Electronic speed controller

An ESC takes a control input from the reciever or the flight controller and uses it to throttle a brushless motor. Brushless motors are digital pulse-width-modulation devices with three 'poles': one pole is positive, one pole is negative, and one pole is being tested for induced current by the ESC, as a sort of speed/position sensor. When the position reaches a certain point, the poles flip and begin targetting their next set of coils with the same attract - repulse - sense combination. The most basic brushless motor uses 3 coils, but to balance things usually more are used, in multiples of three (1/3 are always positive, 1/3 are always negative, and 1/3 are always sensing).

The ESC is in charge of rapidly distributing 12-15V (3-4 cell) DC LIPO power in a square wave to a highly inductive load, at extremely low equivalent series resistance. This is a tough job for an electrical engineer, and it wasn't reasonable to do it until cheap, high-power solid state power transistors came of age. Even so, an ESC is a high-power device that bleeds off a lot of waste heat, and needs to be both rated for a reasonable safety margin, and ventilated adequately to keep it cool. Many otherwise sleek planes with internal control rods mount the ESC somewhere it gets airflow in order to cool it down, or mount it to a heatsink.

ESCs for fixed-wing planes typically come in heat-shrunk wire-dongle packages. ESCs for multirotors are more diverse, and often an autopilot board project will include custom ESC boards as part of a kit.

ESCs can be flashed with new software to improve response time or reduce noise.