Groundschool // Power // 6 min read
ESCs, and what the amp rating means
Continuous against burst, why cell count is a hard limit, and what firmware and BEC actually change.
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What an ESC does
A brushless motor has no brushes, so something has to decide which coils to energise and when. That is the electronic speed controller: it switches DC from the battery into three phases at the right frequency, and it does so by sensing the motor's own back-EMF rather than by being told where the rotor is.
Everything else about an ESC follows from the fact that it is a switch carrying a lot of current.
Continuous, burst, and the honest number
A "50 A ESC" usually means 50 A continuous with adequate airflow, and a burst rating held for a few seconds. Vendors quote whichever is larger and are not always clear which they have quoted.
Size from the motor's maximum current with 20% headroom, and treat burst ratings as a safety margin rather than a design figure. An ESC that runs hot does not fail politely; it fails at full throttle, which is exactly when you were relying on it.
Airflow matters more than the datasheet implies. The same ESC in free air and buried under a battery are different components.
Cell count is a hard limit
The voltage rating is set by the capacitors and MOSFETs, and exceeding it destroys the ESC immediately rather than gradually. A 6S ESC on an 8S pack is not a marginal choice.
Note that cell counts are given as a range, "3-6S", and the top of that range is the charged voltage, not the nominal one. A 6S pack is 25.2 V fresh off the charger, and that is the number the ESC has to survive.
BEC, and when you need one
A battery eliminator circuit is a regulator on the ESC that supplies 5 V or so to power a receiver, a flight controller or servos. On a multirotor with a 4-in-1 ESC and a separate power distribution board, you often do not need it. On a fixed-wing aircraft where the servos draw real current, the BEC rating is a design constraint: four digital servos moving at once can pull several amps, and an under-rated BEC browns out the receiver mid-flight.
An ESC marked "opto" has no BEC at all and expects power from elsewhere.
Firmware and protocol
BLHeli_32 and AM32 are the common firmwares. AM32 is open source and increasingly the default on new hardware. Both give you configurable timing, braking, and telemetry.
DShot is the digital protocol between flight controller and ESC. It replaced analogue PWM and removed a whole class of calibration and noise problems. DShot300 and DShot600 are both fine; the numbers are bit rates, not quality levels.
Bidirectional DShot lets the ESC send rpm back to the flight controller, which enables RPM filtering. On a multirotor this is worth more than most hardware upgrades, because it removes gyro noise at its source.
Rules of thumb
- Continuous rating at least 1.2 times the motor maximum current.
- Cell count is a hard limit, and the limit is charged voltage, not nominal.
- Airflow is part of the rating, even when the datasheet does not say so.
- On fixed-wing, size the BEC for all servos moving at once.
- Bidirectional DShot and RPM filtering are worth more than most hardware upgrades.