TL;DR:
A brushless motor has permanent magnets on the rotor and coils on the stator, and an electronic speed controller (ESC) switches the coils in sequence.
With no brushes to rub, it wears less, runs efficiently and can spin fast, which suits drone propellers.
A motor's KV rating is rpm per volt with no load. Always keep propellers off when testing on a bench.
Take the cover off a toy drone and you will find four small cylinders with three wires each. There is no brush and no commutator in them. These are brushless motors, and the thing that makes them spin is not the motor but the small circuit board connected to them.
Magnets and coils, swapped around
In a brushed motor (see how DC motors turn electricity into motion), the coils sit on the rotor and brushes bring current to them. A brushless motor swaps this around. The permanent magnets are on the rotor, the part that spins. The coils are fixed in the outer stator. Because the coils do not move, no brushes are needed to feed them. Current can be switched electronically.
A typical drone motor has three groups of coils, called phases, which are connected to three wires. If you energize the phases in the right order, the magnetic field they make rotates around the motor, and the magnets on the rotor chase it.
What the ESC does
The ESC, electronic speed controller, is the circuit that does that switching. It takes the battery's direct current and switches it onto the three phases in a repeating pattern. In the simple six-step scheme, two phases carry current at a time, and the pattern repeats six times per electrical cycle.
For this to work the ESC must know where the rotor is, so it can switch at the right moment. Some motors have Hall sensors that detect the magnets. Most drone ESCs have no sensors and instead watch the voltage on the unpowered phase, which the spinning magnets induce (back-EMF). This also means a sensorless motor starts with a short open-loop kick before the signal becomes strong enough to follow.
The flight controller does not drive the coils directly. It sends the ESC a throttle command, and the ESC converts it into the right switching speed and power.
KV rating and electrical speed
Brushless motors are rated by KV: the unloaded rpm per volt. A 1000 KV motor on 1 V spins at about 1000 rpm with no load. It is not a kilovolt rating, and it is not a power rating.
Worked example: a drone motor and its controller
These numbers are hypothetical, to show how to reason. A motor is rated 1000 KV and has 14 magnet poles, which is 7 pole pairs. It runs from a 3S LiPo pack, which is three cells in series at a nominal 3 × 3.7 = 11.1 V.
No-load speed.
1000 rpm/V × 11.1 V = 11,100 rpm. With a propeller attached, the speed is lower, as the prop is a heavy load.Say the propeller holds it to 10,000 rpm. In revolutions per second that is
10,000 / 60 = 166.7 rev/s.Electrical frequency. The field passes one pole pair per electrical cycle, so
f = 7 × 166.7 = 1167 Hz.Commutation rate. Six steps per cycle gives
6 × 1167 = 7000switch events per second, and about 143 µs between them. The ESC must hit that timing reliably, which is why it has a dedicated processor.
Check step 3 in reverse: 1167 / 7 = 166.7 rev/s, and 166.7 × 60 = 10,000 rpm. It matches.
Next consider the larger point of the example. The ESC sets the average voltage on the coils by switching quickly, in the same spirit as PWM on a brushed motor. Lower throttle means lower average voltage and lower speed. Reverse the order of the phases and the motor spins the other way, which is how drones get their pairs of clockwise and counterclockwise propellers.
Why drones use them
Efficiency. No brush friction and no brush voltage drop. More of the battery's energy reaches the propeller, which means more flight time.
No brushes to wear. The bearings are the main wearing parts, so the motor lasts longer.
Power for the weight. The coils sit in the stator where heat can flow out through the motor body into the air, so a small brushless motor can handle a lot of power.
Speed range. Without sparking brushes, they can spin fast and respond fast to throttle changes, which a flight controller needs to stay stable.
The tradeoff is complexity. A brushless motor needs an ESC with the right current rating, and the wiring and firmware are more involved than connecting a brushed motor straight to a battery.
Where it goes wrong
Wrong ESC rating. An ESC that cannot handle the motor's peak current overheats and fails. Check the continuous and burst ratings, and keep margin.
Desync. If the ESC loses track of the rotor, usually after a sudden throttle change, the motor stutters or stops. In flight that is serious.
Overheating. Hard use with little airflow can cook the windings.
Wrong prop. A prop that is too big for the motor and cell count demands too much current.
Safety: A spinning propeller can cut skin deeply and a drone motor can start without warning. Always take the propellers off for any bench test, calibration or first power-up of the ESC. Disconnect the battery before touching the wiring or props. Treat LiPo packs with care: use a proper balance charger, never leave one charging unattended, and never use a puffed or damaged pack.
Try it yourself
Work out the numbers for the motor on your own project: multiply the KV by the cell count times 3.7 V for a no-load speed, then reduce it for the prop. Calculate the electrical frequency from the number of poles, which you can count on the rotor or read from a datasheet. Compare with what a tachometer app or sensor shows with the propeller removed. You will find the real no-load speed a bit under the calculated one, since the battery voltage sags and the motor has losses.
Frequently asked questions
What does the KV on a motor mean?
It is the no-load speed in rpm for each volt applied. A higher KV motor spins faster per volt and generally gives less torque, so it suits a smaller propeller.
Can I connect a brushless motor straight to a battery?
No. It has three wires and needs an ESC to switch the phases in order. Connecting battery power directly will not make it spin and can damage the motor.
Get started
