and momentum-wheel applications, this is exactly what you want — the cup is the rotating mass. For servo applications that need low inertia, we thin the cup wall and hollow the floor to bring the inertia down

• Closed magnetic circuit, low stray field: the cup body closes the magnetic path on the outside. The external stray field is suppressed by 80-95% compared to an open SPM ring. This protects nearby electronics, simplifies EMI compliance, and lets you place the motor close to sensors and controllers

• Structural retention built in: the cup wall itself holds the magnets in place under centrifugal load. There is no separate sleeve to wind or weld. The cup is the retention system, the magnetic return path, and the mounting structure in one piece

Two cup-rotor variants are in production at FAIZEAL:

Radial-Magnetization Cup Rotor — the Standard for Low-Cogging Servo

• Arc segments magnetized radially (N-S-N-S around the ring), typically 8-32 poles

• Simple, robust, lowest cost. The standard for 18-pole / 9-pole-pair BLDC servos paired with 18 / 21 / 27-slot stators

• Segment gaps tuned to your pole-arc ratio to minimize cogging torque

• Standard for robot joint, gimbal, drone propulsion, e-bike hub, and direct-drive torque motor

Halbach Cup Rotor — for Maximum Air-Gap Flux and Zero Back-Side Leakage

• Arc segments with alternating tangential / radial magnetization (Halbach sequence) around the ring

• Concentrates flux on the inner (working) face, cancels it on the outer face — no cup return path needed, but the cup still provides structure

• Higher air-gap flux density (30-50% vs radial) at the same magnet volume; lower back-iron requirement

• Selected when torque density and stray-field suppression both matter (medical imaging, precision instruments, aerospace)

Magnet Selection — Sintered NdFeB, Sized to Your Pole Count and Air Gap

The magnet ring is built from sintered NdFeB (neodymium-iron-boron) arc segments — individually machined to your pole geometry and bonded to the cup inner wall. Each pole is a discrete piece, not a continuous ring. This gives you three engineering controls:

• Pole-by-pole material grade: each arc segment can be drawn from a specific magnet lot. We can match magnet lots pole-by-pole for minimum spread in flux density, or use a single lot for the entire ring if uniformity is sufficient

• Pole-arc optimization: each arc segment has a specific arc angle (pole-arc-to-pole-pitch ratio). We can skew the pole arcs, vary the pole-arc ratio along the ring, or use an asymmetric pole-arc profile to shape the back-EMF waveform and minimize cogging torque

• Magnet grade selection: N35 to N52 (SH / UH / EH for high-temperature; SmCo for sustained >200°C duty). We select the grade to your flux-density target and your operating temperature budget

For an 18-pole / 9-pole-pair servo, the magnet segments are typically radial-magnetization trapezoids. For a Halbach cup, they are a mix of radial and tangential segments in the Halbach sequence. The choice is part of the stator-pair design conversation we have with you at quotation stage.

Cup Body Design — Material and Geometry Matched to Your Load

The cup is the structural, magnetic, and mounting backbone of the rotor. It holds the magnet ring concentric, closes the magnetic return path, transfers torque to the shaft, and defines the rotor's inertia budget. The cup design is matched to your duty cycle:

• Material selection: 6061-T6 aluminum (cost-optimized, good machinability); 7075-T6 aluminum (high-strength, low-inertia); 17-4PH stainless (high-temperature / vacuum / corrosive); titanium Ti-6Al-4V (aerospace / weight-critical). We match the material to your speed and service environment

• Wall thickness: 3-10 mm outer web, sized to the centrifugal load at your maximum continuous speed. Thinner = lower inertia; thicker = higher speed capability. We size the wall to your speed envelope

• Floor thickness: 4-15 mm back plate. For weight-critical builds we hollow the floor or add lightening pockets. For high-torque builds we keep the floor solid for stiffness

• Bore: customer-specified diameter and tolerance. H7 (standard press-fit), H6 (precision), H8 (slip-fit). Keyed or splined on request. Chamfered and burr-free on both faces for clean shaft mounting

• Concentricity: the bore is the reference datum for the magnet array. Machined in a single setup, the cup sets the rotor TIR to typically within 0.05 mm. This is the geometric datum your stator's air gap relies on

For a customer with a fixed stator OD and a target inertia, we engineer the cup wall and floor to that geometry. For a customer with a fixed shaft interface (press-fit, spline, keyed), we machine the bore to that interface and verify concentricity to your spec.

