A brushed DC motor uses mechanical commutators and carbon brushes to flip current direction in the rotor windings as it spins. A BLDC motor inverts this geometry: the permanent magnets are on the rotor, the windings are on the stator, and an electronic controller handles commutation by switching power to the stator coils in sequence. No sliding contacts, no sparking, no brush dust.

Why this matters in practice:

• Efficiency: 85–90% typical, vs. 75–80% for brushed. No friction or arcing losses at the commutator.
• Lifespan: Brushes wear out in hundreds to a few thousand hours. BLDC bearings are usually the only wear item — 10,000+ hours.
• Power density: Heat is generated in the stator (the outer shell), which conducts directly to the housing. You can pack more torque into less mass.
• Controllability: Because commutation is electronic, you get precise speed and torque control "for free."

How commutation works: Most BLDCs are three-phase. The controller energizes two of three phases at a time in a six-step sequence (trapezoidal control), or modulates all three sinusoidally for smoother torque (FOC — field-oriented control). To know when to switch, the controller needs rotor position. Two approaches:

• Hall-effect sensors (three of them, 120° apart) — simple, works at zero speed. Used in drones, e-bikes.
• Sensorless — back-EMF on the unenergized phase tells the controller where the rotor is. Cheaper, no sensor wires, but unreliable below ~5% rated speed. Used in fans, hard drives, HVAC.

Real-world example: A Tesla Model 3 rear motor is a permanent-magnet synchronous motor (PMSM — essentially a sinusoidally-driven BLDC) putting out ~211 kW from ~70 lb of active mass. A comparable brushed motor would weigh 3–4× as much and need brush replacement every few thousand miles.

Rule of thumb — sizing the controller: Peak phase current is roughly motor torque (N·m) divided by the torque constant Kt (N·m/A), times √2 if you're driving sinusoidally. For a 0.05 N·m/A motor at 5 N·m peak torque: 5 ÷ 0.05 = 100 A continuous, ~140 A peak. Your MOSFETs and bus capacitors must handle that — undersized power stages are the #1 cause of "mystery" BLDC failures.

Tradeoffs: BLDCs cost more upfront (magnets + electronics), demand a controller that can fail catastrophically (shorted MOSFET = locked rotor + smoke), and require startup logic for sensorless designs. For a 30-second-per-day toy or a cheap fan, brushed is still cheaper and good enough.