Radial Multi-pole Rings vs. Segmented Arc Magnets: Which Is Better for Your Motor?

There are two primary ways to create a multipole magnetic ring for a motor rotor or stator: manufacture a single sintered ring with built-in multipole orientation (radial multi-pole), or bond individual arc-shaped magnet segments into a ring (segmented assembly). Both approaches have been used for decades, but they produce fundamentally different magnetic field characteristics. Understanding these differences is essential for selecting the right architecture for your motor.

A radial multi-pole ring is pressed and sintered as a single piece in a specially designed orientation die. During pressing, the NdFeB powder is exposed to a magnetic field that curves the grain orientation between poles. The result is a monolithic ring where the magnetic grains follow curved paths from one pole to the next. After sintering, the ring is ground to final dimensions and magnetized. The curved grain orientation produces a naturally sinusoidal surface flux distribution - the magnetic equivalent of a pure sine wave.

• •Single-piece construction: no adhesive joints, no assembly gaps
• •Curved grain orientation: produces sinusoidal flux, not trapezoidal
• •Inherent balance: uniform mass distribution, no heavy/light spots
• •Higher surface flux: 10–20% more flux at the working surface vs segmented designs

Segmented rings are assembled from individual arc-shaped magnets, each pressed and sintered separately with parallel or radial orientation. The arcs are bonded to a back-iron or housing using structural adhesive, sometimes reinforced with a sleeve. This is a mature, well-understood manufacturing process with lower tooling costs and more flexibility in pole count and geometry changes.

• •Modular design: easy to change pole count or arc dimensions without new orientation tooling
• •Lower tooling cost: standard pressing dies, no specialized orientation fixtures needed
• •Proven at scale: billions of segmented motors in production globally
• •Limitation: gaps between segments create flux discontinuities and cogging torque

The performance differences between the two architectures are measurable and significant for high-precision applications. Radial multi-pole rings deliver a smoother torque waveform, lower vibration, and higher motor efficiency. Segmented rings are adequate for applications where cogging torque and vibration are less critical.

• •Torque ripple: Radial multi-pole typically 2–5% vs 8–15% for segmented (motor-dependent)
• •Cogging torque: Significantly lower with radial multi-pole due to continuous flux transition
• •Vibration and noise: Radial multi-pole motors are measurably quieter
• •Efficiency: 1–3% higher motor efficiency due to lower harmonic losses
• •Assembly cost: Radial rings eliminate the bonding and alignment labor of segmented assemblies

The choice between radial multi-pole and segmented depends on your application requirements, production volume, and cost targets.

• •Use radial multi-pole when: Torque smoothness is critical (robotics, servo motors, medical devices), vibration must be minimized, highest motor efficiency is needed, or you want to simplify motor assembly
• •Use segmented arcs when: Cost is the primary driver (appliance motors, commodity BLDC), pole count or geometry may change frequently during development, production volumes are very high and tooling amortization favors segments, or performance requirements are moderate

We produce radial multi-pole rings in 4, 6, 8, and higher pole configurations for servo motors, humanoid robot joints, and high-performance BLDC motors. Our rings achieve <1% flux variance and sinusoidal waveform output, with dimensional tolerances to ±0.02mm on ID. We also produce segmented ring assemblies for applications where that architecture is the better fit. Our engineering team can help you evaluate both options for your specific motor design.