Killing the brush: why electronics took over commutation
A classic brushed DC motor is a beautifully self-sufficient little machine. You feed it DC, and a pair of carbon brushes rubbing against a rotating copper commutator automatically reverses the current in the rotor windings at exactly the right instant to keep torque pointing one way. No electronics, no sensors — the geometry does the thinking. That is why a brushed motor will happily spin the moment you touch it to a battery. It is also why it eventually dies: brushes are tiny blocks of carbon dragged against spinning metal at full motor current, so they spark, wear, throw conductive dust, and demand replacement.
The brushless DC motor (BLDC) does something radical: it turns the motor inside-out. The permanent magnets go on the rotor (the spinning part), and the copper windings move to the stator (the stationary outer shell). Now the windings never rotate, so there is nothing for a brush to slide against — but that creates a new problem. Someone still has to reverse the winding currents at the right moment. That job moves out of the carbon-and-copper world and into silicon: a bank of power MOSFETs switches the windings on and off, and a controller decides the timing by watching where the rotor is.
Reading the rotor: Hall sensors and back-EMF
A brush knows where the rotor is because it is physically touching it. Take the brush away and the electronics are suddenly blind — and a permanent-magnet motor that is energised at the wrong angle produces zero net torque or even runs backwards. So the very first thing any brushless controller must do is answer one question every fraction of a millisecond: where is the rotor right now? There are two classic answers, and they correspond to the two big motor families.
The first answer is Hall-effect sensors: three small chips spaced 120° apart inside the motor, each producing a logic high or low depending on which magnet pole is passing over it. Three sensors give six distinct binary codes per electrical revolution — six "commutation states." The controller reads the 3-bit code and instantly knows which two of the three windings to energise. This is six-step (trapezoidal) commutation, and it is the cheap, robust scheme inside cordless drills, computer fans and most hobby drones.
The second answer needs no sensors at all. When a magnet sweeps past a coil, back-EMF — the voltage a generator produces — appears across that winding, and its waveform encodes the rotor angle. In a six-step BLDC, at any moment one of the three windings is *not* being driven, so the controller can quietly measure the back-EMF on that idle phase. The instant the back-EMF crosses zero, it has detected the rotor passing a known angle, and it times the next commutation from there. This sensorless trick removes three wires and three chips, which is why almost every electronic speed controller (ESC) in a racing drone is sensorless. The catch: at standstill there is no back-EMF, so sensorless motors need a special open-loop "kick" to get started.
Three Hall sensors → 6 commutation states per electrical revolution
rotor angle: 0° 60° 120° 180° 240° 300° (360°)
Hall A: 1 1 1 0 0 0 1
Hall B: 0 0 1 1 1 0 0
Hall C: 0 1 1 1 0 0 0
| | | | | |
state: 5 4 6 2 3 1
drive: A+B- A+C- B+C- B+A- C+A- C+B-
(energise 2 of 3 phases; 3rd phase floats → read back-EMF here)The inverter: a MOSFET bridge that paints a rotating field
Whatever tells the controller where the rotor is, the muscle that actually moves current is the same: a three-phase inverter, six power switches arranged as three half-bridges. Each phase wire connects to the midpoint of a high-side switch and a low-side switch. Turn on the top switch and that phase is pulled to the positive battery rail; turn on the bottom one and it is pulled to ground. With three phases you can sculpt any rotating voltage pattern you like — this is the same inverter topology used everywhere from EV traction drives to solar systems.
+V (battery / DC bus)
│
┌─────┼─────┬─────┐
[Q1] [Q3] [Q5] ← high-side MOSFETs (driven by a gate driver)
│ │ │
A─────B─────C ← three motor phase wires
│ │ │
[Q2] [Q4] [Q6] ← low-side MOSFETs
└─────┴─────┴─────┐
│
GND
NEVER turn Q1 and Q2 on together → "shoot-through": a dead short across +V.
The gate driver inserts a few-hundred-ns "dead time" between top/bottom switching.Two practical details separate a working drive from a smoking one. First, the high-side MOSFET in each leg has its source floating, so its gate must be driven several volts *above* the battery rail — that is the job of a gate driver with a bootstrap or charge-pump supply. Second, you must never turn a top and bottom switch on at the same instant, or you create a dead short from rail to ground through two transistors — shoot-through — which can vaporise a leg in microseconds. The driver enforces a small dead time between handing off from one switch to the other.
How does an inverter make a smooth average voltage out of switches that are only ever fully on or fully off? With pulse-width modulation (PWM). Each switch is chopped on and off tens of kHz, and the *ratio* of on-time sets the effective voltage. Want half the bus voltage on phase A? Switch it on 50% of the time. The motor's inductance averages the chopped square waves into a smooth current, while the high switching frequency keeps the chopping inaudible and the current ripple small. In modern EVs the same job is increasingly handled by silicon-carbide (SiC) devices, which switch faster and run cooler than silicon, squeezing a few more percent of range out of the same battery.