Custom Engineering — Six Dimensions We Open for Your Build

Every rotor is built to a customer envelope. The customization is not a side option. It is the design intent. Six dimensions are always open for engineering input.

1. Rotor Topology and Pole Count

• Topology: outer-rotor cup (standard); Halbach cup on request; axial-flux cup (pancake, magnets on floor face) on engineering review

• Pole count: 8 / 10 / 12 / 16 / 18 / 24 / 28 / 32 poles standard; any pole count feasible within mechanical envelope

• Stator pairing: designed to your stator (existing or new); we recommend the pole-count-to-slot-count ratio that minimizes cogging

2. Magnet Grade and Configuration

• Magnet grade: N35 to N52 sintered NdFeB; SH / UH / EH for high-temperature; SmCo for >200°C sustained

• Magnetization direction: radial (standard for cup servo); Halbach (high-torque / low-leakage); axial (axial-flux cup)

• Anti-corrosion: Ni-Cu-Ni plating (standard); epoxy / parylene (humid / marine / outdoor)

• Pole-arc optimization: skew, asymmetric pole-arc, variable pole-arc along ring — all available

3. Cup Body Material and Geometry

• Cup material: 6061-T6 Al (cost-optimized); 7075-T6 Al (high-strength); 17-4PH stainless; titanium Ti-6Al-4V

• Wall thickness: 3-10 mm outer web; floor 4-15 mm; hollow floor / lightening on request

• Cup OD: 40 mm - 400 mm standard; up to 600 mm on request

• Cup depth: 12 mm - 200 mm standard; matched to your torque envelope

4. Center Bore and Shaft Interface

• Bore diameter and tolerance: customer-specified; H7 (standard) / H6 (precision) / H8 (slip-fit)

• Bore treatment: chamfered, burr-free both faces; press-fit, slip-fit, or adhesive-bonded to shaft

• Keyed or splined bore on request; D-feature or clamping cone on engineering review

• Bore concentricity: within 0.05 mm TIR to magnet array (cup-bore reference datum)

5. Aspect Ratio and Inertia Budget

• Low-profile pancake (aspect ratio 0.15 - 0.8): outer-rotor and direct-drive; short axial stack

• Deep cup (aspect ratio 0.9 - 1.5): high-torque, high-inertia (flywheel / reaction wheel)

• Inertia tuning: thin wall + hollow floor for low inertia; thick wall + solid floor for high inertia

• L/D matched to your torque-speed envelope and inertia-ratio target

6. Balance, Service Environment, and Documentation

• Dynamic balance: ISO 1940 Grade G2.5 (standard); G1.0 (high-speed); G6.3 (large-frame / low-speed)

• Working temperature: -40°C to +150°C (N-grade); +200°C (EH); +350°C (SmCo optional)

• Surface treatment: black epoxy / parylene cup coating (corrosion resistance); Ni / NiCuNi (magnet anti-corrosion)

• Service environment: clean room ISO Class 5-8; vacuum; ATEX; oil & gas; aerospace

• Compliance: CE / REACH / RoHS; per-rotor serial number; full material and process traceability; 10-year file retention

Magnetic Circuit — How Torque and Speed Are Actually Produced

A cup-rotor brushless motor's torque output is set by three coupled engineering quantities: the air-gap flux density (B), the stator current (I), and the synchronous electromagnetic coupling between them (the torque equation T = k . B . I for a well-designed motor). Understanding each one is what lets you specify a rotor that hits your motor's torque-speed envelope.

