# Brushless DC motor

> Source: https://aiwiki.ai/wiki/brushless_dc_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 **brushless DC motor** (BLDC motor) is an electric motor design in which permanent magnets are mounted on the rotating rotor and an electronic controller switches current through stationary stator windings to keep the rotor turning, replacing the brushes and copper commutator ring of an older brush-type motor with solid-state switching and position sensing.[1][2] The design became practical in 1962, when T. G. Wilson and P. H. Trickey described using Hall-effect sensors to detect rotor position and drive transistor switches instead of a mechanical commutator.[1][3] In [humanoid robot](/wiki/humanoid_robot) engineering, one variant, the sensor-based (Hall-effect) permanent-magnet BLDC motor, often built in an outrunner layout, has become the default choice for joint [actuators](/wiki/actuator): it delivers strong torque at the low, controlled speeds robot joints actually use, while continuously reporting rotor position to the controller, something neither a brush motor nor a sensorless BLDC motor can do reliably near a standstill.[4][9]

*In brief: a BLDC motor is a magnet-and-coil motor with no brushes. A controller chip switches power through the coils in the right sequence to keep the rotor spinning, using small sensors, or in cheaper designs its own electrical signal, to know where the rotor is at each instant.*

## How a brushless DC motor works

Every BLDC motor has two main assemblies: a stator, built from stacked steel laminations wound with copper wire, and a rotor carrying permanent magnets that turns inside or around it.[1][2] Passing current through a stator winding turns it into a temporary electromagnet; because opposite magnetic poles attract, the energized winding pulls the nearest rotor magnet toward it.[2][5] If a controller left a single winding energized, the rotor would simply snap into alignment and stop, the way a compass needle settles once it points north. To keep turning, the rotor needs the controller to switch current into the next winding just as it approaches alignment, so the magnetic pull stays slightly ahead of its position. This sequential switching is called commutation, a function every kind of DC motor, brushed or brushless, has to perform somehow.[1][5]

Most BLDC motors built for precision use, including robot joints, have three phases: windings spaced 120 electrical degrees apart around the stator.[5][6] A three-phase controller, typically six power transistors arranged as three half-bridges, energizes two phases at a time in a repeating six-step sequence, producing a magnetic field that steps around the stator for the rotor's magnets to chase.[6] Two figures of merit describe how a motor turns electrical input into mechanical output: the velocity constant Kv (speed per volt, RPM/V) and the torque constant Kt (torque per amp, N*m/A), directly linked by roughly Kt = 9.55 / Kv in SI units. A low-Kv motor is, by the same physics, a high-torque motor, which is exactly the tradeoff that matters in a humanoid's joints: they rarely need high speed, but they constantly need torque, whether lifting a load or holding a pose against gravity.[7]

## Commutation methods

How a motor decides when to switch current, its commutation method, is a consequential design choice, especially in a joint that must move slowly, hold a fixed position for a long time, and resist a push without stuttering.[2][30] Three approaches are in common use.

| Method | How it senses rotor position | Strengths | Weaknesses |
|---|---|---|---|
| Mechanical (brushed) | Carbon or copper brushes slide against a segmented commutator ring, switching current as the shaft turns | Simple, cheap, needs no electronics | Brushes wear, arc, and generate electrical noise; limited life; unsuited to sealed, maintenance-free joints |
| Sensor-based electronic | Hall-effect sensors (or a higher-resolution encoder) detect the rotor magnets' field and report position to a controller, which switches transistors accordingly | Works at any speed, including a dead stop; reliable torque while holding position | Extra sensors, wiring, cost; accuracy depends on sensor placement |
| Sensorless electronic | The controller measures the voltage induced in an undriven winding by the spinning rotor magnets (back electromotive force, or back-EMF) and infers position from where that signal crosses zero | No dedicated position sensor; fewer parts and wires | Back-EMF is proportional to speed, so the signal is too weak and noisy near zero speed; the motor needs an open-loop startup routine first |

