BLDC motors and ESCs

Why this matters

Almost every drone motor is a brushless DC (BLDC) motor, which cannot turn by itself: an ESC (electronic speed controller) must switch current into the windings at exactly the right moments thousands of times a second. Choose too small an ESC and it overheats and cuts out in flight; choose a motor KV that does not suit the voltage and propeller and the motor draws too much current or gives too little thrust.

BLDC motors and ESCs

A BLDC motor has permanent magnets on the rotating part and three-phase windings on the stationary part. The ESC energises the windings one pair at a time in six steps per electrical revolution, so the magnetic field rotates and pulls the magnets along.

The ESC must know where the magnets are to switch at the right moments. Most drone ESCs have no position sensor; instead they use the floating phase in each step to measure the back-EMF the spinning motor generates, as described in Microchip application note AN1175. The catch is that there is almost no back-EMF at start-up, so the ESC needs a special start routine.

KV and the torque constant

KV is the no-load speed per volt (rpm/V): a 400 KV motor at 22.2 V spins at about 8,880 rpm without load. KV relates to the torque constant $K_t$ (torque per amp); following maxon’s documentation, $K_t=60/(2\pi K_V)$ with KV in rpm/V and $K_t$ in N·m/A. A low-KV motor therefore gives more torque per amp, suiting large propellers.

KV alone does not tell you thrust, which depends on the propeller, speed and current at that voltage (module 4).

import math

kv, volts, r_winding, i_noload = 400, 22.2, 0.08, 0.8
kt = 60 / (2 * math.pi * kv)
print(f"Kt = {kt * 1000:.2f} mN·m/A; no-load speed ≈ {kv * volts:.0f} rpm")
for amps in (10, 20, 30):
    rpm = kv * (volts - amps * r_winding)
    torque = kt * (amps - i_noload)
    p_mech = torque * rpm * 2 * math.pi / 60
    p_elec = volts * amps
    print(f"{amps:>2} A: {rpm:.0f} rpm, {torque:.3f} N·m, {p_mech:.0f} W out of {p_elec:.0f} W in, "
          f"efficiency {p_mech / p_elec:.0%}")

Kt = 23.87 mN·m/A; no-load speed ≈ 8880 rpm
10 A: 8560 rpm, 0.220 N·m, 197 W out of 222 W in, efficiency 89%
20 A: 8240 rpm, 0.458 N·m, 396 W out of 444 W in, efficiency 89%
30 A: 7920 rpm, 0.697 N·m, 578 W out of 666 W in, efficiency 87%

Commands and feedback

• Classic PWM commands with pulse width and requires range calibration
• DShot sends digital commands with no calibration, at DShot150, 300, 600 and 1200. ArduPilot recommends DShot600 for most vehicles, and DShot150 for large aircraft with long signal leads
• Bidirectional DShot returns motor speed on the same wire, usable for rpm-based noise filtering
• Separate-wire ESC telemetry reports speed, voltage, current and temperature, useful for spotting a misbehaving motor (see UAT 321)

Widely used open ESC firmware includes AM32 (GPL-3.0, for STM32 and other chips) and Bluejay (derived from BLHeli_S); both support bidirectional DShot. BLHeli_32 was reported discontinued in 2024.

Module lab

Common mistakes

Summary

• A BLDC motor needs an ESC switching its windings in six steps; sensorless ESCs find position from back-EMF
• KV is no-load rpm per volt, and $K_t=60/(2\pi K_V)$
• Speed falls and heating rises at high current
• ArduPilot recommends DShot600, telemetry helps monitor motors, and open firmware includes AM32 and Bluejay

Check your understanding

1. About how fast does a 920 KV motor spin at 14.8 V without load?
2. What is the torque constant of a 500 KV motor (mN·m/A)?
3. With $K_t=0.02$ N·m/A, 15 A and 1 A no-load current, what is the torque?
4. How does a sensorless ESC know where the magnets are?
5. Which DShot speed does ArduPilot recommend for most vehicles?