# Coreless motor

> Source: https://aiwiki.ai/wiki/coreless_motor
> Updated: 2026-07-14
> Categories: AI Hardware, Robotics
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
> From AI Wiki (https://aiwiki.ai), the free encyclopedia of artificial intelligence. Reuse freely with attribution to "AI Wiki (aiwiki.ai)".

A **coreless motor**, also called an ironless motor, is a small electric motor whose rotor, the spinning part, carries a self-supporting winding of copper wire with no iron core wound inside it. Conventional motors wrap their winding around a laminated iron core because iron concentrates magnetic flux and boosts torque, but that core adds weight and creates small magnetic "detents" that make the motor jerk rather than glide at low speed. Cutting the iron out leaves a rotor that is far lighter, has almost no cogging, and can start, stop, and reverse in milliseconds, at the cost of running hotter and producing less torque for its size than an equivalent iron-core motor.[1][2] That tradeoff has made coreless motors a standard choice in precision equipment for decades, from camera autofocus mechanisms to surgical tools, and it is now drawing them into the fingers and wrists of humanoid robot hands, including [Tesla Optimus](/wiki/tesla_optimus)'s.[3][4]

### In brief

A normal small motor's spinning part is essentially a copper-wrapped iron rod: the iron makes it stronger for its size but heavier, and it wants to snap into certain rotational positions the way a compass needle settles toward north. A coreless motor discards the iron rod and keeps only a lightweight cup or basket of copper wire spinning around a fixed magnet. With no iron teeth for the magnet to feel, there is nothing to snap into, so the motion stays smooth even at a crawl, and with so little mass to move, the motor can change speed almost instantly. The tradeoff is that the copper winding has more electrical resistance and a poorer path to shed heat, so coreless motors run hot under sustained heavy load and suit light, precise, intermittent work rather than steady heavy lifting.

## How it works

Most small electric motors share the same basic layout: a stationary magnet surrounds or sits inside a rotating armature, and a commutator and brush pair switch the current direction in the armature's coil as it turns, so the coil's magnetic field keeps pushing against the stationary magnet's field in the same rotational direction.[1] In an ordinary "cored" motor, that armature coil is wound around a stack of thin iron laminations. The iron amplifies the magnetic field for a given amount of copper and current, which raises torque output, but it also adds rotating mass and creates the discrete magnetic teeth responsible for cogging, discussed below.[1][2]

A coreless motor removes that iron entirely from the rotor. Dr. Fritz Faulhaber, a German engineer who founded what became the FAULHABER Group in 1947, patented the technique in 1958: winding the copper coil itself into a self-supporting skewed or honeycomb pattern, commonly called a "Faulhaber winding," then bonding it with epoxy resin into a hollow cup or basket that holds its shape without any internal support.[4] The permanent magnet, rather than spinning, stays fixed inside or around this copper cup, and only the lightweight winding rotates.[1][5] Because there is no iron mass to insulate or cool, coreless brushed motors often use brushes and a commutator made from precious metals such as gold, silver, platinum, or palladium rather than graphite and copper, which lowers contact resistance and electrical noise at the small currents these motors typically run on.[1] In Chinese industry usage, where much of the humanoid-robotics supply chain now sits, coreless motors are commonly called "hollow-cup motors," a direct translation of the same cup-shaped rotor.[19]

Brushless motors can apply the same idea in reverse. A conventional brushless motor keeps its winding on a toothed, slotted iron stator and spins a permanent-magnet rotor around or inside it; a "slotless" brushless motor removes the iron teeth from the stator instead, potting a smooth, largely ironless winding in epoxy and commutating it electronically, typically through Hall-effect position sensors or sensorless back-EMF detection, rather than mechanical brushes.[6][7] Slotless and coreless are, strictly, different designs: one removes iron from the stationary winding, the other from the spinning one. In practice the motor industry blurs the terms and often markets slotless brushless motors as "coreless brushless" motors, since both eliminate the iron teeth that cause cogging.[6][7] A further variant applies the same principle to axial flux motors, where the stator core is replaced with a flat coil wound or etched directly onto a printed circuit board facing a disc of magnets; removing the iron cuts weight and material use, at some cost in magnetic flux linkage, and the approach is beginning to appear in compact, high-speed robotics and drone motors.[8]