Mechanical commutation is rarely used in any modern precision motor.[1][2] Sensorless electronic commutation is common and effective in motors that spin at a roughly constant, moderate-to-high speed, such as drone propellers, cooling fans, and pumps, but it performs poorly at low speed and cannot reliably produce torque from a standstill, since there is no usable back-EMF signal when the rotor is not moving.[8][9] That is precisely the situation a humanoid robot joint is in for much of its duty cycle: holding an arm in place, easing a foot down slowly, or applying steady torque against a load. Sensor-based electronic commutation, almost always using Hall-effect sensors, has therefore become the default in robot joints, which is also why the motor plus its feedback electronics is often called a [servo motor](/wiki/servo_motor): one built to reach and hold a target position, speed, or torque under continuous feedback, rather than simply spinning when given a voltage.[9][30]

## Electromagnetic geometry

### Inrunner and outrunner

Where the rotor sits relative to the stator is the first major geometric choice in a BLDC motor's construction. In an inrunner motor, the magnet-carrying rotor spins inside the stator, with the windings outside it. In an outrunner motor, the arrangement is reversed: the stator sits in the middle, and a cup- or bell-shaped rotor surrounds it and spins on the outside.[10][1]

The choice has a direct mechanical consequence, similar to pushing a door near its hinge versus near the handle: applying magnetic force farther from the axis of rotation produces more torque for the same force, at the cost of top speed, while force closer to the axis favors speed over torque.[10] Because an outrunner's magnets sit at a larger radius, and that larger circumference also leaves room for more magnetic poles, outrunners generally deliver higher torque density and torque per amp (Kt) at a given size, while inrunners generally spin faster per volt (higher Kv) and carry lower rotational inertia.[10][1][12]

| | Inrunner | Outrunner |
|---|---|---|
| Rotor position | Inside, surrounded by the stator | Outside, surrounds the stator |
| Typical Kv | Higher (faster per volt) | Lower (more torque per volt) |
| Torque density | Lower | Higher |
| Pole count | Typically fewer | Typically more |
| Rotor inertia | Lower | Higher |
| Common use | High-RPM propellers, pumps, power tools | Robot joints, direct-drive wheels, gimbals |

Because humanoid robot joints prioritize controllable, holdable torque over rotational speed, outrunner geometries are common in robot-joint motors.[4][12] A third-party teardown of the joint motor in Unitree's Go2 quadruped documented a 36-slot, 42-pole outrunner design with a diametric ring magnet and a velocity constant of about 44 RPM per volt, paired with a roughly 6.2:1 [planetary gearbox](/wiki/planetary_gear_train), an architecture broadly representative of the outrunner-BLDC-plus-modest-reduction approach used across legged and humanoid robotics.[11] The same high-pole-count outrunner "gimbal motor" style, adapted from camera stabilization hardware, was popularized in legged robotics by MIT's Mini Cheetah for its favorable torque-to-cost ratio.[12]

### Radial, axial, and transverse flux

A second, independent geometric axis concerns which direction the magnetic flux travels between rotor and stator.[13]

| Flux path | Shape | Maturity | Notes |
|---|---|---|---|
| Radial flux | Cylindrical; flux crosses the air gap perpendicular to the shaft | Mature, over a century of industrial development | The default choice; well understood, broad supplier base, cost-effective at scale[13] |
| [Axial flux](/wiki/axial_flux_motor) | Flat, disc-shaped; flux runs parallel to the shaft | Commercially growing, harder to manufacture | Enables a shorter, flatter motor for the same torque where axial space is tight; a double-sided stator adds manufacturing difficulty[13][14] |
| Transverse flux | Three-dimensional path: axially through the stator, radially across the air gap, circumferentially through the rotor | Least mature, limited industrial track record | Can reach high torque at low speed from a very high pole count, but structural complexity has kept it out of most production robots[14] |

