ESC

The ESC, or electronic speed controller, sits between the flight controller and the motors. It converts the battery's direct current into the rapidly switching current that a brushless motor needs, and adjusts that output many times a second based on commands from the flight controller.

A quadcopter needs one ESC channel per motor, and these are often combined into a single four-in-one board to save weight and wiring. ESCs run their own firmware as well, and the current rating of an ESC needs to comfortably exceed what the motors will actually draw, especially under hard acceleration, or it can overheat and fail mid-flight.

New builders sometimes focus only on motor size and forget to check that the ESC can handle both the motor's current draw and the battery's cell count. Undersized ESCs are a common cause of mysterious mid-flight power loss, since the board can shut down or fail once it overheats rather than giving a clear warning beforehand.