Brushless DC Motor (BLDC)

How a BLDC motor works

Three things make a BLDC: a permanent-magnet rotor, a three-phase wound stator, and an electronic controller (often called an ESC, electronic speed controller, in hobby contexts). The roles of stator and rotor are swapped from a brushed DC motor — magnets on the rotor, copper on the stator — which means heat is generated where it can be dissipated through the case, not in the spinning part.

The controller uses three half-bridges (six MOSFETs total) to apply DC bus voltage to the three motor terminals in a sequence that drags the rotor field 60° at a time. With a 4-pole-pair rotor, six commutation steps produce 60° of mechanical rotation. The controller has to know rotor angle to commutate at the right moment — either via three Hall-effect sensors mounted in the stator, or by measuring back-EMF on the floating phase between drive pulses.

           DC bus +
              │
        ┌─────┴─────┐
        │           │
       [Q1]        [Q3]   [Q5]      6 MOSFETs in 3 half-bridges
        │           │      │
        ●───── A ───●      │ ←─ phase A → motor coil A
        │           │      │
       [Q2]        [Q4]   [Q6]
        │           │      │
        └─────┬─────┘
              │
           DC bus −

   six-step sequence: AB, AC, BC, BA, CA, CB → rotor turns 360°

Worked example: torque–speed at 24 V

A typical hobby quad-copter motor is rated 920 KV. Drive it from a 4-cell lithium-polymer pack at 14.8 V nominal:

   no-load speed = KV × V = 920 × 14.8 = 13,616 rpm

Now scale up to a 24 V e-bike motor with a 100 KV winding:

   no-load speed = 100 × 24 = 2400 rpm

Torque per amp is the inverse of KV (in the same SI-consistent units): K_t (N·m / A) ≈ 60 / (2π · KV) = 9.55 / KV. So our 100 KV e-bike motor produces about 0.0955 N·m of torque for every amp of phase current. Pulling 30 A peak gives 2.86 N·m at the shaft — multiplied by a 14:1 hub-gear or chainring ratio that's roughly 40 N·m at the wheel, enough to climb a moderate hill.

The output power follows P = T · ω. At 1500 rpm (157 rad/s) and 2.86 N·m the motor delivers 449 W mechanical. Electrical input at 24 V × 30 A = 720 W. The efficiency 449/720 ≈ 62% is on the low side because the example puts the motor near stall; at 80% rated speed the same machine reaches 88–92%.

Trapezoidal vs sinusoidal commutation

• Six-step (trapezoidal, classic BLDC). The controller energises two phases at a time at full DC bus voltage, switching every 60° electrical. Cheap, simple, runs from a basic micro plus comparator. Torque ripple is about 14% peak-to-peak, audible as a low whine. Standard on hobby drones and HVAC fans.
• Sinusoidal (PMSM with FOC). The controller modulates each phase with PWM to track a sinusoidal current reference. Field-oriented control (FOC) splits the current into a torque-producing component (q-axis) and a flux component (d-axis), and regulates each independently. Torque ripple drops to about 1%, efficiency rises 2–4 percentage points, and zero-speed stalled torque is rock-steady. Standard on EV traction, high-end industrial servos, and any robotics joint that has to be silent.
• Direct torque control (DTC). An alternative to FOC that estimates flux and torque directly and switches the inverter to the nearest correct vector. Faster transient response than FOC, slightly noisier acoustically. Used in some industrial drives.

BLDC vs PMSM vs SRM control schemes

Approach	Motor type	Drive complexity	Torque ripple	Position sensing	Typical applications
Six-step (trapezoidal)	BLDC (trapezoidal back-EMF)	Low — six MOSFETs + 8-bit MCU	~14%	3 Hall sensors or sensorless back-EMF	Drones, computer fans, e-bikes
FOC sinusoidal	PMSM (sinusoidal back-EMF)	Medium — DSP/Cortex-M, Park transforms	~1%	Encoder, resolver, or sensorless observer	EVs, robotics, AC compressors
Direct torque control	PMSM or induction	Medium-high	~3%	Encoder or estimator	Industrial drives
Switched reluctance (SRM)	Salient-pole rotor, no magnets	Medium	~10–15% (high)	Position essential	Vacuums, washing machines, niche EV
Stepper micro-stepping	Stepper motor (PM hybrid)	Low-medium	~5%	Open-loop steps	3D printers, CNC, lab automation
Slotless / coreless BLDC	BLDC with no stator iron	Standard FOC	~0.5%	Hall or encoder	Medical pumps, gimbals, optical drives