Radial-flux motors remain dominant in humanoid robot joints because the supply chain, tooling, and design experience are further along, but axial-flux designs are drawing interest where axial length is the binding constraint, such as compact hip or ankle actuators.[13][14]

## Permanent magnet type and geometry

### Surface-mounted versus interior magnets

Within the rotor, magnets can sit on the outer surface, a surface permanent magnet (SPM) design, or embed in slots cut into the iron, an interior permanent magnet (IPM) design.[15][16]

SPM rotors produce a cleaner, more sinusoidal air-gap field, which generally means lower cogging torque (the position-dependent, current-free torque variation from rotor magnets snapping toward stator teeth) and lower overall torque ripple.[15][17] IPM rotors bury the magnets, protecting them mechanically at high speed and adding a second source of torque, reluctance torque, from the rotor iron's own magnetic asymmetry, on top of the usual magnet torque. That combination can push peak torque and power higher, and IPM rotors are also easier to run at high speed using flux-weakening control (injecting current to oppose the magnets' own field so back-EMF does not outrun the drive voltage), a major reason IPM design dominates electric-vehicle traction motors.[16][15] The costs are a more expensive rotor and, in most designs, somewhat higher torque ripple than a comparable SPM motor.[15]

Because a humanoid joint values smooth, low-ripple torque at modest speed over high-speed power, engineering logic favors surface-mounted rotors for joint motors; interior designs are more strongly associated with high-speed traction and industrial motors, though some joint actuators use interior magnets for mechanical durability or extra torque in a constrained size.

### Magnet material: neodymium and its tradeoffs

Almost all high-performance BLDC motors, including those used in humanoid robot joints, use sintered neodymium-iron-boron (NdFeB) magnets, the strongest commercially available permanent magnets.[18][1] A stronger magnet packs more force into a smaller volume: every gram of motor mass in a wrist has to be lifted and accelerated by every joint between the wrist and the torso, so a weaker magnet that forces a larger motor imposes a mass penalty compounding outward along the limb.

NdFeB magnets weaken with heat. Every NdFeB magnet has a Curie temperature, typically 310 to 400 degrees Celsius depending on composition, above which its magnetism disappears entirely; well below that point, heat already causes gradual, partly irreversible loss of field strength, which is why manufacturers sell temperature-rated grades, from standard grades good to about 80 degrees Celsius up through suffixed M, H, SH, UH, and EH grades rated for roughly 100 to 200 degrees Celsius.[19] Higher-temperature grades gain most of their thermal stability by substituting some neodymium with the heavy rare-earth elements dysprosium or terbium, which raise coercivity (resistance to demagnetization) at some cost to magnetic strength and, as discussed below, supply risk.[20] That margin matters for a robot-joint motor, which can spend long periods pushing continuous current to hold a static load, generating heat with little cooling airflow from motion.

Neodymium's other major constraint is supply-chain concentration. China accounts for roughly 60 to 70 percent of global rare-earth mining, around 90 percent of the refining that turns ore into usable rare-earth oxides, and 93 to 94 percent of finished sintered NdFeB magnet manufacturing, according to the International Energy Agency and the Center for Strategic and International Studies.[21][22] Beginning in April 2025, China imposed export-licensing controls on seven heavy rare-earth elements and related magnet products; further restrictions announced that October extended licensing, from December 2025, to components made anywhere using Chinese-origin rare-earth material or technology.[21] Reported effects were significant, with sharp export drops and rising prices outside China through mid-2025 forcing some automakers to slow production.[22][21] Because dysprosium and terbium, used for high-temperature magnet grades, are among the controlled heavy rare earths, the magnets best suited to a hot-running, continuously loaded robot joint sit closest to the exposed part of the supply chain.[20][21]

