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Module 5 — Actuation: motors & ESCs

Between the flight controller's intention and the air stands a three-phase motor and the tiny power-electronics brain that commutates it 100 000+ times per second.

🟢 Foundations. FPV quads use brushless DC motors (BLDC): permanent magnets on the spinning bell, three sets of copper windings on the stator. There are no brushes — the ESC (Electronic Speed Controller) switches current through the three phases in sequence, dragging the magnets around. Motor sizes read like 2807: 28 mm stator diameter, 7 mm stator height. The famous KV rating is unloaded speed per volt:

RPMno-load≈KV×V\text{RPM}_{\text{no-load}} \approx KV \times V

A 1300 KV motor on a 6S pack (~22.2 V) spins toward ~29 000 RPM unloaded. Lower KV = more torque per amp = bigger props = long-range builds; higher KV = 5″ freestyle screamers.

🟡 Practitioner. KV hides a torque constant. In SI units:

Kt=602π⋅KV [N⋅mA]τ=Kt IK_t = \frac{60}{2\pi \cdot KV}\ \left[\frac{\text{N·m}}{\text{A}}\right] \qquad \tau = K_t\, I

For 1300 KV: Kt≈7.35K_t \approx 7.35 mN·m/A — now current draw under load is predictable, not mystical. On the signal side, the FC talks to ESCs with DShot, a digital protocol (no calibration, checksummed). Each frame is 16 bits:

[ 11 bits throttle 0–2047 ][ 1 bit telemetry request ][ 4 bit CRC ]
DShot600 = 600 kbit/s  →  one frame ≈ 26.7 µs

Bidirectional DShot sends eRPM back from each ESC — the data that powers the RPM notch filter in Module 9. ESC firmware is its own world: BLHeli_S → Bluejay (open source), BLHeli_32 (closed) → AM32 (open) — flashable, configurable (timing, PWM frequency, demag protection).

🔴 Advanced. Classic ESCs run six-step (trapezoidal) commutation: at any instant two phases carry current and the third floats; the ESC senses the rotor by the back-EMF zero crossing on the floating phase. That's why sensorless motors stutter at very low RPM — no speed, no back-EMF, no position. Throttle is realized by PWM-chopping the DC bus at 24–96 kHz. A desync is the ESC losing the zero-crossing plot mid-flight (aggressive timing, worn bearings, voltage spikes) — now you can read its causes.

Six-step commutation (1 = high side on, 0 = low side on, · = floating)
step:   1    2    3    4    5    6
A:      1    1    ·    0    0    ·
B:      ·    0    0    ·    1    1
C:      0    ·    1    1    ·    0
        └── back-EMF is sensed on the "·" phase ──┘

⚫ Master. Field-Oriented Control (FOC): measure phase currents, rotate them through the Clarke and Park transforms into a rotor-aligned frame, control torque-producing current iqi_q and flux current idi_d with two PI loops, transform back. Sinusoidal drive, quieter, more efficient at partial load — and fully open to you via projects like VESC or SimpleFOC. Mastery = you can explain why FOC needs current sensing while six-step doesn't, and you've bench-spun a motor with your own commutation code.

Mastery checklist

  • Pick motor size + KV + prop for a stated mission (freestyle / long-range / cinewhoop) and defend the choice with KtK_t and disk-loading arguments.
  • Decode a DShot frame by hand from a logic-analyzer trace.
  • Diagnose a desync from a blackbox log vs a mechanical failure.

🖼️ Image ideas: your own exploded-motor photo (bell, stator, magnets); Wikimedia Commons "Brushless DC motor" animations (several GIFs are CC/PD).

📚 Free resources: Bluejay & AM32 GitHub docs; "How BLDC motors work" open articles; SimpleFOC documentation.