The Magnetization Direction Nobody Explains
When a motor engineer specifies NdFeB arc magnets, the order typically includes grade, geometry, coating, and tolerance. Magnetization direction shows up as a one-word note - "radial" - or gets omitted entirely on the assumption that the supplier knows what to do.
That assumption is correct for simple motors. For high-performance servo, robot joint, and UAV applications, it's the kind of assumption that costs you three prototype iterations to diagnose. Magnetization direction directly determines flux density waveform shape in the air gap, torque ripple, back-EMF harmonic content, and the practical tradeoff between peak torque and cogging torque. Picking the wrong one after the rotor geometry is locked is not a quick fix.
This guide covers the three magnetization approaches used in modern permanent magnet motors - radial, parallel (sometimes called "breadloaf"), and Halbach array - and when each one is and isn't the right choice.
1. Radial Magnetization: The Default That Works Until It Doesn't
Radial magnetization means the magnetic field inside the arc segment points radially - outward from the rotor center in the case of surface-mounted magnets on an inner rotor. It's the standard for good reason: it's manufacturable with standard charging fixtures, it works reliably across a wide range of arc geometries, and it produces a trapezoidal air gap flux density waveform that is well-suited to trapezoidal back-EMF motors.
The trapezoidal waveform has a flat top region where flux density is roughly constant, transitioning quickly at the magnet edges. For brushless DC (BLDC) motors driven with six-step commutation, this is exactly the waveform shape you want - it aligns with the commutation switching and produces relatively smooth torque at the intended operating speed.
The problem shows up in sinusoidally-driven PMSM motors where the control algorithm assumes a sinusoidal back-EMF. Radially magnetized arc segments produce a back-EMF with significant 5th and 7th harmonic content - typically 3–7% of fundamental in well-designed rotors, potentially higher in rotors with few poles or short arc lengths. Those harmonics appear as torque ripple at 6× the electrical frequency. At 3,000 RPM in an 8-pole motor (200 Hz electrical), that's 1,200 Hz torque ripple. In a precision machine tool or a robot joint where mechanical bandwidth is high, it's audible and measurable in the position error spectrum.
Radial magnetization is the right choice for: BLDC motors with six-step commutation, motors where cost and manufacturability dominate over torque ripple, and applications where the electrical frequency is low enough that the 6× harmonic falls below the mechanical resonance of the load.
2. Parallel Magnetization: Closer to Sinusoidal Without the Complexity
Parallel magnetization - also called "breadloaf" magnetization because of how the field lines look inside the magnet - aligns the magnetization direction parallel to a fixed axis rather than radially from the rotor center. The result is a flux density distribution in the air gap that is closer to sinusoidal than the radial case.
The quantitative improvement is modest but measurable. In a typical 8-pole surface-mounted rotor with 150° arc segments, switching from radial to parallel magnetization reduces the 5th harmonic back-EMF component from approximately 5% to 2–3% of fundamental. The 7th harmonic drops from roughly 3% to 1–1.5%. Total harmonic distortion in the back-EMF waveform falls from 6–8% to 3–4%.
For sinusoidally-controlled PMSM motors in CNC axes, collaborative robot joints, and servo drives where torque ripple specification is below 2% of rated torque, parallel magnetization is worth the small cost premium - typically 8–15% over radial for custom arc segments at production volumes.
The limitation is geometric: parallel magnetization works best when the arc segment subtends a relatively small angle. For high-pole-count motors where individual arc segments cover 30–45° of the rotor circumference, the improvement over radial is substantial. For low-pole-count motors where segments cover 60° or more, the flux density at the magnet edges diverges significantly from the intended sinusoidal shape, and the benefit diminishes.
3. Halbach Arrays: High Performance at Real Engineering Cost
A Halbach array arranges magnets so that adjacent segments have progressively rotated magnetization directions - radial outward, then tangential, then radial inward, then tangential in the opposite direction - producing a waveform that is strongly sinusoidal on one side of the array and nearly zero on the other. In a motor rotor, this means nearly all the flux is concentrated in the air gap, and very little leaks through the back-iron.
The performance benefits are real and significant. Compared to a conventional radially magnetized rotor, a Halbach array produces:
- 15–25% higher fundamental air gap flux density for the same magnet volume
- Back-EMF THD below 1% in well-designed configurations
- Near-sinusoidal torque with torque ripple below 0.5% of rated torque
- Reduced rotor back-iron thickness, enabling lighter rotors
These are the reasons Halbach arrays appear in high-performance direct-drive motors for collaborative robots, medical imaging table drives, precision semiconductor handling, and some UAV applications.
The engineering cost is equally real. A Halbach array requires at minimum four magnet segments per pole - and in practice six or eight for good sinusoidal fidelity - compared to one per pole for radial or parallel. Each segment has a different magnetization direction, which means different charging fixtures, higher piece-part cost, and more complex assembly. Positional tolerance requirements are tighter: the angular position of each segment must be held within ±0.5° to avoid degrading the flux waveform. Assembly requires precision jigs and fixtures that are not interchangeable across motor sizes.
At small production volumes - under 500 rotors per month - the tooling and assembly cost premium for a Halbach array is typically 3–5× the magnet cost of a conventional rotor. At high volumes, the premium shrinks to 1.5–2×. The correct decision depends on whether the performance improvement justifies that cost at your production scale and performance target.
4. Selection Matrix
| Motor Type / Application | Recommended Magnetization | Key Reason |
|---|---|---|
| BLDC with six-step commutation (fans, pumps, basic drives) | Radial | Trapezoidal back-EMF matches commutation; radial is cost-optimal |
| PMSM servo, CNC axis, standard industrial robot | Parallel | Reduces torque ripple to 1–2% without complex assembly |
| High-precision servo (semiconductor, medical, optics) | Parallel or Halbach (4-segment) | Torque ripple specification below 1% requires sinusoidal flux |
| Collaborative robot joint, direct-drive torque motor | Halbach (4–6 segment) | High torque density and near-zero cogging in compact form factor |
| UAV propulsion, axial flux motor | Halbach (6–8 segment) | Maximum torque-to-weight; back-iron elimination enables flat rotor design |
| Frameless dexterous hand motor | Parallel or Halbach (4-segment) | Torque smoothness critical for force control; space too limited for 6+ segment Halbach |
5. What to Confirm with Your Magnet Supplier
- ☐ Specify magnetization direction explicitly on the drawing - "radial," "parallel," or "Halbach with N-segment configuration per pole"
- ☐ For parallel magnetization: confirm the supplier has the correct charging fixture for your arc geometry; a fixture designed for a different arc angle will produce a hybrid that is neither radial nor properly parallel
- ☐ For Halbach arrays: request the magnetization direction vector for each segment position in the assembly; verify against your motor simulation before committing to tooling
- ☐ Request flux density mapping data on a sample rotor: the supplier measures magnetic flux at defined positions around the rotor circumference; this validates that the intended magnetization was achieved and provides baseline data for incoming inspection
- ☐ Confirm charging fixture calibration interval; fixture degradation is the most common source of magnetization variation in production runs
HIMAGNET produces radially magnetized, parallel magnetized, and Halbach array arc segment magnets for motor applications from engineering samples through production. For magnetization specification support on your motor design, contact our engineering team with your pole count, arc segment geometry, and torque ripple target.