In response, a small number of companies have begun building non-Chinese NdFeB supply. The most advanced effort is [MP Materials](/wiki/mp_materials), which mines rare-earth ore at Mountain Pass, California, and by early 2025 had brought a magnet plant called Independence online in Fort Worth, Texas, reporting commercial neodymium-praseodymium metal production and early sintered NdFeB magnet output, with General Motors named as a customer.[23][24] Even so, analysts generally expect rare-earth and magnet supply to stay tight through 2026, as new non-Chinese capacity ramps slowly against combined demand from electric vehicles, wind power, defense, and robotics.[21][22]

## BLDC versus PMSM: trapezoidal and sinusoidal drive

"BLDC motor" and "permanent magnet synchronous motor" (PMSM) describe overlapping hardware driven in two different ways, and the industry is not fully consistent about where one term ends and the other begins.[25][5] In the classic distinction, a BLDC motor uses concentrated windings, each coil wound around a single stator tooth, producing a trapezoidal (flat-topped) back-EMF; the controller drives it with roughly square current pulses in a six-step, 120-degree-per-phase pattern timed off Hall sensors.[25][26] A PMSM uses distributed windings, spread across several teeth to approximate a sine wave, producing a sinusoidal back-EMF; it is driven with continuously varying sinusoidal current, usually through field-oriented control, a vector-control algorithm that splits stator current into a torque-producing component and a flux-controlling component. Field-oriented control needs continuous, high-resolution position feedback from a [rotary encoder](/wiki/rotary_encoder) or resolver, rather than the six discrete states Hall sensors provide.[25][26]

The practical tradeoffs follow from those waveforms. Six-step BLDC drive is electronically simpler and cheaper, but the mismatch between an imperfect trapezoidal back-EMF and switched square-wave current produces more torque ripple, noise, and lower efficiency, especially at low speed, exactly where a robot joint spends much of its time.[25] Sinusoidal, field-oriented drive of a PMSM produces smoother torque and generally higher efficiency across a wider speed range, at the cost of finer position feedback and more sophisticated control software.[25][26] In practice, many motors sold as "BLDC" in robotics, including most robot-joint actuators, use near-sinusoidal windings and field-oriented control, blurring the line; "BLDC" has become a catch-all for any electronically commutated permanent-magnet motor, with "trapezoidal versus sinusoidal drive" the more precise description of what actually matters.[5][25]

## Compared with other motor types

### Versus the stepper motor

A [stepper motor](/wiki/stepper_motor) is also a permanent-magnet motor driven by switched current, but built and controlled very differently. Where a BLDC motor typically has a few to a few dozen magnetic poles and needs position feedback to commutate smoothly, a stepper motor is built with many more poles, commonly a toothed rotor with 50 tooth pairs in a two-phase hybrid design, so each electrical pulse advances the shaft by one small, fixed increment, typically 1.8 degrees, or 200 steps per revolution.[27] Because each step corresponds to a defined magnetic equilibrium position, a stepper motor can run open-loop, without any position sensor, simply by counting pulses, which is what makes it cheap to control.[27][28] The tradeoff is that it cannot detect a missed step: if the load momentarily exceeds available torque, the rotor slips a pole and the controller has no way to know its position estimate is now wrong.[28] That failure mode, plus poor efficiency when holding torque continuously, is why stepper motors are common in 3D printers and camera sliders, but rarely used in humanoid robot joints, which must sense and resist unexpected loads, such as a stumble or a bumped arm, rather than assume none will occur.[28][4]

### Versus the coreless motor

A [coreless motor](/wiki/coreless_motor) removes the iron core that a conventional BLDC motor's stator winding is normally wrapped around; instead, the copper coil is wound into a self-supporting hollow cylinder or basket shape strong enough to hold together on its own.[29][31] Removing the iron eliminates cogging torque entirely, since cogging comes from the rotor's magnets being attracted toward the stator's iron teeth, and a coreless motor has no iron teeth to be attracted to; this makes for smooth, low-vibration rotation even at very low speed, and the lighter, iron-free rotor also has much lower inertia, so it starts, stops, and reverses direction faster than an iron-core motor of similar size.[29][31] This does not remove all torque ripple: any energized motor, coreless included, can still show ripple from the interaction between winding current and the magnetic field, known as armature reaction.[17] The tradeoff is that iron amplifies a magnet's field, so without it a coreless motor needs more copper turns to produce the same torque as an iron-core motor of similar size; more turns mean more resistance, which means more waste heat for a given torque, making coreless motors comparatively fragile against sustained high current or stall.[29][31]