BLDC vs PMSM: trapezoid vs sine
Mechanically, a BLDC and a PMSM (permanent-magnet synchronous motor) can look identical — magnets on the rotor, three-phase windings on the stator. The difference is in the *shape of their back-EMF*, and that shape decides how you should drive them. A BLDC is wound so its back-EMF is trapezoidal: nearly flat-topped. It pairs naturally with six-step commutation, where current is held constant in two phases at a time — simple, cheap, and perfectly good for fans, pumps and quadcopters.
A PMSM is wound (often with skewed slots and distributed windings) so its back-EMF is a clean sinusoid. Drive it with sinusoidal currents and the torque comes out almost perfectly smooth, because the product of sinusoidal current and sinusoidal back-EMF has no ripple. Six-step commutation, by contrast, snaps current between phases in abrupt 60° steps, which produces a characteristic torque ripple — fine for a drill, audible and unpleasant in a power-steering motor or an EV at low speed. That is the core trade: BLDC drive is simpler and cheaper; PMSM drive is smoother, quieter and more efficient, at the cost of a more sophisticated controller.
Pole pairs and speed: a worked example
Here is a fact that trips up every beginner: the rotor of a brushless motor does *not* turn at the frequency you feed it. It turns slower, divided by the number of pole pairs. A two-magnet rotor has one pole pair; a 14-magnet outrunner (common in drones) has seven. The relationship between the electrical frequency the inverter produces and the mechanical speed of the shaft is the same synchronous-speed formula you met for the synchronous machine:
n = 120 * f_elec / (2 * p) = 60 * f_elec / p n = mechanical speed [RPM] f_elec = electrical frequency the inverter produces [Hz] p = number of POLE PAIRS (= magnets on rotor / 2) ─── Example: a drone outrunner, 14 rotor magnets → p = 7 pole pairs ─── Desired shaft speed: n = 12,000 RPM f_elec = n * p / 60 = 12,000 * 7 / 60 = 1,400 Hz So the inverter must produce a 1,400 Hz three-phase waveform. Each electrical cycle = 6 commutation steps → 8,400 steps/second. With ~16 kHz PWM, that is only ~2 PWM periods per step at top speed — why high-RPM ESCs push PWM toward 24–48 kHz and use fast MOSFETs.
This formula is the whole reason motor design is a balancing act. Pack in more pole pairs and each electrical cycle moves the rotor a smaller mechanical angle, so you get more torque per amp and smoother running — but to reach a given RPM the inverter must switch proportionally faster, and switching losses climb. A high-pole-count hub motor in an e-bike trades top speed for the grunt to climb a hill directly, gearbox-free; a low-pole-count drone motor spins to 30,000 RPM to whip a small propeller. Same physics, opposite ends of the formula.
Steppers and servos: motion you can trust without a tachometer
A drone motor only needs to spin fast. A 3D printer's extruder, a camera gimbal or a robot joint needs something different: to move to an exact position and *hold* it. Two families of motor specialise in this — and they take opposite strategies. A stepper motor is a brushless motor deliberately built with many pole pairs — often 50 teeth on the rotor — so that energising the windings in sequence advances the shaft in fixed, discrete steps, classically 1.8° (200 steps per full turn). Pulse the windings 200 times and you have turned exactly once. No sensor, no feedback: you command position simply by counting pulses. That open-loop simplicity is why steppers dominate 3D printers, CNC engravers and old disk drives.
The price of open-loop control is that a stepper has no idea whether it actually reached the commanded step. Overload it, accelerate it too hard, or hit an obstacle and it can silently skip steps — the controller still thinks it is at position 1,000 while the shaft is really at 994, and the error never corrects itself. To get torque, steppers also hold full current even while standing still, so they run warm and gulp power compared to their output.
A servo motor takes the opposite bet: add a sensor and close the loop. A servo is any motor — usually a BLDC or PMSM — bolted to a position encoder and run by a controller that continuously compares commanded position against measured position and drives the error to zero. Push the shaft and a servo pushes back; hit an obstacle and it knows immediately, because the encoder reports the discrepancy. That feedback makes servos the choice for industrial robot arms, EV-traction and anything where you cannot afford a silent lost step. The trade is cost and complexity: an encoder, a tuned control loop, and the microcontroller cycles to run it.
- Need raw speed, low cost, simple control? A six-step BLDC with Hall or sensorless commutation — fans, pumps, drones, power tools.
- Need smooth, quiet, efficient torque? A PMSM driven by FOC — EV traction, power steering, premium appliances.
- Need cheap open-loop positioning? A stepper driven by step pulses — 3D printers, CNC, camera sliders.
- Need guaranteed positioning under load? A servo with an encoder and a closed loop — robot arms, gimbals, factory automation.