Slotted vs. Slotless Motors: What is the difference?

Slotted motors have a stator with iron teeth that create spaces called slots. In a slotless motor, there are no teeth - laminated steel is arranged as a smooth ring on the outside of the stator windings. Slotless motors typically have very low or zero cogging and a relatively low inductance. For a more detailed description see Slotless Motors vs. Slotted Motors: What is the difference?.

Why use a Slotless Motor?

Slotless motors have several beneficial attributes:‍

• Low Cogging Torque: Because there is no interaction between the stator’s slots and the rotor’s magnets, slotless motors offer virtually cogging free operation, enabling smooth and precise motion.
• High Efficiency: The absence of iron teeth reduces energy losses from eddy currents in the teeth and the permanent magnets.
• Compact and Lightweight: The slotless design often results in smaller, lighter motors compared to slotted alternatives, making them ideal for portable, lightweight, or space-constrained devices.
• Reduced Noise and Vibration: Slotless motors operate more quietly and smoothly than slotted counterparts, which can be advantageous in medical and metrology applications.
• Thermal Management: The slotless stator design allows better heat dissipation, eliminating sources of thermal inaccuracy for measurement applications. Improved thermal management also increases the motor's lifespan.
• Reliability. Frameless, slotless motors have particularly high mean-time-between failure (MTBF) because they have only two parts – the stator and rotor – and can be integrated into the driven mechanism, using its supporting structure. Any reduction in MTBF from motor bearings is eliminated because frameless motors do not use their own bearings - they typically rely on the host equipment’s bearings and mechanical enclosure.

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Where are slotless motors used?

The key attributes of slotless motors make them suitable for a variety of high-performance applications:  

Defense & Aerospace

Slotless motors are ideal for many applications including azimuth and elevation drives in gimbals for weapon aiming, ISTAR (Intelligence, surveillance, target acquisition, and reconnaissance) equipment and SATCOM (satellite communication) systems. Space constraints often require a compact motor with a large through bore and allow for only slightly larger outer diameter. Surveillance and monitoring equipment frequently has tight thermal limits to avoid measurement distortion and so the low heat dissipation of slotless motors is a major advantage. Accurately tracking moving objects at long range also requires smooth motion at low rotational speeds to avoid the image ‘jittering’ in the field of view and so low-cogging, slotless motors are ideally suited.  

Medical Devices

Slotless motors are used in a wide range of medical applications, including:

• Robotic arms for minimally invasive surgery, endoscopic and laparoscopic tools. High precision, smooth motion, and minimal cogging ensure steady and precise     instrument manipulation.
• Rotational stages in scanners, such as in computed tomography (CT) gantries or high-resolution ultrasound probes. Slotless motors provide smooth rotation, minimizing vibration and improving image quality in devices like CT, MRI and ultrasound machines.
• High-speed dental drills and polishing tools. Slotless motors enable lightweight, compact designs capable of high speeds without vibration or noise, enhancing     patient comfort.
• Insulin pumps, intravenous infusion systems, and other wearable medical devices. Precise control of motor speed and position ensures accurate delivery of fluids or medication.
• Advanced prosthetic limbs and powered exoskeletons for rehabilitation. Smooth motion and reduced torque ripple provide a natural feel and efficient operation.
• Centrifuges, analysers, and automated pipetting systems. Silent and vibration-free operation, along with high precision, is critical for sensitive instruments.
• Portable ventilators and CPAP (Continuous Positive Airway Pressure) machines. Silent operation, efficiency, and reliability make them ideal for continuous patient support.

Marine Propulsion

Slotless motors are revolutionizing underwater applications, providing energy efficient, low-noise propulsion for ROVs, AUVs, and other marine vehicles. Their high torque-to-diameter ratio and inherent cooling properties make them particularly suited for harsh underwater conditions. The thin active parts of slotless motors allow perfect integration into the thruster rims (for rim drives). Slotless motors are well suited to applications where energy efficiency is important, most notably in battery powered devices.

Metrology

For metrology, slotless motors deliver unmatched precision, which is particularly important in directly driven applications without gears. Cogging-free operation enables high levels of precision, especially on slow motion scanning and point to point motion trajectories.

Advances in Slotless Motors

Small but powerful neodymium (NdFeB) magnets in a Halbach array with advances in stator construction, enable slotless motors with particularly high torque in a compact form. Alva’s FiberPrinting™ introduces a groundbreaking approach to manufacturing slotless motors that produces stators with a very high copper fill factor. By integrating continuous fiber reinforcement into the motor’s structure, this technique offers enhanced mechanical properties and lightweight construction, while maintaining the precision and efficiency. FiberPrinting ensures optimized placement of the motor’s windings, improving the motor's thermal management and reducing weight without compromising performance.

Conclusion

The transition from slotted to slotless motors marks a significant step forward in motor technology. While slotted motors generally have superior motor constants and torque density, recent advances in slotless motor construction enable new levels of performance, precision, and compactness. This innovative approach is reshaping traditional applications and unlocking new possibilities.

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Contact

Nicolas Giraudo

Chief Commercial Officer

nicolas@alvaindustries.com

+49 40 63 69 8152

+49 175 340 11 74

Fossegrenda 1, 7038 Trondheim, Norway

www.alvaindustries.com