Thursday, April 20, 2017

Beginners Guide Part 2 - ESC and BEC


ESC means Electronic Speed Controller.
Every electric RC vehicle has its power system which consists of motor, ESC and LiPo battery.
ESCs connects between the motor and the battery. They have more important functions.
First, the ESC gives impulses to the motor without which the motor could not operate. Second, it controls the motor speed, according to the throttle position. Thirdly, it powers the other RC devices in the model (receiver and servos) so that no special battery is needed. Fourth, monitors that the voltage in the LiPo battery does not fall below 3 volts per cell, which would cause problems with the power of other RC components and may even destroy the battery.
ESCs today are a small computers and allows programming of a large number of parameters, ranging from impulse length to minimum battery voltage.

They generally come in 2 versions, with built-in BEC (Battery Eliminating Circuit) for powering the receiver and without the BEC (OPTO ESC).
In addition, two values are important for each ESC.

Maximum continuous current in A and maximum burst current in A that you should not exceed if you do not want to "burn" the ESC. Thus, the ESCs come in values of 5A, 8A, 10A, 25A, 35A, etc., which indicate the maximum continuous current, the maximum burst current usually stands in a bracket eg. 25A (30A burst). Additionally, the maximum voltage at which the ESC can work is also important, usually marked by the number of cells (10-14 NiMH / 2-4S Lipo).

ESC usually has three groups of wires:

  • Three wires for the motor
  • Two thick wires for the battery
  • Three twisted wires for the receiver, similar to cables on servos and other RC devices.
The two wires for the battery are usually red and black, and there is a need to be careful that the ESC and the battery do not connect vice versa because of short circuit where ESC will burn and be destroyed.
It is best to use connectors that do not allow incorrect connection. In this regard, a beginner should avoid the so-called "Bullet" gold connectors for connecting the battery.

The three wires for the motor can be connected with identical connectors to the motor wires (best to use bullet connectors, but also you can weld them and insulate with shrink tube if you do not intend to move the ESC to other model). Through these three wires, the ESC supplies power to the motor but also controls the motor's performance. The information flow is bi-directional - not just from the ESC to the motor but vice versa, because the ESC must know at any time the position of the rotor relative to the stator. A detailed explanation of how this is done is quite complicated for beginners and not necessary. The first brushless motors had special sensors that informed the ESC about the current position of the rotor. Today's modelling motors are "sensorless", meaning they do not have special sensors, but ESC receives feedback thanks to magnetic induction in the windings.

Some ESCs, especially those of higher power, still have two or three wire strands with a RC switch at the end. This is nice for security reasons, so that the receiver would not drain the battery until the model is prepared for flying. Of course, the battery should always be unplugged from the model when the flight ends, because of danger for the battery, that itdoes not consume too much battery power, after which it may become unusable. Of course we are talking about LiPo batteries.

Another thing that most newer ESCs and even some of the cheapest ones have, is the protective mechanism against the accidental starting of the motor.
What does this mean l?
This means that when you connect the battery to your model, if you leave the throttle stick on radio in a position that is not neutral (the lowest), the motor will not start,it will produce a range of sounds to alert you that something is wrong.
Otherwise,if that's not the case, the motor might start to spin the propeller while your fingers are near, which can lead to wounding and even loss of some extremities.
Many better and more expensive radios and their receivers have this mechanism built into the receiver as well.

Therefore, it is very important before turning off the radio to disconnect the battery (disconnect it from the ESC), so that it does not repeat the above written scenario and have undesirable consequences.

Connection diagram is presented with the following picture:

High quality ESCs (Jeti, Kontronic, Himax ...) come with a whole list of additional functions that can be programmed. Programming is performed either by the throttle stick on radio, or, more comfortably and safely, by a special programming card. One of the most important functions of the ESC is to give a warning before the first motor start. Usually, one or more "beeps" are heard a second or two, after which the propeller starts to spin. So you have enough time, for example, to pull your fingers away if you unintentionally launched the motor.
There are also a braking functions (essential for electric gliders with folding propellers), motor start-up (quick start, softer or faster) and much more that I would not go into because a novice with a simple model will not need it at the very beginning.

Is it possible to reverse the direction of rotation of the motor and the propeller? 
If the ESC does not have this function, the wires must be reconnected. Most brushless motors currently on the market are of outrunner type. When you connect the motor to the ESC and the battery and notice that the propeller is turning in the wrong direction, all you have to do is switch any 2 wires (of three) between ESC and motor. And that's it.

