The previous lesson left a motor that cannot run on its own, because the coils need their current switched in a repeating sequence timed to the rotor's position, and there are no brushes to do it. The electronic speed controller, the ESC, is what does it. One sits at the base of each arm, wired to the battery on one side and to the three motor phases on the other, and it is a small computer with power electronics attached. Understanding it removes most of the mystery from a quadcopter's electrical faults, because the ESC is where the battery's raw direct current becomes the three timed phases the motor turns on.
Six switches and a sequence
The heart of an ESC is six power transistors, MOSFETs, arranged as three half-bridges, one per phase. Each phase wire can be connected by its pair of transistors either up to the battery positive or down to the battery negative, or left floating. With three phases each in one of these states, the controller can drive current from any phase to any other. Commutation in the simplest scheme, called six-step or trapezoidal, energises two phases at a time and leaves the third floating, then steps to the next pair, and so on through six steps that complete one electrical cycle. At each step the stator field points in a fixed direction, the rotor swings toward it, and just before it arrives the controller steps to the next pair so the field keeps leading. Done fast and in order, the six steps become a smoothly turning field and a smoothly turning motor.
Knowing where the rotor is, without a sensor
The steps must be timed to the rotor, so the controller has to know the rotor's angle. Some motors carry position sensors, but multirotor ESCs are almost all sensorless, and they read the rotor from the motor itself. The trick is the floating phase. A permanent magnet moving past a coil induces a voltage in it, the back-EMF, and while two phases are driven the third is left floating precisely so the controller can measure the back-EMF on it. The moment that voltage crosses the midpoint, the zero crossing, tells the controller exactly where the rotor is, and it schedules the next commutation step a fixed fraction of a step later. The motor is its own position sensor, read on the wire that happens to be idle.
This is also why a brushless motor can stutter or fail to start. At rest and at very low speed there is no back-EMF to read, so the controller has to guess the first few steps blindly, nudging the rotor until it spins fast enough to generate a signal it can lock onto. A motor that twitches instead of starting, or that stalls under sudden load, is usually a controller that has lost the back-EMF and its place in the sequence. It is the sensorless scheme showing its one weakness, at the low speeds where a quadcopter fortunately spends little time.
Setting the speed
Commutation sets the timing; something else must set the speed. The controller varies the motor's speed by switching the driven phases on and off very fast, tens of thousands of times a second, and varying the fraction of time they are on. This is pulse width modulation, and the on fraction, the duty cycle, sets the average voltage the motor sees and hence its speed. A throttle of half is the controller holding the phases on for half of each tiny interval. The motor's inductance smooths the chopped voltage into a steady current, so the motor feels an average and not the switching itself, though the switching is audible as the characteristic whine.
Example. A four cell battery reads 15.0 V under load. The flight controller commands 40 per cent throttle. Ignoring losses, what average voltage does the motor see, and if the motor is 1800 Kv, what unloaded speed does that imply?
At 40 per cent duty the average is V. At 1800 Kv the unloaded speed is rpm. Under a propeller's load the real speed is lower, but the duty cycle is the throttle's grip on the motor.
Now you. The same setup is commanded to 75 per cent throttle on a 15.0 V pack. What average voltage reaches the motor, and the unloaded speed at 1800 Kv?
Answer
V, and rpm unloaded.
How the flight controller talks to the ESC
The flight controller sends each ESC a throttle command hundreds or thousands of times a second, and how it sends it has evolved. The oldest scheme borrows the servo signal of radio control, a pulse whose width from one to two milliseconds means off to full, updated only fifty to five hundred times a second. It works but it is slow and analog, so a little electrical noise shifts the pulse and the throttle. The modern replacement, DShot, sends the throttle as a digital number in a short burst of bits, immune to noise, checksummed so a corrupted command is discarded rather than obeyed, and fast enough to update every control loop. Digital telemetry can even flow back the other way, the ESC reporting the motor's real rpm and temperature. Choosing DShot over the old pulse scheme is one of the easy correct decisions in a modern build, and it removes a whole class of twitchy throttle faults.
Current, heat and the rating that matters
The number printed largest on an ESC is its current rating in amps, and it is the ceiling on how hard the motor may be driven. Every transistor has a small resistance, and current through a resistance makes heat at a rate of , that square again, so doubling the current quadruples the heating in the ESC. Push a motor and propeller that draw more current than the ESC is rated for and the transistors overheat and fail, often taking the motor's timing with them. This is the same that governs the wiring in the next lesson, and it is the single most common way a propulsion system destroys itself: an aggressive propeller, a high Kv motor, and an ESC without the current headroom to survive them. Size the ESC above the worst current the motor and propeller will ever draw, with margin, and it becomes the reliable servant it should be, faithfully turning a throttle number into a spinning field.