What a brushless motor and its speed controller are

A brushless motor on a hobby drone is an outrunner: the outer bell carries the magnets and spins, the inner stator carries the copper windings and stays still, and there is no physical contact between them apart from two small bearings. Nothing rubs, so nothing wears out at the speed a brushed motor's carbon brushes do, and a set of four will outlast several sets of propellers and several batteries. The speed controller is the part that makes this possible, because a brushless motor cannot be driven by simply applying voltage: something has to energise the three windings in sequence, thousands of times a second, in step with where the bell currently is.

What distinguishes this pair from every other part of the aircraft is that they are the only components that convert one form of energy into another, and they are consequently the only parts that get genuinely hot. A motor that is warm after a flight is normal. A motor too hot to keep a finger on is telling you that something in the match between the motor, the propeller and the speed controller is wrong.

The numbers on a motor and an ESC, and what they mean

Motors carry a four digit stator size and a KV figure, and speed controllers carry a current rating and a cell count range. Those four numbers, read in that order, tell you almost everything you need before buying.

The stator is the part that produces torque, so its dimensions matter more than the outside of the motor. A wider stator makes more torque and swings a larger propeller. A taller stator adds power without adding diameter and suits fast, punchy flying. KV is the figure most often misread: it is not power and it is not quality, it is simply how fast the motor turns for each volt it is given with nothing on the shaft. A high KV motor turns a small propeller quickly, a low KV motor turns a large propeller slowly, and both can produce the same thrust.

On the speed controller, the continuous current rating is the one that counts and the burst rating is marketing arithmetic that assumes a few seconds at a time. The cell count range is a hard limit set by the components on the board: connect a six cell pack to a speed controller rated to four and it will fail immediately and expensively. Speed controllers also list a digital protocol such as DShot300 or DShot600, which is the language the flight controller uses to send throttle commands, and both ends must speak it.

How motors and speed controllers fail

Motors fail at the bearings and at the windings, and speed controllers fail all at once. The bearing failure is the common one and it is slow: grit works past the seal, the bell develops a rough feel when you turn it by hand, and eventually it grinds. Winding failure comes from heat, and announces itself with a smell of scorched varnish that you will recognise instantly the first time you meet it. Wire fatigue at the point where the three motor leads leave the stator is the third, and it is caused by vibration rather than by current.

In the air, these failures have distinct signatures. A dragging bearing shows as one corner of the aircraft running hot and the flight time dropping. A desynchronisation, where the speed controller loses track of where the bell is, shows as a sudden stutter and a violent flick to one side, usually during a hard throttle change, and it is the failure most likely to end in a crash. A dead speed controller shows as an aircraft that flips over on take off before it has left the ground, because one corner is producing nothing at all. A speed controller that has partially failed will get hot enough to soften the tape underneath it.

Choosing and replacing motors and speed controllers

To choose a motor and a speed controller, work inwards from the propeller. The propeller size is set by the frame, the propeller sets the stator size, the intended cell count sets the KV, and the current the whole combination draws sets the speed controller rating. Working the other way round produces a build that is fast on paper and unflyable in fact.

1. Fix the propeller size first, because the frame has already decided it.
2. Pick the stator size to suit that propeller, larger stators for larger blades.
3. Choose KV against the pack you intend to fly, lower KV for more cells, higher KV for fewer.
4. Rate the speed controller comfortably above the highest current the motors will draw, not level with it.
5. Check the mounting pattern, since most 5 inch class motors use M3 bolts on a 16 mm square and smaller ones use M2 on a 12 mm square.
6. Confirm the bolts are the right length, because a bolt that is too long reaches the windings and destroys the motor the first time you power it.

Replacing a motor means desoldering three wires and unbolting four screws, and the order matters: unplug the battery, unbolt the motor from the arm, then desolder, so the motor is not hanging on its wires. Direction is not set by the wiring on a modern build. Swapping any two of the three wires reverses a brushless motor, but the usual method is to change the direction in the speed controller's configuration software instead, then confirm by spinning each motor from the bench with the propellers removed. Replace all four motors together if the aircraft has real hours on it, since a single new motor on three worn ones gives the flight controller a corner that behaves differently from the others.

Motors also need cleaning, and it is the one piece of maintenance that genuinely extends their life. Blow the grit out with a dry air duster, brush the bell and the gap between bell and stator with a soft brush, and use a cotton bud with isopropyl alcohol on the shaft. Do not soak a motor, do not run water anywhere near it, and do not oil the bearings, since the oil holds dust against the very surfaces you were trying to clear.

The alternatives

There are three real choices here: brushed against brushless, four separate speed controllers against one four in one board, and buying the parts separately against buying a matched combination. Brushed motors are still used on the smallest indoor machines and on toy grade drones, because they are cheap, they need no speed controller of their own and they are quiet. They also wear out, they lose power as they wear, and they cannot be repaired. Brushless is better in every way that matters except one, and that one is price at the very bottom of the market.

A four in one board puts all four speed controllers on a single square that bolts under the flight controller on a 30.5 mm or 20 mm mounting pattern. It is tidier, it needs far less soldering, and it is the standard way builds are done now. Four separate speed controllers mounted on the arms are heavier and messier, but they have one clear advantage that the tidy option cannot match: when one fails, you replace one, and on a four in one board a single failed corner means replacing the whole board and unsoldering everything attached to it. For a first build that is going to be crashed regularly, that is not a small point.

Motor and ESC combos are sold as matched kits, and for a first build they are the sensible purchase, because the manufacturer has already done the current matching that beginners get wrong. A four motor kit with a matched four in one board sits in the middle price band for drone parts: appreciably more than a season of propellers, appreciably less than a camera drone. Buying separately makes sense only when you already know exactly which motor you want and why.

Are new motors and speed controllers worth fitting?

No on a working aircraft, and yes when a corner has actually failed. Motors are the part of a drone least likely to reward an upgrade, because the difference between a mid range motor and an expensive one is a few percent of efficiency and a smoother bearing, neither of which a flyer notices in the air. The money buys a receipt rather than a different flight. A set of propellers, a second battery or a better handset all change the flying more, for less.

The upgrade that does change the flying is a change of specification rather than a change of quality: moving to a lower KV motor on a larger pack for a long range build, or to a smaller stator for a lighter aircraft that fits under the 250 gram boundary the Civil Aviation Authority scheme uses, a figure the CAA publishes and revises. That is not an upgrade, it is a different aircraft, and it usually means a new frame as well. Where the money genuinely goes further is elsewhere in the kit: FPV goggles change what flying feels like, landing pads keep the grit out of the very bearings this page is about, and the parts of a drone shows where the rest of the budget could go. The controller covers the handset end of the same system.

Where to go next

PARTSThe parts of a droneWhat an aircraft is made ofPARTSFPV gogglesThe view from the aircraftPARTSLanding padsWhy the take-off spot mattersFLYINGThe controllerThe sticks and what each one does