Choosing Between Slotted and Slotless Motors for Aerospace Applications

In the world of aerospace, the need for robust and reliable designs that can withstand extreme environmental conditions is critical. This is particularly true of the motion solutions driving these applications forward, as they must not only deliver the necessary power and long life required for these applications, but must do so in a small, lightweight envelope. Two of the most popular motor technologies utilized throughout the Aerospace industry are slotted brushless DC motors, where the wire coil is inserted into lamination slots, and slotless brushless DC motors, which use a self-supported coil design. Here we cover the unique advantages that each of these motor topologies bring to aerospace systems.

Slotted Motors

In slotted motors, a slotted lamination stack is assembled, and the coil is wound within the slots of the stator. This construction allows for a small air gap between the laminations (stator) and magnet, resulting in highmagnetic induction and which may allow the use of a smaller magnet diameter. The volume of the copper is limited by the slot space available, which impacts the capability to wind within the slots. This motor topology is inherently very robust, given that the coil is inserted in the lamination; the design can be further ruggedized by overmolding the wound stator.

Slotless Motors

Opposing slotted motors are slotless motors, where windings are frequently self-bonded and self-supportive; the coil is then inserted directly into the air gap, resulting in a larger air gap and lower magnetic induction of the coil. The motor design and copper volume are optimized for a given diameter to achieve the ideal magnetic induction. The typical design utilizes a larger air gap, and the loss of induction is compensated for by leveraging a larger, high energy permanent magnet (like a NdFeB magnet). Smaller (<0.5”/12.7mm) and shorter slotless designs may also be simpler to manufacture at high volumes. Finally, with minimal substantially lower iron losses, mechanical power is maximized and the delivers greater efficiency and cooler operation.

Comparing Slotted and Slotless Motors for Aerospace Applications

Engineers must balance a variety of factors when designing new systems, including the need to optimize designs for Size, Weight, and Power (SWaP); prioritize precision and smoothness of operation; account for efficiency and power requirements; and meet strict environmental requirements to achieve reliability. While both slotted and slotless motors can be used for the same application, each design has unique benefits that may identify them as the ideal motor selection for a given application:

• Torque Density. Both BLDC slotless and slotted motor technologies are designed to provide high torque and deliver 10X the maximum continuous torque without magnetic saturation. With that said, slotted designs (due to having the coil inside the stator slots) feature lower thermal resistance than slotless designs, and the slotted coil is therefore able to provide higher power/torque thanks to better heat dissipation.
• Operating Temperature. Slotted coils can generally handle high temperatures up to 220°C, enabling more torque generation and unlocking specific high-temperature applications. Slotless designs can handle temperatures up to 150°C, but the self-supported nature of the design means that they may lose their integrity and collapse at extreme temperatures.
• Cogging Torque. The rotor in a slotted motor has preferred magnet positions around the lamination, generating a cogging or detent torque. While there are methods to decrease the detent torque, such as skewing the lamination. The rotor in a slotless motor sees a continuous permeance and therefore can be considered zero cogging.
• Inertia. When comparing slotted and slotted designs, both can be designed with relatively low inertias when compared to an outer rotor design. Slotless motors may use a larger magnet to compensate for the lower magnet induction of the coil, resulting in higher rotor inertia. This is another consideration when deciding which motor technology to use as certain applications such as trim actuators for autopilot systems may require a very low inertia to achieve high sensitivity.
• Robustness. A slotted motor with an overmolded stator is best suited to handle harsh environments, such as high shock and vibration, due to its very robust stator and rotor design. Slotless motors can also be designed to survive many of the same high humidity and salt fog conditions required under DO-160 standards but will have inherently lower survivability than their slotted equivalents.

Conclusion

Choosing between slotted and slotless motor technologies is a critical decision in the Aerospace industry. Slotted motors excel in durability and high temperatures, ideal for demanding applications. Slotless motors offer simplicity and efficiency, though with slightly lower tolerance for extreme conditions. Engineers must carefully weigh factors like torque density, temperature, cogging torque, inertia, and robustness to select the best motor for each aerospace application, ensuring seamless integration and meeting industry demands for performance and reliability. Our engineers are ready to assist; reach out to us here to start collaborating!