Key takeaways
- Radial multi-pole rings are manufactured as a single piece with curved grain orientation, sinusoidal surface flux, <1% flux variance.
- Arc segments are bonded assemblies of individual arc-shaped magnets, adhesive joints create flux discontinuities and higher torque ripple (typically 8–15% vs 2–5% for radial).
- Radial multi-pole rings require higher tooling investment but eliminate assembly labor, making them more economical at volumes above ~500 units/month.
- For humanoid robot joint motors, collaborative robots, and high-performance servo drives, radial multi-pole is the correct default.
- For industrial BLDC motors, wind turbine generators, and cost-sensitive programs, arc segments are frequently the right choice.
Overview
Overview
Radial multi-pole rings and arc segment assemblies are the two dominant architectures for multi-pole permanent magnet rotors. Radial rings are single-piece sintered NdFeB with curved magnetic orientation between poles, delivering a sinusoidal surface flux distribution. Arc segments are built by bonding individual arc-shaped magnets into a ring, with adhesive joints between each segment.
The choice between them affects torque smoothness, motor efficiency, assembly cost, and long-term reliability, and the correct choice depends strongly on the application's torque-ripple tolerance and production volume.
Side-by-side
Side-by-Side Comparison
Wins by criterion
Radial Multi-pole Ring leads on more criteria
7
Radial Multi-pole Ring
3
Arc Segment Assembly
0
Even
| Criterion | Radial Multi-pole Ring | Arc Segment Assembly |
|---|---|---|
| Typical Torque Ripple | ✓2–5% | 8–15% |
| Surface Flux Waveform | ✓Sinusoidal | Trapezoidal (with harmonics) |
| Surface Flux Variance Ring-to-Ring | ✓<1% | 2–5% typical |
| Flux Discontinuities (air gaps) | ✓None | Present at adhesive joints |
| Tooling Cost | Higher | ✓Lower |
| Pole Count Flexibility | Fixed by tooling | ✓Modular, easy to change |
| Assembly Labor | ✓None (single piece) | Manual bonding |
| Economic Volume Threshold | >500 units/month | ✓Any volume |
| Vibration and Noise | ✓Lower (smooth flux) | Higher |
| Motor Efficiency | ✓1–3% higher typical | Baseline |
✓ indicates the better option for the criterion. Winner assignment reflects typical engineering practice; your application may weight criteria differently.
Use cases
When Each Is the Right Choice
When Radial Multi-pole Ring wins
- Humanoid robot joint motors (Figure, Tesla Optimus, Agility Digit class)
- Collaborative robot actuators
- High-performance servo motors
- Premium EV auxiliary motors (EPS, compact traction)
- Production volumes above 500 units/month
- Applications where vibration or noise matters
When Arc Segment Assembly wins
- Industrial BLDC motors with moderate performance requirements
- Wind turbine permanent-magnet generators
- Designs where pole count changes frequently during development
- Low production volumes where tooling amortization is unfavorable
- Cost-sensitive commodity applications
How to choose
Decision Framework
Decision framework
If the application demands smooth torque, humanoid joints, surgical robots, precision servos, radial multi-pole is non-negotiable. The 2–5% torque ripple vs 8–15% for segmented assemblies is the difference between fluid motion and jerky control. For applications with relaxed torque-smoothness requirements and production volumes below 500/month, arc segments are frequently more economical. Both architectures can deliver on durability; the difference is primarily performance and assembly economics.
Specs
Related NdFeB Grades
N42SH
150°CWorkhorse SH-grade NdFeB for 150°C traction motors, robotics actuators, and high-duty servo drives.
N45SH
150°CHigh-flux SH-grade NdFeB for compact, high-torque motors operating continuously up to 150°C.
N48SH
150°CPremium SH-grade NdFeB, the gold-standard magnet for high-performance EV and robotics motor rotors.
Industries
Related Applications
Robotics
Radial multi-pole rings, joint-motor magnets, and high-torque servo-motor assemblies for humanoid robots, collaborative robots, and industrial robotic systems.
EV Motors
High-performance NdFeB magnets for electric vehicle traction motors, auxiliary drives, and e-axle systems, with the temperature stability and flux density required for continuous high-torque service.
Industrial Automation
NdFeB magnets for stepper motors, servo drives, linear actuators, magnetic couplings, and factory automation equipment across European and North American manufacturing.
Questions
Frequently Asked Questions
When should I use radial multi-pole rings instead of arc segments?
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Use radial multi-pole rings when torque smoothness, low cogging, or motor efficiency are important design factors, humanoid robot joints, collaborative robots, surgical devices, high-performance servo motors. The sinusoidal surface flux eliminates the torque ripple that segmented assemblies inherently produce. Production volumes above 500 units/month also favor radial rings economically because assembly labor is eliminated.
How much higher is torque ripple with arc segments vs radial multi-pole rings?
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Typically 8–15% torque ripple with arc segments vs 2–5% with radial multi-pole rings, depending on motor design and pole count. The difference comes from the flux discontinuities at adhesive joints between segments, segmented rings produce a trapezoidal surface flux distribution with higher harmonics, while radial rings produce a nearly pure sinusoidal flux that drives much smoother torque.
Are radial multi-pole rings more expensive than arc segments?
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Per unit, typically yes, radial multi-pole rings carry higher tooling cost (specialized orientation dies) and require premium manufacturing processes. At production volumes above 500 units/month, the per-unit cost converges or reverses because arc segments require manual bonding and alignment labor that radial rings eliminate. For high-volume production of smooth-torque motors, radial rings are often the more economical choice on a total-landed-cost basis.
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