Electronic speed control
An electronic speed controller 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. 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 with many smaller RTF models.
Brushless vs Brushed
Most ESCs sold are now intended for use with brushless motors. These are not compatible with brushed motors, and brushed ESCs are not compatible with brushless motors. A brushless ESC has 3 motor connections instead of 2. The task performed by a brushless ESC is much more complicated, as it needs to switch power between the three different coils at the right speed to match the rotation of the motor. Early brushless ESCs required sensors on the motor, however, these are now largely obsolete, but brushless ESCs are sometimes called sensorless for this reason.
ESCs are rated for the maximum current they can provide to the motor. Be aware that some manufacturers may advertise the maximum peak current not the maximum sustained current as the prominent figure. It is usually sensible to choose an ESC that leaves a reasonable margin above the maximum current from the motor/prop cominbation.
If a BEC is integrated, it is important to check the current rating for this is sufficient for the receiver, servos, and any other equipment powered from the receiver. (eg Fly Cam Eco)
Some R/C manufacturers that install proprietary hobby-grade electronics in their entry-level aircraft, vehicles or vessels use onboard electronics that combine the two on a single circuit board. Perhaps the most sophisticated example of that type can be found on the E-flite Blade mCX helicopter with its five-function module combining servos, 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.
Controllers 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. Aircraft controllers may have their braking capabilities overriden as well for use in electric helicopters and airplanes. On surface models the dynamic braking may provide the benefits of antilock braking.
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.
Most brushless ESCs are programmable using the transmitter or a programming card. Common options include enabling/disabling brake, low voltage cutoff voltage, cell type and/or count, and timing. Programming using the transmitter involves moving the throttle stick to respond to sounds emmitted through the motor. The instructions are needed for reference for this method. Programming cards can be more straightforward.
Many ESCs have integrated BECs that power the receiver and servos via the throttle servo connector.