The propeller lessons treated the rotation rate as a free choice. It is not free; it is delivered by a motor, and the kind of motor a quadcopter uses is worth understanding in its own right, because its behaviour sets the voltage of the battery, the current the wiring must carry, and the marriage between motor and propeller that a good design lives or dies by. Almost every multirotor uses a brushless DC motor, and the name is best read as a correction of the older brushed kind it replaced.
Why brushless
A brushed motor spins because a rotating coil is fed current through sliding contacts, the brushes, that flip the current's direction twice per turn to keep the push going. The brushes work but they wear, they spark, and they limit the speed and the life of the motor. A brushless motor removes them by turning the machine inside out. The coils are fixed to the stationary part, the stator, and the permanent magnets are on the spinning part, the rotor. Nothing rubs. The catch is that something must now do the brushes' old job of switching the current at the right moments from outside, and that something is the electronic speed controller of the next lesson. The motor and its controller are one system, and neither makes sense alone.
Multirotor motors are almost all outrunners, meaning the rotor is the outer bell that spins, wrapped around a stationary inner stator. The bell carries the magnets and the propeller bolts to its top. This layout puts the magnets at a large radius, which gives high torque at modest speed, exactly what a propeller wants, in contrast to the inrunner arrangement used where raw speed matters more than torque.
Three phases and the rotating field
The stator's coils are wired in three groups, the three phases, usually called A, B and C. Driving current into the phases in the right repeating sequence makes the magnetic field of the stator point in a direction that steps around the circle, and the rotor's permanent magnets chase that direction, so the bell turns. This is why a brushless motor has three wires and not two: two would give a field that only flips back and forth, while three give one that rotates. The controller's whole task is to energise the three phases in the correct order and at the correct moments, an action called commutation, and it is doing electronically and hundreds of times a second what the brushes did mechanically.
The rotor does not turn once for every cycle of the phases. It carries several pairs of magnetic poles, commonly seven pairs on a small multirotor motor, and the field must cycle once for each pole pair to move the rotor one full turn. The electrical frequency the controller produces is therefore the pole pairs times the mechanical rotation rate, which is why these motors need such fast switching: a motor with fourteen poles spinning at thirty thousand rpm asks the controller to run its phases through three and a half thousand electrical cycles a second.
The Kv rating
The one number that a builder chooses a motor by is its Kv, the velocity constant. It states how many rpm the unloaded motor turns for each volt applied, in rpm per volt. A 2400 Kv motor on a fully charged four cell battery, about 16.8 V, spins near rpm with no propeller. Under load it turns slower, but Kv still sets the ballpark and, crucially, it sets how motor and battery must be matched. A high Kv motor spins fast on a low voltage and suits small propellers; a low Kv motor spins slowly and suits large ones. Choosing a high Kv motor and a large propeller together is a classic destructive mismatch, because the motor tries to spin the big propeller fast, the current soars, and the motor or the controller burns.
Kv has a hidden partner. The same construction that gives a high Kv gives a low torque constant, the torque produced per amp of current, and the two are reciprocal: once consistent units are used. A motor wound for high speed per volt produces little torque per amp, and a motor wound for high torque per amp turns slowly per volt. There is no winding that gives both, which is the electrical face of the same trade the propeller makes between speed and load. Fast and gentle, or slow and strong, is a choice made once in the motor's copper and never escaped.
Example. A builder has a 2207 motor rated at 1800 Kv and wants a rough top speed on a fully charged six cell battery, about 25.2 V. Ignoring load, what is it?
rpm unloaded. A propeller loads it down to perhaps sixty to seventy per cent of that in the air, so a working figure near 28,000 to 32,000 rpm, which is a sensible speed for the 5 inch propellers a 2207 motor is built for.
Now you. The same 2207 frame is offered in a 2400 Kv winding. On a four cell battery, about 16.8 V, what is the unloaded top speed, and is that higher or lower than the 1800 Kv motor on six cells above?
Answer
rpm unloaded, lower than the 45,360 of the 1800 Kv on six cells. Higher Kv on lower voltage often lands near the same place, which is why builders can reach a target speed from either direction and then choose by current and efficiency.
Reading a motor's size
Motors are named by a four digit number like 2207 or 2306, and it is not arbitrary. The first two digits are the stator's diameter in millimetres and the last two its height, so a 2207 has a stator 22 mm across and 7 mm tall. The stator is where the copper and the iron that make torque live, so its volume, not the bell's size, is what sets how much work the motor can do and how much heat it can shed. A taller or wider stator makes more torque and runs cooler at a given load, at the cost of weight. The pairing of stator size with propeller size is the heart of a build: a 22 by 7 stator is the standard partner for 5 inch propellers, a larger stator for larger propellers, and the Kv is then chosen to hit the wanted speed on the chosen battery voltage. Get the three to agree, stator size to propeller, Kv to voltage, and voltage to the current the wiring can carry, and the propulsion system is settled. The current itself, and the controller that meters it, are the next two lessons.