These properties point coreless motors toward a different role than the iron-core BLDC motors used in shoulder, hip, or knee joints. Because [dexterous hand](/wiki/dexterous_hand) fingers need to move quickly and delicately, add minimal inertia, and rarely sustain high stall torque, coreless motors are common in humanoid hand and finger actuation, while the higher-torque, more continuously loaded joints elsewhere in the body almost always use conventional iron-core BLDC motors.[31][4]

## Motor types compared

| | BLDC (trapezoidal, Hall-sensored) | PMSM (sinusoidal, FOC) | Stepper | Coreless DC | Brushed DC |
|---|---|---|---|---|---|
| Commutation | Electronic, sensor-based (usually Hall effect) | Electronic, sensor-based (usually encoder or resolver) | Open-loop pulse counting; no feedback required | Electronic, sensor-based or sensorless | Mechanical (brushes and commutator) |
| Drive waveform | Trapezoidal, six-step square wave | Sinusoidal, field-oriented | Stepped DC pulses | Trapezoidal or sinusoidal | DC, self-commutated by brush position |
| Torque at zero speed | High, if sensor-based | High | High holding torque, but no load feedback | High, if sensor-based | High |
| Cogging torque | Present | Present | Present (detent torque) | Absent (no iron core) | Present |
| Maintenance | None (no wear parts) | None | None | None | Periodic brush replacement |
| Typical humanoid use | Shoulder, hip, knee, and other high-torque joints | Increasingly used for the same joints under FOC drive | Rarely used | Hand and finger actuators | Rarely used |

## The motor's place inside a robot actuator

A bare BLDC motor spins quickly and produces relatively little torque; almost nothing in a humanoid robot connects a motor's shaft directly to a joint.[2][30] Instead, a robot actuator packages the motor with the other parts needed to turn raw rotation into safe, controllable joint motion: a reducer, most often a [harmonic drive](/wiki/harmonic_drive) or planetary gearbox, that trades speed for torque; a rotary encoder or resolver for position feedback beyond what Hall commutation alone provides; often a dedicated [force-torque sensor](/wiki/force_torque_sensor); precision bearings to carry structural loads; and driver electronics, frequently built as one integrated module rather than separate parts bolted together.[30][32][33] Because the motor is built to be embedded directly into that structure rather than sold as a stand-alone product, robot-joint BLDC motors are often sold and described as "frameless" or "torque" motors: bare rotor-and-stator sets, with the machine's own bearings and housing supporting the rotor.[35][36]

Tesla's [Tesla Optimus](/wiki/tesla_optimus) illustrates the pattern. Published specification summaries describe its rotary joint actuators as combining a custom, self-developed permanent-magnet motor with a harmonic-drive reducer, a torque sensor, an encoder, driver electronics, and [cross-roller](/wiki/crossed_roller_bearing) and angular-contact bearings in a single housed unit, in three rotary variants reportedly rated around 20, 110, and 180 newton-meters, alongside three linear variants built around [planetary roller screws](/wiki/planetary_roller_screw) for the legs.[34][32] [Unitree](/wiki/unitree)'s joint modules follow a similar recipe: frameless torque motors integrated with low-backlash reducers, high-resolution encoders, and force or torque sensors into single joint units, matching the outrunner BLDC motor and planetary gear stage the Go2 teardown documented directly.[33][11]

