Electronic Speed Controllers (ESCs)

Electronic Speed Controllers are dedicated motor control modules which control a motor's speed. There a number of things that need to be considered when choosing an ESC

Brushed or Brushless?
If you're controlling a brushed motor you will need a brushed ESC. If you're using a brushless motor you will need a brushless ESC. Its easy to tell the two apart as generally a brushed ESC will have two output leads while a brushless ESC will have three output leads.

BEC or No BEC?
A BEC or battery eliminator circuit is a DC power converter which takes the high voltage level of the main battery and converts it into a lower voltage level that can be used to power the receiver. The alternative is to use a separate battery to power the receiver instead. There are three types of BECs When using linear regulators you can put them in parallel to double your available current. However switching regulators can interfere with each other so the 5V connector on all but one ESC should be removed.
 * BEC (often use linear regulators)
 * s-BEC (switching BEC as they use switching regulators which are more efficient than linear regulators)
 * u-BEC (ultimate BEC, a trade name given to external BECs often switching regulators)

Opto or Non Opto
There's some confusion around this term (see below). Opto refers to opto-isolation, a method of coupling electric signals using light which avoids electrical noise being transferred. Often the source of the noise is in the high voltage power lines used to drive the motors and can distort the low level signals in the receiver.

Optical isolation seems like a sensible design choice. However, with modern 2.4 GHz transmitters the general opinion is that it's not a necessity. Additionally for the optical isolation to be effective you must use a separate battery for the receiver as using a BEC will create a common ground making the optical isolation redundant.

Opto Confusion
The term opto has some confusion surrounding it as it been used as a marketing tool to describe ESCs which cannot supply the receiver with power. The correct term for ESCs which can supply power to the receiver is that they are with BEC which means they contain a Battery Eliminator Circuit. Hence in some guides for quadcopters you may see things like ‘you need at least one non-opto ESC'. Care needs to be taken as to which meaning is being used especially with cheap brands.

Current Rating
The current rating of an ESC is the maximum current the ESC can drive the attached motor with. The current rating of your ESC shouldn't exceed the stall current from your motor. If you draw more current than the current rating through an ESC you could damage it. One common cause of this is a short circuit between the ESC output leads, hence isolating these connections is essential.

Programmability
Some ESCs have an amount of programmability. This means using some software and a connector you can alter the limits and settings in the ESC. This allows you to fine tune the ESC to your particular motor, maximising its performance.

Calibration
Most ESCs have a calibration procedure. This determines how the ESC interprets the signals passed to it. This is essential if you want bidirectional control.

The procedure will consist of something like this: This is just an example configuration procedure, most ESCs will come with a leaflet explaining the procedure for that particular model.
 * 1) Turn off the power to both transmitter and receiver and ESC. Removed anything attached to the rotor and ensure all connections are isolated.
 * 2) Turn on power to transmitter and move the channel that the ESC is on to maximum.
 * 3) Turn on power to ESC and receiver and listen for tone.
 * 4) Move control to minimum and listen for tone.
 * 5) Move control back to maximum and wait for a tone.
 * 6) Return control to the neutral position.
 * 7) Remove power to receiver and ESC
 * 8) The ESC is now configured

Reversible
Some ESCs only control motors in one direction. ESCs which can control a motor in both directions often will say they are “with reverse” or “bidirectional”. Motor direction can always be reversed by swapping the electrical connections. If the ESC is bidirectional then the motor can also be reversed using the servo reverse switch on the transmitter (depending on the model).

Brake
ESCs which feature brakes use electronic braking to slow the motor down quickly. If you rotate the rotor of a motor (or it rotates due to an inertial load) it acts a generator as the armature (windings) pass through the magnetic field inside the motor. This generates a voltage which if connected to a circuit allows current to flow. By dissipating this electrical energy over a resistive load as heat, rotational energy is removed from the rotor this is known as dynamic braking. If you short circuit the motor terminals this resistive load is the motor's internal resistance in the motor windings and should only be used in low power applications.

Having a brake is useful for drive systems as it allows the faster decelerations and hence manoeuvrability.