Working principle
A brushless DC motor turns the classic DC machine inside out: the permanent magnets sit on the rotor and the windings sit in the stator, where they are easy to cool. There is no commutator and there are no brushes. Instead, a power-electronic inverter energises the stator phases in sequence, using Hall sensors or sensorless back-EMF detection to know where the rotor is. This electronic commutation is what the letters DC in the name really refer to — the supply is DC, the winding current is not.
Commutation and control
In the common six-step scheme the drive energises two of the three phases at a time and switches to the next pair every 60 electrical degrees, keeping the stator field roughly 90 degrees ahead of the rotor magnets where torque per amp is greatest. PWM on the DC bus sets the effective voltage and therefore the speed. Field-oriented control refines the same idea with sinusoidal currents, producing smoother torque and lower acoustic noise.
Efficiency and thermal behaviour
Because the rotor carries magnets rather than windings, there are no rotor copper losses and almost all the heat is generated in the stator, right next to the housing. Efficiency of 85–95 % is routine, and the torque-to-weight ratio is excellent. Removing the brushes also removes their friction, sparking and wear, so a BLDC motor can spin far faster and run for tens of thousands of hours without maintenance.
Where it is used
Brushless DC motors dominate anywhere efficiency and size matter: drones and RC models, electric vehicles and e-bikes, computer and server fans, cordless power tools, drive units in appliances, and hard-disk and pump spindles. The cost of the required electronic drive is the main drawback, but as controller prices have fallen the BLDC motor has steadily replaced brushed machines across consumer and industrial products.