Real-world specs

• DJI 2312E (Phantom 3 era). 920 KV outrunner, 14 magnetic poles, 27.5 g, draws up to 17 A peak from a 4S LiPo. The "920 KV" number is the standard label for an entire family of consumer-drone motors.
• Tesla Model 3 rear motor. Interior-permanent-magnet PMSM, 211 kW peak, 18,000 rpm max shaft speed, 96% peak efficiency, neodymium-boron-iron rotor magnets. Driven by silicon-carbide MOSFETs in a six-pack inverter.
• Dyson V11 cordless vacuum motor. 125,000 rpm, switched-reluctance design (a BLDC cousin), about 600 W mechanical from a 26 V battery pack. Small enough to fit in your fist.
• Computer CPU cooler fan. 12 V, 2-pole BLDC, 3000 rpm typical, sensorless six-step commutation in a tiny IC potted into the hub. Cost target: under $0.50 per motor at OEM volumes.
• Bafang BBSHD e-bike mid-drive. 1500 W rated, 48 V system, ~13 KV after the internal planetary reduction, peak shaft torque about 30 N·m. Hall-sensored six-step commutation.

Common failure modes

• MOSFET shoot-through in the controller. If high-side and low-side switches in the same half-bridge turn on simultaneously — even for a few hundred nanoseconds during a commutation transient — they short the DC bus straight through silicon. Result: instant blow-up, often taking the gate driver with it. Cure: dead-time insertion in PWM, robust gate-driver isolation, and a TVS diode across the bus.
• Magnet demagnetisation. Neodymium-iron-boron magnets demagnetise at around 80°C (standard grade) up to 200°C (high-Curie SH grades). Sustained high-current operation, especially with field-weakening, raises rotor temperature past these limits and permanently weakens the magnets. Cure: stay within thermal envelope, use SH or higher-grade magnets for high-power EVs.
• Hall sensor misalignment or failure. A loose Hall PCB, a cracked sensor, or interconnect corrosion sends rotor-position errors that desynchronise commutation. The motor stalls, draws excessive current, and overheats. Cure: thermal-aged epoxy bonds, redundant sensors, or sensorless fallback.
• Bearing failure. Same as in induction motors — but BLDCs run faster (often 10,000+ rpm), so bearings see more cycles per minute and die sooner. Drone motors typically survive 100–300 hours; quality bearings from NMB or SKF push that to 1000+ hours.
• Encoder/resolver loss in PMSM servos. FOC requires accurate rotor angle; lose the encoder, lose the motor. Industrial drives include encoder-loss detection and a graceful coast-down. Less-careful drives will spin wildly out of control until the bus fuse opens.

Historical context

The first commercial BLDC dates to 1962, when T.G. Wrathall demonstrated a six-transistor commutator built from then-new germanium power transistors. Cost was prohibitive. The technology stayed niche — military, instrumentation, hard-disk spindles — until two waves of progress hit. First, rare-earth permanent magnets: samarium-cobalt in the 1970s, then neodymium-iron-boron in 1984, raised flux density 5× over ferrite at the same volume. Second, cheap power MOSFETs and microcontrollers from the 1990s onward turned a six-FET driver into a single $5 IC.

By the 2000s BLDC dominated computer cooling fans, hard drives (the 7200 rpm spindle motor in every PC was a BLDC), and the first wave of consumer R/C aircraft. The 2010s drone explosion put hundreds of millions of small outrunners in the air; the EV boom of the 2020s does the same at megawatt scale. Today, "rare-earth-free" BLDC and PMSM designs — switched-reluctance, induction, ferrite-magnet — are the active research front, since neodymium supply has become a strategic concern.