This integration is also why sensor-based commutation matters so much for humanoid joints specifically. A joint actuator's controller does not just need to know which of six 60-degree sectors the rotor is in; it typically layers a finer encoder on top for closed-loop position and, indirectly, torque control.[30][33] But even that finer encoder depends on the motor being commutated smoothly at every speed down to zero, including while holding a pose or easing into contact, exactly the regime where sensorless back-EMF sensing fails.[8][9] A related architecture, [quasi-direct drive](/wiki/quasi_direct_drive), pairs an outrunner BLDC motor with a deliberately low gear reduction (single digits up to a few dozen to one, rather than a harmonic drive's typical 50:1 to 100:1), so the motor's own torque, not gearing, supplies most of the joint's output. The lower reduction sacrifices torque density but keeps the joint backdrivable, which matters for impact tolerance in legged locomotion and for [reinforcement learning](/wiki/reinforcement_learning)-trained controllers that sense contact through the joint itself.[12]

## Suppliers and landscape

No single company builds a complete humanoid-robot joint-motor stack from mined ore to finished actuator; the supply chain splits between magnet producers, frameless-torque-motor makers, position-sensor makers, and robot builders that increasingly design their own motors in-house.

| Company | Role | Notes |
|---|---|---|
| [Nabtesco](/wiki/nabtesco) / [Harmonic Drive Systems](/wiki/harmonic_drive_systems) | Reducers, not motors | Best-known strain-wave reducer makers, often paired with robot-joint BLDC motors |
| [Moog](/wiki/moog) | Motors and actuators | Long-established maker of brushless-motor rotary servo actuators for aerospace, defense, and industrial robotics[38] |
| [Novanta](/wiki/novanta) (Celera Motion) | Frameless motors and encoders | Sells frameless direct-drive BLDC motor kits and non-contact encoders for robot joints[36] |
| [Damiao](/wiki/damiao) | Integrated joint actuators | Chinese maker of integrated BLDC-plus-reducer-plus-encoder joint-motor modules used in robot-arm and humanoid research[39] |
| [Melexis](/wiki/melexis) | Position-sensor ICs | Hall-effect and linear Hall/TMR sensor chips used for BLDC commutation feedback[35] |
| [TDK](/wiki/tdk) | Magnets and sensors | Sintered NdFeB and ferrite permanent magnets, plus Hall/TMR sensor ICs used in motor commutation[37] |
| [MP Materials](/wiki/mp_materials) | Rare-earth mining and magnets | Most advanced vertically integrated US miner-to-magnet NdFeB producer, supplying General Motors from its Fort Worth facility[23][24] |
| [Unitree](/wiki/unitree) | Vertically integrated robot maker | Designs its own frameless torque motors, reducers, encoders, and drivers in-house[33] |
| Tesla | Vertically integrated robot maker | Designs its own permanent-magnet motors and actuator modules in-house for Optimus[34] |

Other established brands with a long track record in high-performance BLDC and coreless motors for robotics, such as Kollmorgen, Maxon, and T-Motor, are not yet covered as standalone AI Wiki articles but supply widely used components. Beyond individual companies, industry researchers generally describe China as supplying a large majority of the humanoid-robotics motor and actuator supply chain, mirroring its dominance of magnet manufacturing described above, while Western and Japanese suppliers concentrate more on frameless motor kits, sensors, and specialty materials.[33][21]

## See also

- [Actuator](/wiki/actuator)
- [Servo motor](/wiki/servo_motor)
- [Coreless motor](/wiki/coreless_motor)
- [Stepper motor](/wiki/stepper_motor)
- [Axial flux motor](/wiki/axial_flux_motor)
- [Harmonic drive](/wiki/harmonic_drive)
- [Quasi-direct drive](/wiki/quasi_direct_drive)
- [Rotary encoder](/wiki/rotary_encoder)
- [Humanoid robot](/wiki/humanoid_robot)
- [Tesla Optimus](/wiki/tesla_optimus)

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