Slotted Motor vs. Slotless Motor: What's the Difference?

We are back with the second part of the blog article to explore deeper the key differences between a coreless motor and slotted motors.

In this part, we will move beyond the basics and explore the practical implications of these differences—how they impact performance and how they align with specific application requirements.

We will cover critical aspects such as low inductance, iron losses, manufacturability, and conclude with a summary of what truly sets Mirmex Motor apart from the market.

Low Inductance

Slotless motors typically have lower magnetizing inductance, which means they require less energy to establish a rotating magnetic field. This is because, in slotted motors, the magnetic field is guided through steel grooves (slots), which can slow down the responsiveness of the field. By eliminating these slots, slotless motors allow the magnetic flux to respond more directly and quickly to the electrical input in the windings—resulting in a faster electrical response to the signals from the motor controller.

Another key difference is how torque behaves with increasing current. In most motors, torque increases proportionally with current. However, in slotted motors, this relationship eventually flattens out—reaching a torque plateau. This happens because the steel in the slots can only carry a limited amount of magnetic flux. Once it saturates, adding more current doesn’t increase torque. In slotless motors, this saturation doesn’t occur, so torque continues to scale with current, offering more consistent performance (possible to see in the picture below).

Finally, because slotless brushless motors, whether in axial flux or radial flux configurations, have lower inductance and avoid strong, localized magnetic flux concentrations, they maintain a more uniform magnetic field around the airgap. This reduces the risk of demagnetizing the permanent magnets, even under high current conditions.

These advantages make slotless motors or slotless windings especially useful in applications that require high torque at low speeds, without relying on gearboxes. Their precise controllability also makes them ideal motors for medical applications, high-end manufacturing, collaborative robots (cobots),  laser positioning systems, and control systems where smooth and accurate motion is critical.

Low Iron Losses

Whenever we look at an electric motor, we can break down the energy losses into different categories based on the active components involved in converting electrical power into mechanical power. These losses typically fall into three main types: copper losses, iron losses, and magnet losses.

Magnet losses are usually very low and mostly negligible, unless you are dealing with motors that use non-permanent magnetic materials—which is not the case here. That leaves us with copper and iron losses, which are much more interesting to analyze.

As explained earlier, slotless motors contain significantly less iron material due to the absence of slots—although they often still include a laminated iron stack around the windings. Because the magnetic flux passes through less iron, we can expect lower iron losses overall.

However, these iron losses become more pronounced at higher speeds. This is because the magnetic field inside the iron core changes direction more rapidly, increasing the rate of hysteresis and eddy current losses. Eddy currents are small loops of electrical current induced within the iron, which swirl around like water eddies and generate heat. The faster the motor spins, the more rapidly the magnetic field changes, and the greater these losses become.

This behavior is clearly illustrated in the graph below. These characteristics make slotless motors particularly advantageous in applications with high-speed duty cycles, where minimizing heat build-up is critical. For example, for motors made for surgical robots, excessive heat from motor losses could compromise precision—so a cooler, more efficient motor design is essential.

Manufacturability

From a manufacturing perspective, slotted motors require a more complex stator design. 

The stator must include grooves (slots), which are typically created by stacking multiple layers of laminated metal sheets to form a cylindrical shape. This process allows the slots to guide the magnetic flux and hold the windings in place.

Slotless motors, while they also use a laminated stack around the windings to enhance magnetic performance, have a much simpler cross-sectional geometry—often resembling a clean circular crown. As shown in the cross-sections of the top picture above, the difference in complexity is clear.

This complexity in slotted designs becomes a major limitation when working with very small diameters. At micro scales, it's impractical to manufacture precise slots, making slotted motors unsuitable for such applications. As a result, the true micromotor market (below 15 mm in diameter) is currently dominated by slotless designs, which are easier to produce at miniature sizes. Mirmex Motor for example manufactures ultra small diameter motors with this technology.

In summary, slotted motors typically tend to have higher inertia and suffer from cogging torque, higher inductance (which reduces electrical responsiveness), greater iron losses, and are impractical to manufacture at very small diameters. Instead, slotless brushless motors like those from Mirmex are always zero cogging motors.

This leads to a clear distinction in motor selection based on the specific requirements of each application. Whether it's precision, size constraints, or thermal performance, the choice between slotted and slotless designs depends on what matters most for the use case.

At Mirmex Motor, our custom made motors with slotless winding technology—optimized by AI software—allow us to achieve much higher power density than traditional slotless designs.

This innovations make our solutions extremely competitive in performance, even in demanding applications.

Do you have an application in mind where you'd like to assess whether our slotless technology could compete with slotted designs? Feel free to reach out.