## Cogging torque, and why coreless motors avoid it

Cogging torque is the torque a permanent-magnet motor produces with no current flowing at all, caused by magnetic attraction between the rotor's permanent magnets and the discrete iron teeth of a slotted stator.[2] The magnets are pulled toward alignment with the low-reluctance iron teeth and pulled away again as the rotor keeps turning, producing a small but repeating torque ripple. It is worst at low speed, where the rotor's own inertia is too small to smooth it out, and it is what gives an ordinary small motor its faint jerky, notchy feel when turned by hand.[2] In a typical slotted iron-core motor, cogging torque runs on the order of one to five percent of peak torque, according to one manufacturer comparison, and it can be reduced but not eliminated by skewing the stator or magnets or by choosing a fractional slot count, usually at some cost to running torque.[9]

Removing the iron teeth removes the mechanism itself. With no ferromagnetic teeth for the magnet to be attracted to, there is no periodic reluctance variation to resist rotation, so a coreless or slotless motor's cogging torque is effectively zero rather than merely reduced.[1][2][3] Manufacturers point to this first because of what it enables: a coreless motor can turn smoothly and predictably at a fraction of a revolution per second, and the torque it produces tracks drive current in a close to straight line, which makes the motor easier to control precisely in a servo loop.[3]

## Types and variants

| Variant | Where the iron is removed | Commutation | Typical use |
|---|---|---|---|
| Coreless brushed DC | Rotor: a self-supporting copper cup or basket spins around a stationary magnet | Mechanical brushes and commutator, often precious metal for low-current designs | Precision micromotors: medical infusion pumps, camera and lens actuators, small robot joints[1][5] |
| Slotless brushless ("coreless brushless") | Stator: largely ironless winding bed; permanent magnets still spin on the rotor | Electronic, via Hall-effect sensors or sensorless back-EMF detection | Surgical robotics, drones, high-speed spindles, brushless dexterous-hand actuators[6][7] |
| Coreless (ironless) axial flux | Stator core replaced with a flat wound or PCB-etched coil disc | Usually electronic and brushless | Thin, high-speed, low-profile drives; an emerging option in compact robotics[8] |

## Cored versus coreless: the tradeoffs

Removing the iron core is a bundle of tradeoffs, not a pure upgrade. The table below compares a coreless (ironless) motor with a conventional cored (iron-core) motor of similar size.

| Property | Cored (iron-core) motor | Coreless (ironless) motor |
|---|---|---|
| Rotor | Copper winding wound on a laminated iron core | Self-supporting copper winding, no iron |
| Rotor inertia | Higher | Very low, enabling fast starts, stops, and reversals[3][5] |
| Cogging torque | Present; roughly 1 to 5 percent of peak torque in a typical slotted design[9] | Effectively zero[1][2][3] |
| Torque per unit size | Higher; iron concentrates the magnetic field | Lower; one manufacturer comparison puts it at roughly 70 to 85 percent of an equivalent iron-core design[9] |
| Winding resistance for equivalent torque | Lower | Higher; more turns of thinner copper wire are needed to make up for the missing iron[10] |
| Efficiency, especially at light or partial load | Held down by fixed iron losses (hysteresis and eddy currents) that persist regardless of mechanical output | Higher; with no iron losses, efficiency has been reported up to about 90 percent in some designs[1][3][11] |
| Heat dissipation path | Winding heat conducts through the iron core and motor frame | Winding heat must cross an air gap before reaching the housing, a more limited path[12] |
| Behavior under sustained high current | Generally tolerant | Thermally fragile; coil temperature limits are typically around 100 to 125 degrees C, and sustained near-stall current can melt winding insulation and short the coil, a failure mode manufacturers call burnout[12] |
| Manufacturing | Simpler, lower cost at volume | More complex precision winding and epoxy bonding process, plus high-energy magnets, which raises unit cost[10] |
| Best suited to | High continuous torque, cost-sensitive, industrial-duty applications | Precision positioning and light, intermittent, delicate loads |

