An electronic speed control or ESC is a device mounted onboard an electrically-powered R/C model in order to vary its drive motor's speed, its direction and even to act as a dynamic brake in certain controllers, perhaps even antilock braking. An ESC can be a discrete unit which plugs into the receiver's throttle control channel or incorporated into the receiver itself, as is the case in most toy-grade R/C vehicles. Some R/C manufacturers that install proprietary hobby-grade electronics in their entry-level vehicles, vessels or aircraft use onboard electronics that combine the two on a single circuit board. A sophisticated example of that type can be found on the E-flite Blade CP helicopter with its four-function module combining control surface mixing as well as acting as a receiver, gyroscope and speed control.
Regardless of the type used, an ESC interprets control information not as mechanical motion as would be in the case of a servo, but rather interprets the information in a way that varies the switching rate of a network of field-effect transistors, or "FET's." The rapid switching of the transistors is what causes the motor itself to emit its characteristic high-pitched whine through its brushes, especially noticeable at lower speeds. It also allows much smoother and more precise variation of motor speed in a far more efficient manner than the mechanical type once in common use. The use of three-phase, AC brushless motors absolutely requires an ESC of the proper type. Those designed for use with DC brushed motors will not work.
ESC's designed for sport use in surface models generally have reversing capability; newer sport controls can have the reversing ability overridden so that it can be used in a race. Controls designed specifically for racing, for use in electric gliders with folding propellers and even some sport controls have the added advantage of dynamic braking capability. Simply put, the ESC forces the motor to act as a generator by placing an electrical load across the armature. This in turn makes the armature harder to turn, thus slowing or stopping the model. Of course, stopping the propeller of an electric glider does not stop the model itself.
Some controllers add the benefit of regenerative braking. This puts the voltage being generated by the motor during dynamic braking back to work recharging the vehicle's drive batteries. On full-sized vehicles, regenerative braking is used in electric and hybrid golf cars and hybrid automobiles while dynamic braking is used in diesel-electric locomotives to help slow trains on long downgrades.