robotics//actuator//brushless motor

A brushless motor is an electric motor with permanent magnets on the rotor and windings on the stator, whose current is switched between phases by electronics instead of by brushes and a commutator, and it is the motor of drones, electric vehicles, robot joints and industrial servo drives. Taking the brushes out removes the part that wears and sparks and lets the heat leave through the stationary windings, so a brushless motor gives more power for its weight and lasts longer; the price is that it cannot run without its electronics.

A brushless motor is an electric motor with permanent magnets on the rotor and windings on the stator, whose current is switched between phases by electronics instead of by brushes and a commutator, and it is the motor of drones, electric vehicles, robot joints and industrial servo drives. Taking the brushes out removes the part that wears and sparks and lets the heat leave through the stationary windings, so a brushless motor gives more power for its weight and lasts longer; the price is that it cannot run without its electronics.

The electronics are a controller in their own right. On a drone each motor has an ESC, a small microcontroller that switches the three phases at tens of kilohertz with PWM and decides when to commutate, usually without any position sensor, by reading the back-EMF the spinning magnets induce in the unpowered phase. In an industrial servo drive the motor carries an encoder (rotary encoder) and the drive runs a current loop at tens of kilohertz with field-oriented control, the innermost loop of the whole machine. The flight controller or the PLC never sees the phases; it sends a speed or torque command and the motor's own loop makes it happen.

A drone motor is specified by its Kv rating, the no-load speed per volt. Speed is roughly proportional to the voltage applied and a propeller's thrust grows with the square of speed, T=kTω2T=k_T\omega^2T=kT​ω2, so a battery that sags from 4.2 to 3.6 V per cell costs about a quarter of the thrust at the same command, a slow change in the plant that the altitude loop's integral term has to absorb (propeller thrust).

Motor and propeller are a lag in the loop. The rotor needs 10 to 100 ms, depending on size, to change speed, which is a time constant rather than a pure delay but eats phase all the same (loop delay); the minimum thrust is positive, since a fixed-pitch propeller cannot pull downwards.

Two families share the name. A BLDC motor in the narrow sense is driven with trapezoidal currents and suits the sensorless ESCs of drones; a permanent-magnet synchronous motor (PMSM) is driven with sinusoidal currents and suits the smooth torque of servo axes, on hardware that is often nearly the same.

Sensorless commutation has a failure of its own: under an abrupt throttle change the ESC can lose track of the rotor and the motor stutters or stops (a desync), one of the few ways a healthy motor can drop a drone. The bearings are its wearing part, and their vibration is what vibration analysis watches.