Robotics

BLDC

ActivePublished: 21 October 2026Updated: 21 October 2026Published

Key innovation

Replacing the mechanical brush commutator with electronic commutation, which removed wearing brushes and sparking while keeping the controllability of a DC motor.

How it works

A permanent-magnet rotor turns inside (inrunner) or outside (outrunner) a wound stator, most often with three phases connected in a wye (Y) or delta configuration. A controller with a transistor bridge (a three-phase inverter) energizes successive winding pairs so that the stator field leads the rotor field and sustains torque. To know which windings to energize, the controller must know the rotor position: it uses Hall-effect sensors or an encoder, or runs sensorless, inferring position from the back-EMF induced in the undriven winding. In trapezoidal control (six-step commutation) two of the three phases are energized at any instant. In sinusoidal control, typically using field-oriented control (FOC), the phase currents are modulated sinusoidally for smoother torque and quieter operation.

Problem solved

A classic brushed DC motor needs a mechanical commutator and brushes that wear down, spark, generate dust and electromagnetic interference and require periodic maintenance. This limits durability, efficiency and maximum speed and prevents operation in sealed or explosive environments. The BLDC motor solves this by moving the commutation function from the brushes to an electronic controller driven by rotor position.

Components

Permanent-magnet rotorRotor field source

The rotating part carrying permanent magnets (often neodymium). In an inrunner it spins inside the stator, in an outrunner around it.

Wound statorRotating field source

The stationary core with windings, usually three-phase in a wye (Y) or delta configuration, producing the rotating field.

Controller / inverterElectronic commutation

The electronic controller with a transistor bridge (three-phase inverter) that switches the phase currents — the electronic equivalent of the commutator.

Official

Rotor position sensingPosition feedback

Hall-effect sensors or an encoder that measure rotor position, or sensorless operation based on the back-EMF induced in the undriven winding.

Hall-effect sensorsDirect measurement of rotor position.

EncoderPrecise angle measurement, required e.g. for FOC.

Sensorless (back-EMF)Inferring position from back-EMF; difficult startup at standstill.

Official

Implementation

Reference implementations

SimpleFOC

C++ · SimpleFOC Community

Implementation pitfalls

Sensorless startupHigh

At standstill there is no back-EMF, so a sensorless controller does not know the position and startup can be unreliable.

Fix:Start from a forced open-loop phase, or add Hall sensors / an encoder.

Commutation timing errorsMedium

Incorrect phase advance relative to rotor position reduces torque and efficiency and can cause vibration.

Fix:Calibrate the angle and tune phase advance to speed.

Overheating and demagnetizationHigh

Excess heat weakens the permanent magnets and damages winding insulation; power is thermally limited.

Fix:Current limiting, cooling and selecting magnets with a higher Curie temperature.

Cogging torqueLow

Interaction between magnets and stator slots causes torque ripple, troublesome at low speed.

Fix:Skewed slots/magnets and sinusoidal control (FOC).

Evolution

1962

Electronic commutation becomes possible

Inflection point

The development of solid-state electronics in the 1960s made it possible to replace the brush commutator with transistor switching.

1984

Neodymium (Nd-Fe-B) magnets

Inflection point

The introduction of neodymium magnets substantially increased the power density and torque-to-weight ratio of brushless motors.

2016

QDD actuators in legged robotics

The spread of quasi-direct-drive actuators based on BLDC motors (e.g. in MIT legged robots) established BLDC as a standard in dynamic legged robotics.