ESCs and brushless motors

Every electric RC model has the same power chain: battery, electronic speed controller (ESC), motor. Getting those three matched is the difference between a model that runs cool for years and one that cooks its electronics in a single afternoon.

What an ESC does

The ESC sits between the battery and the motor. It reads the throttle signal from the receiver and switches battery power to the motor many thousands of times a second to set its speed. A brushless ESC also works out the motor's position so it can drive each of its three wires in turn.

Most ESCs also include a BEC (battery eliminator circuit), which supplies a regulated voltage to the receiver and servos, so the model needs only one battery.

Brushed and brushless

Brushed motors use carbon brushes to switch the current inside the motor. They are cheap, simple, and wear out over time. You still find them in ready-to-run cars and micro helicopters.

Brushless motors do the switching electronically in the ESC. They are more efficient, more powerful for their size and need almost no maintenance. For cars, the trade-offs are covered in brushed vs brushless RC cars.

kV explained

A brushless motor's kV is its approximate no-load revs per minute per volt.

RPM ≈ kV × battery voltage

So a 1000kV motor on a 3S pack (11.1V nominal) spins at about 11,100rpm with no load. The same motor on 4S (14.8V) spins at about 14,800rpm.

• Low kV motors turn slower with more torque, suited to big propellers, large helicopter rotors, crawlers and higher voltages.
• High kV motors spin faster with less torque, suited to small propellers, fast cars on low cell counts, and small helicopters.

Note the lower-case k and capital V doesn't mean kilovolts. It's a constant, not a voltage.

Turns, sizes and motor types

Turns. Brushed car motors and some brushless car motors are described by the number of wire turns: fewer turns means a faster, hungrier motor. A 12T is faster than a 27T.

Sizes. Car motors are often named by can size (540, 550). Aircraft motors are usually named by stator size, such as 2212: a 22mm diameter stator, 12mm tall.

Inrunner and outrunner. Outrunners have the outside case spinning and suit propellers and helicopters. Inrunners spin on the inside and are common in cars.

Sensored and sensorless

Sensored brushless motors have a cable telling the ESC exactly where the rotor is. They pull away very smoothly from a standstill, which suits racing and crawling. Sensorless systems are simpler and tougher, and fine for bashing, planes and helicopters.

ESC ratings

• Continuous current (amps): what the ESC can deliver indefinitely with good cooling.
• Burst current: what it can handle for a few seconds.
• Cell count or voltage: the battery range it accepts, for example 2-3S or 2-6S.
• BEC output: voltage and current for the receiver and servos.

Pick an ESC whose continuous rating is comfortably above the motor's maximum current draw, at least 20% higher, and whose voltage rating covers your battery. Our guide to LiPo batteries explained covers cell counts and voltages.

Matching motor, ESC and battery

1. Start from the model's recommended battery: cell count and capacity.
2. Choose a motor whose kV gives the speed (or head speed) you want on that voltage.
3. Check the motor's maximum current at that voltage with the propeller, rotor or gearing you'll use.
4. Choose an ESC rated for that voltage and at least 20% above that current.
5. Check the battery's C rating can supply the current.

Settings by type of model

Helicopters. Use governor mode if your ESC or flybarless unit offers it: it holds the rotor speed constant as the battery voltage falls. Set a gentle soft start so the rotor doesn't spin up violently. The gyros and flybarless units guide covers governors in flybarless controllers.

Planes. Calibrate the throttle range when you first fit an ESC, set the low-voltage cut-off to reduce power rather than cut it dead, and set the brake on for folding propellers or off for fixed ones.

Cars. Set the battery type (LiPo or NiMH) so the cut-off works, choose drag brake and reverse settings for the way you drive, and turn down "punch" if a new driver keeps flipping the car.

Heat is the warning sign

After a run, the motor and ESC should be warm, not too hot to touch. If they're too hot to hold a finger on for a few seconds, something is overloaded: in cars, usually gearing that is too tall; in planes, too large a propeller; in helicopters, too high a head speed or a binding mechanism. Most motors are damaged above about 80°C.

For the rest of the power chain, see the Electronics hub.