The efficiency and thermal rows explain each other. An iron-core motor loses a roughly constant slice of its input power to hysteresis and eddy currents in the core no matter how lightly it is loaded, so its efficiency sags noticeably below full load. A coreless motor has no core to lose energy in, so it keeps a flatter, generally higher efficiency curve down into light loads, which is exactly the duty cycle most precision and robotic-finger applications run.[1][3][11] The same missing iron, however, is what concentrates flux and keeps winding resistance down in a cored design; without it, a coreless motor needs more copper turns of thinner wire to hit a given torque, which raises resistance and, with it, the resistive heating produced by any given current.[10] Because that heat has to cross an air gap rather than conduct through a solid iron core, coreless motors tolerate sustained high current far worse than cored motors of similar size, which is why manufacturers commonly recommend keeping continuous torque to a fraction, often cited as around 80 percent, of a coreless motor's rated maximum and reserving the rest for short pulses.[12]

## Use in humanoid robot hands

Humanoid robot joints span a wide range of size and duty. A hip or shoulder actuator has to move a heavy limb continuously and hold position against gravity, which favors a cored brushless motor, often paired with a high-reduction gearbox such as a [harmonic drive](/wiki/harmonic_drive) or run at a lower ratio in a [quasi-direct-drive](/wiki/quasi_direct_drive) configuration. A fingertip joint in a [dexterous hand](/wiki/dexterous_hand) faces close to the opposite problem: it needs to move only a few grams of fingertip and sensor mass, but it has to do so quickly, precisely, and, when the finger touches something, compliantly, letting an external push move the joint back through the transmission instead of crushing whatever it touched.[5][13] Coreless motors fit that problem well. Their low rotor inertia means the motor itself is not what slows the finger down, their lack of cogging means there is no detent fighting fine position control, and their low winding inductance means the current, and so the torque, responds to a command almost immediately.[5][13][14] Because friction and cogging inside the motor are what usually fight against being backdriven, a coreless motor paired with a modest gear ratio is far easier to backdrive than a cored motor driving a high-reduction gearbox, though the transmission and gear ratio chosen for a given joint still matter at least as much as the motor itself in determining how backdrivable the finished joint ends up being.[13]

Tesla's [Optimus](/wiki/tesla_optimus) hand is the most visible current example. Earlier Optimus hand generations were reported to use around six actuators per hand, mounted in the hand itself and pulling tendons to the fingers.[14] The Gen 3 design, described in Tesla's hardware patents and in independent teardown coverage, instead moves the motors out of the hand and into the forearm, packing in 17 motors that drive tendons down into a hand with 22 degrees of freedom. That relocation lightens the hand for faster movement and gives the motors more room and airflow to shed heat.[15][16] Other dexterous-hand programs pursue a similar remote-actuation strategy regardless of motor type: [Shadow Robot](/wiki/shadow_robot)'s tendon-driven Dexterous Hand mounts 20 DC motors away from the fingers and pulls 24 joints by cable, which, like Tesla's forearm relocation, keeps mass and inertia out of the fingertip itself.[17] Coreless motors and remote tendon actuation are complementary answers to the same design problem.

Because coreless motors are a substantial line item in a hand's bill of materials, estimates of exactly how large a share vary by source and by hand generation, and should be read as estimates rather than settled figures. One industry teardown analysis of Tesla's Optimus hand, reported by 36Kr and by TechBuzz China's China Humanoid Robotics Tracker, put hollow-cup (coreless) motors at roughly 48 percent of the hand module's component cost;[15][19] a separate industry account put a comparable figure closer to 55 percent of hand-actuator cost.[14] Cost pressure has drawn new entrants into the coreless-motor supply base specifically for robot hands: Shanghai MOONS' Electric has marketed a domestically produced coreless motor priced well below imported equivalents, according to the same tracker, and Shenzhen-based Zhaowei Machinery and Electronics, which also sells dexterous-hand modules under its [ZW Hand](/wiki/zwhand) brand, builds integrated motor-and-gearbox units aimed at the same market.[19][20]

