Unlocking Performance Density in Slotless Motor

Magnets are at the centre of today’s global discussions—for both good and bad reasons. 

These motor magnets play a critical role in motor performance, efficiency, and size optimization, making them essential in modern design. At the same time, they are deeply tied to complex geopolitics and trade restrictions, which make sourcing challenging.

This is why understanding magnet selection and motor architecture is key to delivering high-quality products. In a nutshell, using neodymium magnets for slotless brushless motors enhances torque density and supports high-performance electric motors.

Building on our previous blog article about the differences between slotted and slotless motors, this article explores how magnets influence motor design. Radial flux slotless brushless motors and axial flux "flat" slotless brushless motors are gaining attention for their ability to deliver zero cogging torque, smoother motion, and better efficiency—especially when magnet grade is optimized. While slotless designs benefit from higher magnet grades, these choices can introduce trade-offs in slotted motors, making this an important discussion.

Ultimately, two design factors stand out:

• Number of pole pairs
• Magnet grade

These choices directly impact torque, speed, and power density, making them essential for optimizing performance in slotless brushless DC motors.

Pole pairs combinations

Firstly, going back to the basics: the number of pole pairs refers to the count of North–South magnetic pole pairs arranged around the rotor (or stator, depending on the design).

From physics, the final speed of the motor depends inversely on the number of pole pairs for a given supply frequency. The synchronous speed formula is:

n_s=(120×f)/"number of poles" =(60×f)/"number of pole pairs"

Where:

n_s= synchronous speed (RPM)

f= supply frequency (Hz)

Key principles:

• A high number of pole pairs typically provides higher torque capability but lower speed.
• A low number of pole pairs results in higher speed but lower torque.

Of course, this varies depending on the applications:

• Applications like ventilators need high speed, so they use fewer pole pairs.
• Applications requiring high torque, such as actuation systems, need more pole pairs.

Increasing the number of pole pairs helps minimizing the magnetic flux per pole and reduce iron losses, which greatly improves torque smoothness. However, achieving this requires careful winding design—and for slotless designs with micro-dimension wires, this can be challenging. In fact, it often means a high CAPEX investment in winding machines before any customization can even be considered.

This is where the Mirmex motor coil winding technology makes a difference. Our patented imprinting process simplifies winding manufacturing and enables easy customization for each application and pole-pair combination. No more fear of customizing windings!

Magnet Grade Specifications

When designing high-performance electric motors, one factor that can significantly influence performance is the magnet grade. But what does magnet grade actually mean? In simple terms, it refers to the classification of a magnet’s performance, usually indicated by a code that combines:

• A number (e.g., 35, 52, even up to 60) representing the maximum energy product the magnet can store, measured in Mega Gauss Oersted (MGOe).
• A letter code (e.g., N, H, SH, UH) defining the temperature capability of the magnet.

For example, a magnet named N52UH has an energy product of about 52 MGOe and can operate at ultra-high temperatures up to 180°C. As suppliers push boundaries, magnet grades are reaching N60, offering even more performance potential.

Increasing magnet grade significantly enhances electric motor performance by boosting magnetic flux, which improves power and torque density—critical for compact, efficient designs.

However, in slotted motor designs, there is a limitation: stronger magnets can cause magnetic saturation in the teeth, reducing the benefit of higher grades. In contrast, slotless motor designs fully leverage stronger magnets. 

Because slotless motors have a larger apparent air gap, the magnetic field in that gap is naturally lower. This leaves room to boost the field with higher-grade magnets, enabling more compact and smooth designs that distribute torque evenly and deliver significant performance gains.

Efficiency also improves with higher magnet grades. To generate mechanical power, a motor needs torque and speed, both of which depend on the magnetic field. If most of that field comes from the magnet itself—a “free” source—the motor requires less electrical input, reducing losses and increasing overall efficiency. This is particularly valuable in applications where energy savings and thermal management are critical.

Ready to Upgrade Your Design?

While slotted motors are already limited by saturation, slotless motors still have margin to grow.

By combining high-grade magnets with Mirmex optimized coil winding technology, slotless motors can achieve torque constants and motor constants closer to those of slotted motors, while maintaining their inherent advantages: zero cogging, low inductance, and smooth torque delivery. This means slotless motors can now compete head-to-head with slotted designs, offering superior performance without sacrificing efficiency or smoothness.

If your project currently uses a slotted motor but requires low ripple and zero cogging, it may be time to consider switching to slotless technology. Contact us to explore how our advanced designs can help you achieve better results.