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Conventional and Slotless Motors: What You Need to Know

17 Jun 2021
Kollmorgen Experts

Conventional brushless and slotless motors both have their strengths and weaknesses when it comes to performance, but when choosing between the two, it’s not a question of which motor is best. It’s a question of which motor is best for the application.  

Many applications that require a smooth and precise mechanical system can benefit from motors that deliver high torque in a compact package. Take for example electro-optical/infrared systems. In an EO/IR system, a stable, responsive platform with accurate and precise movement is needed for tracking objects even under jarring conditions due to road shock, air turbulence, vibration and other forces.  

However, mechanical and environmental considerations—such as torque ripple or the forces on the system in on-the-move applications—can disrupt the visual or infrared sensor feedback, requiring additional software to stabilize and smooth the image. A smooth and stable mechanical system that corrects for the environment at the point of data collection can reduce the computing processing required to stabilize images.  

Since cogging torque and torque ripple can be addressed through motor design and the control system, a motor’s torque per volume is a better indication of performance in an EO/IR system. Conventional motors provide higher torque per volume than slotless motors, and for low speed systems (<1,000 rpm), such as an EO/IR system, they deliver higher available peak and continuous torque for more responsive performance.   

How Do Cogging Torque and Torque Ripple Come into Play? 

To understand the differences between conventional motors and slotless motors, it’s important to understand the forces of cogging torque and torque ripple and how the two play out in an EO/IR system.  

Cogging torque occurs in the non-energized state of a conventional motor when the attraction between the permanent magnets on the rotor and the steel teeth of the stator laminations create a “jerking” motion when it’s rotated. Slotless motors don’t experience this jerking as there are no teeth in the stator and the magnets are attracted to the lamination throughout the rotation.  

Torque ripple occurs in the energized state of a motor, both conventional and slotless. This is due to variances in the electromagnetic fields as the rotor and stator interact. Even though cogging torque can be reduced in a slotless motor, torque ripple is always present and must be addressed through high-resolution feedback and the use of advanced control algorithms. (Learn more in our blog “Cogging Torque and Torque Ripple: What You Need to Know.”)

For low-speed systems, such as EO/IR, cogging torque is generally not the most important feature for a servo motor. Even though eliminating cogging torque might have some impact on torque ripple, it is the behavior of the motor when it’s energized that matters most. And since all motors have torque ripple, including slotless, focusing on this attribute over other motor design attributes obscures the benefits of a conventional motor over slotless. Torque per volume is the leading performance indicator over cogging torque or torque ripple when selecting a motor for an EO/IR application.

The Push for Slotless 

Unlike conventional motors, slotless motors do not have steel teeth on the stator (this is why a conventional motor is sometimes called a slotted motor). Instead of teeth to support the windings, the stator lamination is constructed of steel rings stacked together with copper coils mounted to them and then encapsulated. As a result, the coils are positioned in the gap between the stator lamination and the rotor magnets.   

A large air gap between the rotor and the stator limits the amount of torque a motor can produce.  Creating a smaller air gap—the space between the magnet and the stator—can generate more torque. Halving this distance will get you four times the torque. Slotless motors try to narrow this gap as much as possible to squeeze out the most torque. However, too tight of an air gap creates a problem. To close the gap, most manufacturers will use larger magnets. But this increases cost. Conventional brushless motors can produce greater torque without having to push the limits (and costs) of manufacturing.  

As you can imagine, having the windings encapsulated in the stator lamination provides several advantages in a slotless motor. As mentioned above, it has no cogging torque. Since there are no teeth in the lamination to interact with the rotor magnet, the motor exhibits smooth running characteristics. Torque production is predictable and highly controllable because it is directly related to the current supplied to the winding. Slotless motors have low core losses at high speeds (not generally seen in EO/IR applications) and can be effectively used for low-weight, low-torque and stable-condition applications. They are less effective in highly dynamic EO/IR applications.  

Even though cogging torque is eliminated in a slotless motor because of the lack of teeth in the lamination, it still experiences torque ripple. Because the motor needs to be in an energized state to produce torque, the benefit of having no cogging torque is minimized. For precise and sensitive applications like EO/IR, torque ripple still needs to be eliminated through feedback controls.

Conventional Brushless Motors 

Conventional motors with tooth laminations, sometimes called slotted motors, have slotted steel laminations that are stacked together with copper windings inserted into these slots. The part of the stator closest to the rotor is called the tooth, and it focuses the electromagnetic flux toward the rotor magnets, concentrating the energy better than a slotless design.  

Compared to a slotless motor, conventional motors provide a good balance between torque output, motor constant, efficiency and manufacturability. For their size, conventional brushless motors have a high motor constant with high efficiency and high acceleration rates with lower inertia. This gives a conventional motor less armature reaction at high current, less torque ripple, and higher continuous torque at low speeds. 

There are some challenges with conventional motors that have a high pole count. They are generally less efficient and have lower torque at high speeds. But the biggest issue raised against conventional motors is the presence of cogging torque. However, as we’ve explained above, when it comes to low-speed, high-torque applications, cogging torque is less of an issue than torque per volume in an energized state.

Overcoming Cogging Torque 

Conventional motors, with their higher torque per volume, efficiency and output, are best suited for low-speed systems such as highly dynamic EO/IR applications. Where they also stand out is in their manufacturability. Through various co-engineered and standard designs, we’ve been able to minimize cogging torque in conventional motors. For example, choosing the right slot/pole combinations can minimize cogging torque. Other considerations include: 

  • Kw or winding factor, which determines the effectiveness of the interface between the magnet flux and winding.  
  • Winding insertion, such as needle winding with moderate slot fill or hand winding with a higher slot fill factor. However, higher slot/pole combinations take longer to wind and have longer end turns. 
  • Armature reaction. Lower slot/pole combinations have more armature reaction. 

There are several other design options and steps that we can take to minimize the impact of cogging torque. They might reduce the efficiency of the motor, but based on the system design they might not be necessary: 

  • Pole span (width of magnets). 
  • Magnet shaping. 
  • Slot opening and tooth tip design. 
  • Stacking skew for up to 90% reduction of cogging torque.

Conventional Motors Still the First Choice 

In systems where smooth running, high acceleration rates and high torque constant are needed, conventional brushless motors are still the first choice. When the load is low, a slotless motor can be a good alternative. However, they don’t have the range of performance for aggressive higher-torque applications. Low-speed, high-torque EO/IR systems need superior torque per volume to create smooth, stable systems that correct for environmental disturbances and reduce computing processing requirements. Conventional brushless motors can produce higher peak torque, while slotless motors don’t perform as well in these medium- to high-torque applications.  

Kollmorgen has the co-engineering expertise to design conventional motors that meet the needs of EO/IR systems while minimizing cogging torque. Ready to discover what your application is capable of? 

About the Author

Kollmorgen Experts

Kollmorgen Experts

This blog was a collaborative effort among a team of motion and automation experts here at Kollmorgen, including engineers, customer service and design experts. Wherever you are in your project, we’re here to help.

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