Not every dexterous-hand designer uses electric motors at all. [Clone Robotics](/wiki/clone_robotics), for instance, builds its hands and limbs around hydraulic "Myofiber" artificial muscles rather than motors, trading the coreless motor's heat and torque limits for the different complexity of valves and fluid management.[21] For most humanoid-hand programs, though, coreless motors, whether sourced from long-established Swiss and German precision-motor makers or from newer Chinese suppliers competing on price, remain the default answer wherever a joint needs to move light, fast, and smooth rather than heavy and strong.

## Suppliers and landscape

A small group of specialist manufacturers, most tracing back to Swiss precision-mechanics roots, has supplied coreless motors for decades; the humanoid-robotics boom has since pulled in new entrants, mostly out of China, competing largely on price.

| Maker | Headquarters | Notes |
|---|---|---|
| Maxon | Sachseln, Switzerland | Founded in 1961 as Interelectric AG; commercialized the ironless-rotor DC motor. Its RE series spans 6 to 65 mm in diameter and 0.3 to 250 W, with efficiency reported up to about 90 percent[22][23][11] |
| FAULHABER | Schönaich, Germany, with Swiss manufacturing | Dr. Fritz Faulhaber patented the self-supporting skew-wound coil in 1958. Its Swiss coreless-motor arm, formerly the independent company Minimotor SA (founded 1962, acquired by Faulhaber in 2008), merged into FAULHABER SA in 2023[4][24] |
| Portescap | West Chester, Pennsylvania, US (originally a Swiss precision-motor maker; part of Regal Rexnord since 2023) | Makes coreless brush DC motors and related low-inertia disc magnet motors[25][26][27] |
| Nidec (via Nidec Copal and Nidec Precision) | Kyoto, Japan | Makes its own separate line of coreless motors, mainly for camera lens actuators and automotive uses, unrelated to the Faulhaber and Minimotor lineage it is sometimes grouped with[28][29] |
| Shanghai MOONS' Electric | Shanghai, China | Domestic Chinese coreless and slotless motor lines marketed as lower-cost alternatives to imported motors[7][19] |
| Shenzhen Zhaowei Machinery and Electronics | Shenzhen, China | Integrates coreless motors with micro gearboxes and encoders into dexterous-hand drive modules under its [ZW Hand](/wiki/zwhand) brand[20] |

International makers such as Maxon and FAULHABER still set the performance benchmark for precision coreless motors, but the same trackers that document Chinese cost competition also note that Chinese suppliers do not yet match imported motors' performance consistency at scale, a gap several are explicitly racing to close as humanoid-robot hand orders grow.[19]

## See also

- [Brushless DC motor](/wiki/brushless_dc_motor)
- [Servo motor](/wiki/servo_motor)
- [Axial flux motor](/wiki/axial_flux_motor)
- [Actuator](/wiki/actuator)
- [Dexterous hand](/wiki/dexterous_hand)
- [Humanoid robot hands](/wiki/humanoid_robot_hands)
- [Tendon-driven](/wiki/tendon_driven)
- [Quasi-direct drive](/wiki/quasi_direct_drive)
- [Harmonic drive](/wiki/harmonic_drive)
- [Tesla Optimus](/wiki/tesla_optimus)
- [Artificial muscle](/wiki/artificial_muscle)

