Electronic speed controllers

Summary¶

An electronic speed controller (ESC) converts digital throttle commands from the flight controller into three-phase AC power for a brushless motor. Inside, six N-channel MOSFETs form a three-phase bridge — at any instant two MOSFETs are active, directing current through two of the three motor windings and creating a magnetic field the rotor chases. The ESC switches this pattern thousands of times per second, keeping the rotor in continuous pursuit. libdrone uses the Pilotix 75A AM32 4-in-1 6S ESC: four independent motor controllers in one unit, running AM32 open-source firmware with bidirectional DShot support.

Concept¶

The three-phase bridge¶

Inside the ESC, six N-channel MOSFETs are arranged in three half-bridge pairs, one per motor phase:

Battery+ ── HI_A ──┬── LO_A ── Battery− (Phase A) Battery+ ── HI_B ──┬── LO_B ── Battery− (Phase B) Battery+ ── HI_C ──┬── LO_C ── Battery− (Phase C)

At any moment, exactly one high-side and one low-side MOSFET are active. This connects two phases through the motor windings, creating a magnetic field. The third phase floats — its back-EMF is monitored to detect the rotor position without sensors. As the rotor approaches the active field position, the ESC advances to the next commutation step, moving the field forward. The rotor chases continuously.

MOSFET selection is critical. N-channel MOSFETs have low on-resistance (Rds_on) — typically 2–5 mΩ for modern FET ESC designs. A MOSFET with 2 mΩ Rds_on carrying 30 A dissipates only 1.8 W as heat. The same current through an older bipolar transistor would dissipate several times more. Low Rds_on = high efficiency = cooler ESC.

Sensorless commutation and back-EMF¶

The ESC does not use sensors to detect rotor position. It infers position from the back-EMF — the voltage the spinning rotor induces in the floating (inactive) phase. When the back-EMF on the floating phase crosses through the midpoint between supply rails (the zero-crossing), the rotor is at a known position relative to the active field. The ESC uses this zero-crossing to time the next commutation step.

At very low RPM (startup), back-EMF is too small to detect reliably. The ESC uses a startup sequence — briefly running in open-loop, forcing commutation at a fixed rate to accelerate the motor until back-EMF becomes detectable. This is why motors take ~0.5 s to reach minimum stable RPM after an arm command.

AM32 firmware¶

AM32 is open-source 32-bit ESC firmware running on an STM32F051 microcontroller inside the ESC. It handles commutation timing, DShot protocol decoding, bidirectional DShot eRPM reporting, and current limiting. It runs at 48 kHz switching frequency — each MOSFET switches on and off 48,000 times per second.

48 kHz provides approximately 14 switching cycles per commutation step at 30,000 RPM (3,500 Hz electrical frequency). This gives smooth current waveforms with low ripple, which means cooler motor windings and less electromagnetic noise compared to lower switching frequencies.

AM32 is the open alternative to proprietary BLHeli_32. Its source code is publicly available; the community adds features and fixes bugs. → See open-source-philosophy.

Current rating and safety margin¶

The ESC must handle the peak current any single motor-propeller combination draws at full throttle. The BrotherHobby 2507 draws 40–55 A per motor at peak. The Pilotix 75A ESC provides a 35–85% safety margin over the measured peak.

A safety margin is necessary because: motor current increases as battery voltage drops during a flight (lower back-EMF headroom means more current for the same mechanical output); simultaneous full-throttle events on all four motors can create brief current spikes above the steady-state maximum; and ESC temperature derating reduces the continuous rating by 10–20% when warm.

Capacitor placement¶

The 1000 µF low-ESR electrolytic capacitor soldered directly to the ESC power pads is not there for energy storage — it stores only ~0.24 J (trivial). Its function is suppressing back-EMF voltage spikes when motors rapidly decelerate. A decelerating motor briefly acts as a generator, pushing voltage back into the power bus. At 6S, unclamped spikes can exceed the MOSFET drain-source breakdown voltage and destroy the ESC.

Critical: every millimetre of wire or trace between the capacitor and the ESC pads adds approximately 1 nH of inductance. At a 50 A/µs transient, each 1 nH adds 50 mV to the unclamped spike. The capacitor must be soldered directly onto the ESC power pads — no pigtail wire. The TVS diode (SMBJ28A, integrated in the Pilotix ESC) provides additional clamping at nanosecond timescales.

Reference¶

Pilotix 75A AM32 4-in-1 6S — libdrone specification¶

Thermal management¶

The ESC sits at the centre of the Platform middle layer. The always-on Gdstime 3010 fan at the Platform rear draws air front-to-rear across the ESC/FC stack. This active cooling keeps ESC junction temperatures within operating range during sustained full-throttle climbing and aggressive manoeuvres. Without the fan, sustained high-throttle flight at ambient temperatures above 25°C may trigger the ESC's thermal current-limiting, which reduces thrust asymmetrically and degrades flight performance.

Procedure¶

ESC startup verification¶

1. Connect battery. Do not arm for 5 seconds — wait for gyro calibration.
2. Arm. All four motors should initialise simultaneously (brief beep sequence).
3. In Betaflight Motors tab (props removed): advance master slider to 10%. All four motors should spin up simultaneously. If any motor lags by more than ~1 second, BiDi DShot may not be fully synchronised — rebind or check DShot configuration.
4. Verify eRPM reporting: at ~20% throttle, all four motors should report non-zero eRPM values proportional to commanded speed.

Post-crash ESC inspection¶

1. Visually inspect the ESC PCB for burnt components, lifted pads, or discolouration from heat.
2. Check motor solder joints — crash forces can crack solder joints on motor phase wires.
3. After any hard crash, run the motor verification above before the next flight — a damaged ESC MOSFET may still function at low throttle but fail at high current.

Rationale¶

Why a 4-in-1 ESC rather than four individual ESCs¶

A 4-in-1 ESC consolidates four controllers onto one PCB, sharing the power input and ground connections. This reduces connector count, wiring length, and total mass compared to four separate ESCs. The single shared power pad accepts one set of battery lead connections and one capacitor location. With four separate ESCs, each needs its own power tap, capacitor, and mounting — adding ~15–20 g of wiring and connectors and introducing multiple additional failure points. The 4-in-1 design also uses the stack mounting pattern directly, eliminating the need for ESC mounting plates.

Connections¶

requires: - brushless-motors related: - propellers - dshot-protocol - rpm-filter leads_to: - dshot-protocol