One of the frequent questions about the brushless power systems is whether it is enough to have one ESC for two motors?

No - each motor must have its own ESC. So if you are building twin engine plane, both motors must have their own ESC, although both ESCs can then be connected to the same battery.

Also, something that is very confusing to beginners is when you connect the battery for the first time. ESC releases only sound "beeps" and motor does not respond to throttle.
This is because the new ESC first needs to "learn" how much is the throttle range of your radio and receiver.
The "learning" procedure is extremely simple and is the same in most cheap Chinese ESCs. Those more expensive come with instructions, so it is recommended to stick to the instructions.

Before you continue, remove propeller from motor, just in case.

In short, it is necessary as always, first turn on the radio, and push the throttle to the maximum.
Then connect the battery to the ESC(which is connected to motor) and wait for the first "beep" or two to be heard. After that (the same second), lower the throttle stick to the lowest position.
Now the ESC makes one or several "beeps" (it depends on the ESC), which indicates that it has remembered the highest and lowest throttle and the entire range of it, and no other sound warnings from it anymore. If this does not work, check the instructions of the ESC itself.
This procedure doesn't need to be repeated if everything went well.

As i have mentioned before, many today's ESCs have BEC installed.
So let's explain, what BEC is and what it is used for.

BEC is an abbreviation for the Battery Eliminator Circuit.

The receiver as well as the servos used to run the control surfaces, the flaps, the retractable landing gear, bomb drops and more, use some voltage and current for their work.
But that voltage and current are not as good as it is needed to run the motors.
All receivers work at 4-6 V voltage and they power everything that is connected to them, so each servo is powered by the receiver, with the same voltage as the receiver.
In order to get the required 5 or 6V rated voltage with battery rated voltage of 11.1V, the battery voltage must be "lowered" to the one that is intended for the receiver and servo operation. The higher the voltage would burn both the receiver and the servo, which can lead to significant cost.
That's why there is a BEC, it can be embedded in a ESC, or can be purchased separately.
Another important item in BEC is the maximum current in the amperes that can be given at rated voltage (5V or 6V). What does this mean now?
So every RC component, of course, consumes electricity. So the receiver consumes a certain amount of current, say 100mA, each idle servo consumes some (say) 100mA, but when it is under load, the current rise up to 400mA or more, it depends on the servo. Small servos consume less, but they also have less power. Bigger servos consume more acordingly.
And now, if you have a model with 2 small servos and receiver built in, you will need a BEC of 1A.
But if there are more servos (4-6), and even more like the retractable landing gears and some LED lighting , such a BEC will not be sufficient, it will be necessary to use a BEC that can supply the receiver with a current of 3-5A or more. Again, it all depends on how large components (servos) are built into the model.

There are several types of BECs:

Linear BEC - regulates the voltage by an analog circuit using an active element, never using just a resistor.
Except for a large loss (heating) regulating only with the resistor is not actually a regulator, as sevros consume more current , voltage on resistor drops down and ultimately the servos get less voltage. The main problem of Linear BECs is the cost of manufacturing. For high currents they need expensive parts and compacted circuits.Switching BEC is now appearing in the form of a single chip and costs are very low.
Linear BEC is predominant for small currents and small voltage differences, and in that it does not emit any radio pulse disturbances.

UBEC - stands for Universal BEC and it signifies a BEC that is not embedded in ESC, ie. a self-powered electronic power supply. The type may be linear or switching. SBEC - stands for the Switching Battery Eliminator Circuit

SBEC is an improved version of BEC that uses switching technology as a circuit, and can therefore provide more current in amps. Many SBECs can also be used with much higher input voltages than standard BECs (batteries above 6 cells).
Switching BEC, due to its contouring and mode of operation, emits small but noticeable radio impulse disturbances.
Thus, the recommendation to any ESC with or without built-in SBEC, or SBEC only in your model,is to put away from the receiver as far as possible to be less disruptive to the receiving signal from your radio.

Connecting an ESC with an external BEC is shown in the illustration. If you have an ESC with a built-in BEC that is too weak and you want to add an external BEC, you need to disable the middle wire (from the 3 color) on the connector to the receiver by removing it from the connector.

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