• Air-gap flux density (B): set by the magnet grade, the magnet arc geometry, the cup wall thickness, and the air gap. Higher-grade magnets (N48, N50, N52) push more flux into the gap. Tighter magnet-to-stator tolerances push the magnet closer to the air gap. We design the magnet array and the cup grinding spec together to hit your B target

• Stator current (I): not part of the rotor spec, but the rotor must be able to handle the stator's continuous and peak current without saturating or heating the magnet array. We verify the magnet grade and the cup return-path geometry against your stator's current envelope

• Synchronous coupling: the rotor magnet ring's pole count must match the stator's slot count to deliver smooth torque. An 18-pole rotor / 21-slot stator pairing (fractional-slot concentrated winding) gives low cogging and high winding factor. We pair the pole count to your stator's slot count at quotation stage

The cup is the magnet side and the return path of the magnetic circuit. The stator is the current side. We work with your stator designer (or we spec a stator to our rotor) to deliver a motor that hits its torque-speed envelope without saturating, without cogging, and without thermal runaway.

End-to-End In-House Production — From Magnet Lot to Balanced Cup Rotor

The rotor is built end-to-end in our factory. The steps that define performance are not outsourced:

• Sintered NdFeB (and SmCo) production, grain-boundary diffusion, and pulse magnetization through controlled, audited supply chains

• Multi-axis grinding of magnet arc segments: radial / tangential / Halbach magnetization geometry matched to your pole spec; arc dimension tolerance +/-0.02 mm; pole-arc angle +/-0.5 degree

• Magnet anti-corrosion plating: Ni-Cu-Ni (standard) or epoxy / parylene coating for harsh environments

• Cup machining: 6061-T6 / 7075-T6 aluminum or 17-4PH stainless or titanium; CNC turned cup with inner wall, outer wall, floor, and bore in a single setup for concentricity; bore machined to H7 tolerance with concentricity to 0.05 mm TIR

• Magnet ring assembly: arc segments laid out and bonded to the cup inner wall with vacuum-degassed structural epoxy; magnetized; end-to-end field mapping for flux-density uniformity

• Cup OD grinding: precision cylindrical grinder; cup OD matched to your air-gap spec (typically Ra 0.4 um or better, concentricity 0.05 mm TIR)

• Dynamic balance: ISO 1940 Grade G2.5 (standard) or G1.0 (high-speed); balancing by precision cup OD grinding; final balance report with per-rotor vibration spectrum

• 100% final test on every rotor: Hi-Pot test (insulation integrity); insulation resistance; magnet ring integrity; cup OD concentricity; balance report; visual inspection

Technical Specifications

All values are typical ranges — every parameter is customized on request.

Frequently Asked Questions

What is a cup rotor?

A cup rotor (also called a can rotor or outer-rotor rotor) is a brushless motor rotor where the permanent magnet ring is built as discrete arc segments mounted on the inner cylindrical wall of a machined metal cup. The cup spins around a stationary stator that sits in the center bore. The cup body closes the magnetic return path on the outside, suppresses the external stray field, and provides structural retention for the magnets — all in one piece. Cup rotors are the standard for low-profile, high-torque-density, high-inertia motors: robot joints, gimbals, UAV propulsion, e-bike hub motors, direct-drive torque motors, and high-speed spindle drives.

How is a cup rotor different from an inner-rotor (conventional) motor?

In a conventional inner-rotor motor, the magnet ring is on the rotor surface and the stator surrounds it. In a cup (outer-rotor) motor, the magnet ring is on the inside of a rotating cup and the stator is fixed in the center. The cup rotor puts the magnets at the largest possible radius, so torque scales with radius squared (T ~ B . I . r^2) — more torque per unit axial length. Cup rotors also have higher natural inertia (good for flywheels / reaction wheels) and a closed magnetic circuit (low stray field). Inner-rotor motors are better when you need low rotor inertia and high speed; cup rotors are better when you need high torque density in a short axial stack.

