Diametrically Magnetized Disc vs Multipole Ring Magnets Performance Guide

Magnetization Patterns Explained

Diametrically Magnetized Disc Magnets

• Magnetized across the diameter with a clear North-South polarity.
• Magnetic field lines exit one semicircle and enter the opposite semicircle.
• Commonly made from high-performance materials like NdFeB (Neodymium Iron Boron) and SmCo (Samarium Cobalt).
• Typical grades range from N35 to N52 for NdFeB and 2J85 for SmCo.

Multipole Ring Magnets

• Feature multiple alternating poles arranged evenly around the circumference—usually 4, 6, 8, or more poles.
• Two main orientations: axial (poles aligned along the magnet’s axis) and radial (poles pointing outward radially).
• Manufacturing methods include sintered magnets, offering high strength and precision, and bonded magnets, which provide complex shapes and flexibility.

NBAEM specializes in delivering both diametric discs and precision multipole ring magnets tailored for your exact application needs.

Magnetic Field Performance Comparison

When comparing diametrically magnetized disc magnets to multipole ring magnets, the differences in magnetic field performance stand out clearly:

Diametric discs produce a clear north-south field across the diameter, but edge effects can cause uneven flux distribution. On the other hand, multipole ring magnets offer superior radial field uniformity, thanks to their alternating pole layout around the ring. This design smooths out pole transitions, delivering better sinusoidal magnetic fields crucial for reducing torque ripple in applications like BLDC and spindle motors.

Finite Element Analysis (FEA) simulations confirm that multipole rings provide stronger and smoother air-gap flux density, making them the preferred option for high-performance motor designs requiring consistent torque output.

For detailed guidance on magnet performance and material grades like NdFeB, check out our comparison of N52 vs. N35 magnet grades, which dives into how material choice affects field strength and stability.

Torque & Cogging in Rotating Applications

When it comes to torque and cogging, diametrically magnetized discs tend to show higher cogging effects. This makes them a good choice for 2-pole stepper motors where precise positional holding is needed but torque ripple isn’t a big issue. The sharp 180° pole transition causes noticeable torque jumps as the rotor turns.

On the other hand, multipole ring magnets—with their multiple smooth pole transitions—offer near-zero cogging torque. This makes them ideal for servo and spindle motors requiring smooth rotation and consistent torque output. Thanks to their sinusoidal flux density, torque ripple is minimized significantly.

In fact, real test data and torque ripple formulas show that 6- and 8-pole ring magnets reduce torque ripple by up to 70% compared to diametric discs. This means quieter, more efficient motors especially useful in high-performance BLDC applications.

For those seeking reduced cogging torque and smoother motion, multipole rings are the go-to option. If you want to dig deeper into magnetic materials that withstand higher operating temperatures, the detailed SmCo magnet data sheet covers options that fit challenging motor designs.

Mechanical & Assembly Considerations

When it comes to mounting diametrically magnetized discs, press-fit or adhesive mounting methods are commonly used. However, these discs demand a strict pole alignment tolerance—any misalignment over 0.05 mm can cause noticeable performance drops. This sensitivity to pole misalignment requires careful handling during assembly.

On the other hand, multipole ring magnets benefit from more advanced mounting techniques like injection overmolding or shrink-fit, which offer stronger mechanical stability and better protection. NBAEM enforces a tight tolerance of ±0.03 mm on pole placement for multipole rings, ensuring excellent radial field uniformity and consistent magnetic performance. This high accuracy reduces risks during motor assembly and improves overall reliability.

Industry-Specific Application Matrix

When it comes to real-world use, choosing between diametrically magnetized discs and multipole ring magnets depends heavily on the application’s performance demands and cost targets.

• Drones: The 6-pole multipole ring magnet is ideal for gimbal motors, delivering smooth, precise control with about a 30% reduction in cogging torque. This means better camera stabilization and longer flight times thanks to less motor vibration.
• Automotive EPS (Electric Power Steering): Diametrically magnetized discs are a cost-effective choice here. With a simple 2-pole design, they support high-volume production—upwards of 100,000 units per year—while maintaining reliable performance in steering systems.
• HDD Spindle Motors: High-speed spindle motors benefit from 8-pole multipole ring magnets. Their superior pole placement accuracy keeps runout below 0.5%, ensuring stable RPMs and longer hard drive life.

For more details on magnetic solutions tailored to motor applications in various industries, check out NBAEM’s expert insights on the application of magnets for motor.

Cost & Scalability Breakdown

When it comes to tooling, multipole ring magnets take longer—typically 4 to 6 weeks—due to the complexity of precise pole placement. In contrast, diametrically magnetized discs require about 2 weeks, making them quicker to produce.

At a volume of 10,000 pieces, the per-piece cost difference ranges from $0.15 to $0.80, with multipole rings generally being the more expensive option because of intricate manufacturing and tighter tolerances. However, this extra cost often pays off in performance for applications needing smooth torque and sinusoidal flux density.

NBAEM offers a minimum order quantity (MOQ) of 100 prototype pieces, complete with full pole validation to ensure accurate magnetization and reliable performance before scaling up production. This service helps mitigate risks and speeds up product development.

Selection Checklist for Diametrically Magnetized Disc Magnets and Multipole Ring Magnets

Choosing the right magnet depends on your specific needs. Here’s a quick guide to help you decide:

• Need fewer than 4 poles? Go with a diametric disc magnet—simple and effective for basic two-pole setups.
• Looking for smooth, sinusoidal back-EMF (BEMF)? Multipole ring magnets are your best bet, offering higher radial field uniformity and reduced torque ripple.
• Operating temperature above 120°C? Consider SmCo multipole magnets for better thermal stability and performance.

For precision builds, especially with tight pole placement tolerances, you can contact our CTA team to get a detailed pole placement report within 24 hours.

This checklist ensures you pick a magnet that fits your application—whether it’s for cost-effective EPS steering or high-performance spindle motors.

For more on high-performance materials like SmCo, check out our detailed article on SmCo magnets.