The basics of electric power: ESCs
|A big ESC, the YGE 90A, image courtesy of f3aunlimited.com|
WHAT IS AN ESC
After talking about batteries, and brushed motors it is time to talk about the speed controllers, or ESC, which feeds the power of the battery to the motor(s) and the system.
The first distinction to be made is between 'brushed' or 'brushless' ESCs. This refers to the application in which the ESC will be used for and the choice of one type or the other will be determined by the type of motor that we intend to use. Nowadays the vast majority of motors is brushless, and so is the ESC, so we can happily assume that, unless you specifically looked for it, you have a brushless system.
Within the brushless group we can find two more types of ESCs, the ones with the BEC and the ones without. What is a BEC? It is the short term for 'Battery Elimination Circuit'. As the name says it is a circuit which converts the power coming from the battery to a voltage which is safe to power your receiver and servos. BECs can be integrated in the ESCs or can be purchased separately.
BECs are very useful because often the batteries we use deliver up to 50V (a mighty 12S, used in big EDF jets or 700 class helicopters), whereas the majority of receivers work in the range of 5V. So either you use a second battery to power up the receiver leaving your main one for the motor, or you use a BEC, which converts the high power of the battery to a lower one, while delivering the full power to the motor. At the same time. Cool huh?
There are endless discussions about wether it is better to use an ESC with an integrated BEC or to have them separate. Some say that keeping them separate would let you to keep your receiver and servos powered in case the ESC failed, allowing you to keep controlling the surfaces for an emergency landing. The other group says that a separate BEC is just another component that can fail. I'll let you decide, but I feel comforted knowing that my biggest planes have the added security of a separate BEC.
ESCs are referred to using an 'A' value. That 'A' stands for Ampere, and identifies the maximum power they can deliver to the motor. Usually the given 'A' value is a range, such as 25-30. This means that that particular ESC is capable of delivering up to 25A constantly with short bursts of 30A to the motor before giving up and melt.
HOW DOES AN ESC WORK
How does an ESC work? This is very boring so feel free to skip ahead, but in case you are interested...well keep going.
With fuel engines you have a servo that acts on a valve on the engine which regulates the amount of fuel that gets inside the cylinder, determining the final rotational speed. The ESC acts as this system as a whole. It is based on pulse width modulation, meaning that it transforms the inputs you give with your thumbs into pulses of different width (or duration) which drive the motor speed. Imagine that the motor turns because of tiny tiny impulses of power. Now, if the duration of these pulses increases, the speed of the motor will too, and vice versa.
If you are interested in how an ESC does all these things I suggest you pay a visit to this page, which goes through this topic in great depth.
HOW DO WE USE IT
Wiring an ESC is simple, it has 3 sets of wires coming out from the sides, one set goes to the motor, and it is composed of three wires; one set goes to the battery and it is made of two wires and the last goes to the receiver, and it looks like a servo wire with a plug at the end.
|Typical wiring diagram for an electronic speed controller, image courtesy of Castle Creations|
- It is the motor that drives the choice of the ESC, every motor has some specs, one of which is: 'max. Amps'. Say your motor has a 'max. amp.' value of 15A: this means that your motor will absorb 15A when your throttle stick is at maximum. You will want an ESC which capable of delivering at least 25% more of that value, which would bring you to get a 20A ESC.
- It is always safer to use a bigger ESC, there is no danger whatsoever in using an over capable ESC, the only limit is the weight your plane can carry.
- Be super careful to solder the battery plug the correct way, you will almost always have to solder a plug yourself, which is good so you can match your batteries, but is bad because you could get it wrong. Check twice before you power it up.
- Swap two of the three wires to reverse the motor direction, there is no polarity here, and no wrong way of plugging the motor, so power it up, and if the propeller/fan/rotor spins the wrong way swap two of the wires. I like to mark them with heat shrink tubes of different colours after I determined which wire goes where.
- ESCs need ventilation, usually they get warm, or even hot, so a good airflow is always good.
- Separate ESC and receiver, the first produces radio interferences so you want to keep them as far away from each other as possible.
- It is always better to extend motor wires rather than battery wires, the pulses we discussed before can generate spikes of current which will kill your ESC overtime, more info here
- Always check your ESC can manage the input voltage, it is written on the specs, you don't want to connect a 6S LiPo to an ESC rated up to 3S, or chances are you'll see some smoke
Hope this was helpful, stay tuned for the next episode of 'The basics of electric power'