What pole count should I use for my cup motor?

Pole count is paired with the stator slot count, the stator winding topology, and the motor's torque-speed envelope. Common pairings: 18-pole rotor / 21-slot stator (fractional-slot concentrated winding, low cogging, high winding factor — the standard for robot-joint cup servos); 18-pole / 27-slot; 16-pole / 18-slot; 12-pole / 9-slot (fractional-slot, very low cogging); 8-pole / 12-slot (integer-slot, simpler winding, higher cogging). Tell us your stator's slot count and winding topology, and we will recommend the pole count that minimizes cogging and maximizes winding factor.

What is the difference between a radial and a Halbach cup rotor?

A radial cup rotor has arc segments magnetized radially (N-S-N-S around the ring). It is simple, robust, and lowest cost — the standard for 18-pole BLDC servos. A Halbach cup rotor has arc segments with alternating tangential and radial magnetization (the Halbach sequence). The Halbach arrangement concentrates flux on the inner (working) face and cancels it on the outer face, giving 30-50% higher air-gap flux density at the same magnet volume and lower back-iron requirement. Halbach is selected when torque density and stray-field suppression both matter (medical imaging, precision instruments, aerospace). We will recommend the right topology for your envelope.

What is the difference between radial and axial magnetization for a cup rotor?

Radial magnetization means the magnets are on the inner cylindrical wall of the cup, magnetized perpendicular to the wall (pointing inward). This is the standard cup-rotor construction for radial-flux motors. Axial magnetization means the magnets are on the cup floor face, magnetized along the motor axis. This is the construction for axial-flux (pancake / disc) motors, where the stator sits face-to-face with the magnet floor. Radial is the standard for cup rotors; axial is the standard for axial-flux disc motors (wheel-hub, in-wheel, low-profile direct-drive). Tell us your motor topology and we will build the right cup.

Can I use SmCo instead of NdFeB?

Yes. Samarium-cobalt (SmCo) gives sustained operation up to +350°C (vs +150°C for N-grade NdFeB) and is more corrosion-resistant than NdFeB (no plating required for most service environments). The trade-off is energy product: SmCo has roughly 60-70% of the energy product of N-grade NdFeB at the same volume, so you need a larger magnet ring to hit the same flux-density target. SmCo is the right choice for high-temperature rotor service (downhole tools, aerospace actuators, high-temperature turbomachinery). We will spec the SmCo grade to your flux and temperature budget.

What is the maximum speed of a cup rotor?

With a 6061-T6 or 7075-T6 aluminum cup and N-grade NdFeB magnets, the typical continuous speed limit is 3,000 rpm - 12,000 rpm depending on the cup OD, wall thickness, and magnet grade. With a thinner cup wall and higher-grade bond, 15,000 rpm+ is feasible. The cup wall thickness is sized to your maximum continuous speed and the magnet centrifugal load. We return a feasibility note and a recommended cup spec within 2 working days.

What is the maximum OD of a cup rotor you can build?

Standard OD range is 40 mm - 400 mm. We have built cup rotors up to 600 mm OD for large-frame outer-rotor and in-wheel hub applications. Above 600 mm OD, the cup machining and magnet-ring assembly become engineering-specific; we will review the application and return a feasibility note within 2 working days.

How is the cup rotor dynamically balanced?

Dynamic balance is achieved by precision grinding of the cup outer diameter (the reference surface). Cup-OD grinding gives balance correction in micron-precision radial cuts, with no balance weights and no balance holes — preserving the rotor's geometry and concentricity. Final balance grade is ISO 1940 G2.5 (standard), G1.0 (high-speed cup), or G6.3 (large-frame / low-speed). Every rotor ships with a balance report showing residual unbalance and vibration spectrum.

