From RC Helicopter Wiki
A brushless motor (more correctly, a brushless direct current (BLDC) motor) is a type of electric motor that employs a stationary armature around (or within) which the magnets move. Since the power wires connect to the stationary portion of the motor, brushes are not required, removing the friction and electrical losses they cause. Brushless motors generally last longer than brushed motors, and give better performance. A brushless motor typically has three power leads, where a brushed motor will have only two. A brushless motor is controlled by the special brushless electronic speed controller switching on and off the windings at the correct time as it senses the motor turning, thus replacing the commutator in a brushed motor.
The speed at which a brushless motor can rotate for a given battery voltage is known as it's KV rating. By varying the motor's pinion, motors with a range of KV ratings can be used in a given helicopter.
Brushless motors come in two basic forms, the outrunner motor, where the magnets revolve around the armature, and the inrunner motor where the magnets revolve within the armature. Helicopters tend to use outrunner motors, as they offer better torque and are believed to cool better.
To make a brushless motor turn in the opposite direction, swap any two wires from the ESC to the motor.
Virtually all brushless motors used in model helicopters are what are known as sensorless motors, in that they use the back EMF generated by the spinning motor to determine the motor's position---necessary for the controller to know when and which windings to switch on. Unfortunately, this can cause issues with some motor and controller combinations, leading to erratic or impossible starting and/ or running.
The enemy of brushless motors is heat. As the magnets heat up above a critical temperature (typically above 100C), the magnets rapidly lose their magnetism; this reduces the back EMF they cause in the armature, allowing more power in which heats the motor more, leading to its rapid death.
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