Slotted Motor vs. Slotless Motor: What’s the Difference? (Part 1)

If you are reading this post, chances are you are already familiar with various types of electric motors—and you may have encountered the term 'slotless motor' along the way. If you have not, don’t worry— this post will start with a quick explanation.

Like many other technological components, electric motors have their own family tree that can separate them apart.

Just as you can differentiate between a brushless vs. a brushed motor, electric motors can also be categorized as slotted or slotless.

As implied by the name, and akin to brushless motors which lack motor brushes, slotless motors are crafted without the slots (the grooves in the stator core) present in conventional designs. In the picture above, you can observe this concept in a simplified manner through a brushless electric motor diagram.

Typically, a slotted motor design includes grooves in the stator core. These grooves physically hold the current-carrying windings, which act as electromagnets - when current passes through them, they generate electromagnetic fields.

Besides their structural role in securing the windings, these grooves also help guide the magnetic flux toward the airgap.

The airgap, which is the space between the rotor and stator, is the critical region where magnetic flux from both the stator windings (via the slots) and the rotor magnets interact. This interaction is what ultimately drives the motor. This concept is clearly illustrated in the image below.

In contrast, slotless motors or as it is commonly called ''coreless motor'' use a different approach. Slotless motors or coreless motors are those sold by Mirmex.

These slotless motors do not contain a steel core with grooves and rely solely on the geometry of the motor windings to amplify the magnetic flux. Typically, the winding layout is simpler, as shown below. In general, the slotted design enhances the creation of electromagnetic fields around the copper windings, resulting in higher magnetic flux in the airgap.

This specific geometry brings both advantages and disadvantages, which we will explore further throughout this article. These pros and cons can be particularly relevant depending on the application.

Cogging Torque

One key difference between slotted and slotless motors lies in their rotational smoothness. 

In slotted motors, as explained earlier, the slots redirect the magnetic flux through their shape, locally amplifying it. This creates regions of higher magnetic flux that interact with the rotor's magnetic poles. As the rotor turns, these interactions cause a periodic disturbance known as cogging torque - a "hiccup" effect that disrupts smooth rotation.

Slotless motors, on the other hand, eliminate this issue. Since there are no slots to concentrate magnetic flux in specific regions, the magnetic field remains more uniform. As a result, a slotless electric engine exhibits zero cogging torque, leading to perfectly smooth rotation.

This smoothness translates into good NVH performance such as low noise, minimal vibration, and enhanced controllability, allowing for more precise motion control. These characteristics make slotless motors particularly attractive for applications that demand high precision and quiet operation, such as motors for prosthetics, motors for surgical tools and motors for medical tools in general.

After introducing the core concepts behind slotted and slotless electric motor designs, and highlighting the key differences between them, stay tuned for the second part of this blog series, where we will dive deeper into the specific performance factors that set them apart.

While our motors are cogging-free, make sure you’re cogged in for the next post!