What encoder / resolver can I integrate on the cup rotor?

We can integrate an encoder magnet ring, a resolver target ring, or a sensor mount surface on the rear face of the cup (the floor opposite the magnet ring) or on the cup outer diameter. Common integrations: Hall-sensor magnet ring (3-pole pair, sin/cos or digital output); resolver target (variable-reluctance or magnetized); incremental encoder target (optical or magnetic); absolute encoder target (BiSS / EnDat / SSI / SSI24 / Hiperface). Tell us your encoder / resolver spec and we will integrate the target to your drawing.

What is the shaft interface — how does the cup rotor mount to my shaft?

Standard interface is interference press-fit at H7 tolerance (H6 for precision, H8 for slip-fit). We machine the bore to your specified diameter and tolerance, and verify concentricity to the magnet array within 0.05 mm TIR. For zero-backlash applications (precision servo), we bond the rotor to the shaft with a Loctite-grade structural adhesive. For high-torque applications, we add a keyway (DIN 6885) or a spline. For frequent service or field-replaceable assemblies, we can machine a tapered bore or a bolted flange interface. Tell us your shaft interface and we will match it.

What are your typical lead times?

Prototype / sample cup rotor: 15-25 working days after drawing freeze. Production: 30-60 working days depending on volume, materials, and pole count. Aluminum cup rotors with radial magnetization typically run on the shorter end of this range; Halbach cups, titanium cups, and large-OD rotors may run longer. Repeat orders from a frozen design typically ship in 20-40 working days.

Can you reverse-engineer an existing cup rotor for our production line?

Yes. Send us your existing cup rotor (or its drawings, or its CAD model, or its magnetic field map), and we will reverse-engineer the pole count, the magnet grade, the cup material and wall thickness, the bore, and the floor geometry. We will return a quote for an exact-replacement cup rotor (drop-in replacement on your existing shaft) or for a performance-upgraded cup rotor (higher energy product, thinner wall, lower inertia) within 2 working days. We support replacement of OEM cup rotors from Kollmorgen, Parker, Yaskawa, Estun, Maxon, Faulhaber, and most other brushless servo manufacturers.

Can you keep spare cup rotors for our production line?

Yes. We offer scheduled batch-rotation stock for OEM customers, with a 5-year material and process reproducibility guarantee per fleet reference. Tell us your expected service interval and your safety stock target, and we will hold the right number of finished cup rotors in our bonded warehouse for call-off.

Get a Custom Quote — Fast

Send us:

• Cup OD and cup depth (or your existing motor's dimensions)

• Pole count and stator slot count (or we will recommend, based on your duty cycle)

• Magnet grade (N35-N52 / SH / UH / EH / SmCo) — or let us specify to your flux target

• Magnetization direction (radial / Halbach / axial) — or let us recommend

• Cup material preference (6061-T6 Al / 7075-T6 Al / 17-4PH stainless / titanium) — or let us recommend

• Wall thickness and floor thickness (or we will size to your speed and inertia budget)

• Bore diameter and tolerance (H7 / H6 / H8 / keyed / splined / bonded) — or we will spec to your shaft drawing

• Maximum continuous speed and peak speed

• Working temperature range (rotor-side)

• Dynamic balance grade (G2.5 / G1.0 / G6.3)

• Encoder / resolver target mounting requirements (or we will spec to your drive)

• Surface treatment (black epoxy / parylene / Ni / NiCuNi)

• Service environment (clean room / ATEX / oil & gas / aerospace / vacuum)

• Quantity and target delivery date

We typically return a feasibility and indicative pricing note within 2 working days, and a firm quotation within 5-7 working days.

FAIZEAL — custom cup rotors for outer-rotor BLDC motors, segmented NdFeB arc magnets on cup inner wall, closed magnetic circuit, built to your pole count, cup geometry, and bore.