## References

1. Motion Control Tips, "What are coreless DC motors?" https://www.motioncontroltips.com/what-are-coreless-dc-motors/
2. Wikipedia, "Cogging torque," https://en.wikipedia.org/wiki/Cogging_torque
3. Electromate, "Coreless vs Iron-Core: Why maxon Uses Coreless Motor Design In Precision Motion Control Applications," https://www.electromate.com/news/post/coreless-vs-iron-core-why-maxon-uses-coreless-motor-design-in-precision-motion-control-applications
4. FAULHABER, "FAULHABER History," https://www.faulhaber.com/en/about-us/company/history/
5. FAULHABER, "DC-Motors with precious metal commutation, SR series," https://www.faulhaber.com/en/products/dc-motors/sr/
6. Motion Control Tips, "Slotted versus slotless DC motors," https://www.motioncontroltips.com/whats-the-difference-between-slotted-and-slotless-motors/
7. MOONS' Industries, "Slotless and Coreless products," https://www.moonsindustries.com/c/slotless-coreless-products-a06
8. University of Kentucky SPARK Lab, "Review of Coreless Axial Flux Permanent Magnet Machines for Electric Aircraft Propulsion," IEEE Transactions on Transportation Electrification, 2025, https://sparklab.engr.uky.edu/sites/spark/files/2025-06/2025%20IEEE%20Trans%20TTE%20UK%20SPARK%20Review%20Coreless%20Axial%20Flux%20Permanent%20Magnet%20Machines%20Electric%20Aircraft%20Propulsion.pdf
9. Servotecnica, "Coreless motors vs. iron-core motors," https://servotecnica.com/en/motori-coreless-vs-motori-ironcore/
10. Progressive Automations, "Cored Vs Coreless DC Motors," https://www.progressiveautomations.com/blogs/products/cored-vs-coreless-dc-motors-which-should-you-choose
11. maxon group, "Highly efficient brushed DC motors," https://www.maxongroup.com/en/drives-and-systems/brushed-dc-motors
12. Process and Control Today, "Understanding the thermal parameters of coreless DC motors," https://www.pandct.com/news/understanding-the-thermal-parameters-of-coreless-dc-motors/
13. Emergent Mind, "Backdrivable Actuators in Robotics," https://www.emergentmind.com/topics/backdrivable-actuators
14. LAM Motor, "Coreless Motors: Why They Are the Heartbeat of Humanoid Robotics," https://lammotor.com/coreless-motors-in-humanoid-robotics/
15. 36Kr, "Elon Musk's Plan to Build One Million Robots: How Many Motors, Reducers, and Lead Screws Are Made in China?" https://eu.36kr.com/en/p/3780414717129481
16. Droids (Substack), "The Forearm Is the New Hand: Inside Tesla's Optimus V3 Patents," https://droids.substack.com/p/the-forearm-is-the-new-hand-inside
17. Shadow Robot, "Dexterous Hand Series," https://shadowrobot.com/dexterous-hand-series/
18. Basenor, "Tesla Optimus Gen 3 Hands: 22-DoF, 50 Actuators Explained," https://www.basenor.com/blogs/news/tesla-optimus-gen-3-hands-22-dof-50-actuators-explained
19. TechBuzz China, "The State of Robot Hands in China," https://robotics.techbuzzchina.com/reports/robot-hands-china.html
20. ZHAOWEI, "Which Dexterous Hand Motor Is the Best?" https://www.zwgearbox.com/company-faq/which-dexterous-hand-motor-is-the-best
21. Clone Robotics, "Hand," https://clonerobotics.com/hand/
22. Electromate, "RE Series Brushed DC Motors," https://www.electromate.com/re-series-brushed-dc-motors/
23. Wikipedia, "Maxon Group," https://en.wikipedia.org/wiki/Maxon_Group
24. FAULHABER, "FAULHABER SA: Merger of Swiss companies to ensure future success," https://www.faulhaber.com/en/motion/swiss-faulhaber-companies-unite-into-one/
25. Portescap, "Stepper Motors, Disc Magnet Motors," https://www.portescap.com/en/products/stepper-motors/disc-magnet-motors
26. Portescap, "22S78 Brush DC Coreless Motor," https://www.portescap.com/en/products/brush-dc-motors/22s78-brush-dc-coreless-motor
27. Wikipedia, "Regal Rexnord," https://en.wikipedia.org/wiki/Regal_Rexnord
28. Nidec Corporation, "Coreless motor" (glossary), https://www.nidec.com/en/technology/motor/glossary/item/coreless_motor/
29. Nidec Copal, "Coreless motors" (product category), https://www.nidec.com/en/nidec-copal/product/search/category/B105/M104/S119/NCPL-Coreless%20motors/