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how the electric system works?

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Old 04-13-2007, 02:01 AM
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spunkler
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Default how the electric system works?

Hello all,
I'm fairly new to the sport and have done some flying w/ electrics but have purchased all components based on wwhat my hobbyshop told me so I don't fully understand the relationship between all of the componants. Lets start w/ the battery. I have a TP 1320 11.1 volt battery when it talks about Max continuos discharge being 13C and Max Burst being 17C what does C stand for? It also states max continuous current of 17amps . Does this refer to the maximum amps my motor should draw to be safe using this battery?
Also the esc do you base the selection on what your motor draws in max and continuous current only or does the battery figure into the equation as well. I have a 1 lb slow stick w/ a Eflite 370 outrunner w/ eflite 20 amp esc w/ the TP batt mentioned earlier. It works fine I just am looking for some help understanding the basics of setup guidlines. I understand how to figure total watts of power and that I'm well overpowered for what Ive got. Thanks
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Old 04-13-2007, 09:14 AM
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Default RE: how the electric system works?

Go here:

[link=http://www.ampaviators.com/index.php?option=com_content&task=blogcategory&id=15&Itemid=31]Beginners Guides[/link]

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Old 04-13-2007, 09:41 AM
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Default RE: how the electric system works?

Well C rating is how a battery output is listed, C times the capacity, in your case 1.32 amps give you your packs constant available output. 13 x 1.32= 17.16 amps. Burst is rated same
17 x 1.32 = 22.44 amps. Now, that said, couple of things to throw at you, #1 don't always believe the C ratings mfgs put on their packs, many do not perform as stated. Rule of thumb I use is when selecting amp rating for a pack is to use the 80% rule. You motor should only draw 80% of the packs capabilities. So if your pack is good for 17 amps constant you should look to make your set up draw around 13.6 amps. Does two things, helps increase the pack life, as it's not being run at it's full ability, and two it also allows a little lee way in case the pack isn't quite up to snuff on the C rating.

Now then, on a brushless motor, the amp draw is the same no matter what position the throttle is in. Umm, the esc on a brushless set up is a basic on/ off switch. Firing each leg of the motor, for a specific length of time to give you your speed. Unlike a water faucet where the more you turn it open the more water that flows, a brushless set up is a wide open or off faucet. Folks get fooled into thinking the amp load drops when reading a meter. A meter averages the amp draw so other then at wide open throttle the delay between the esc firing causes the meter to be fooled into showing a lower amp reading. Wide open throttle is the reading to use for your amp spec. Also something else I just thought of on the 80% rule above. On most set ups, a prop unwinds slightly when the bird is in the air. What I mean by unwind is that the amp draw does in fact drop slightly, from what you read sitting on the ground running a static test. BUT, some props do the exact opposite, they pull even more then when sitting still. The extra 20% you allowed for in your calcs will keep you from blowing a pack if you run into a situation like this.

And if not to confuse you even more, a pack can only provide the amp draw specified on it for a set amount of time, before it gets to cut off voltage. Go below this and you will damage the pack. Going with a bigger capacity pack, will allow you to run longer, but adds weight.
I have a Slow Stick myself, I run a 3S2100 pack on mine, so mine has a bigger fuel tank, so to speak.

Here's an easy way to remember 2 things on packs, C rating is like the size of a water hose. Low C rated packs only allow a small dribble of water to run out, a higher C rated pack is like running a bigger diameter hose, more water flows.
On capacity, like your 1320 pack that would be a small water tank, a 2100 would be a larger tank, a 3700 would be an even larger tank.
C rating is the size of the hose, capacity is the size of the tank the hose is connected to.
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Old 04-13-2007, 11:09 AM
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Default RE: how the electric system works?

so when buying a esc if my motors max burst current is 11a all i need to do is make sure my esc can handle at least 11amp max draw and it doesn't matter what i'm getting for power from my battery?
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Old 04-13-2007, 03:10 PM
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Default RE: how the electric system works?

Ummmm, kinda yes and no. Yes you need an esc to handle at least what the motor draws.
BUT and it's a big one. Depending on where your esc is located sometimes you are better off going with a higher capacity esc then your system demands. Remember how I said it's an on/ off switch? Well in the off position the esc has to get rid of the voltage it's holding on to rather then releasing it to the motor. This causes heat, and heat has to be dissapated or the esc will over heat and shut down or fail. A bigger capacity esc can handle more heat then a smaller one. But a smaller one will work if there is enough airflow over it to keep it cool, like on the SS. It's out in the air so you should be fine. But the same set up in say something like a Cub, where the esc and motor and even the pack aren't cooled as well could cause you a problem.
Now then 2 other things on an esc. One they are rated for the max amount of voltage they can handle, exceed this and all the magic smoke leaks out of them. 2 ratings on an esc one with the bec hooked up, one with it not. Incase you don't know the bec is a built in Battery Eliminator Circuit. It steps down the voltage to the 4.8 or 6 volts your receiver and servos have to operate on. And guess what it gets hot too, as it's bleeding off voltage to drop it down to run your system. Just another item you need to consider in an electric. Also the bec is limited to how much of a load it can handle, such as how many servos it can operate, again over load it and the smoke comes out.
Isn't this fun?
Another curve ball for ya, if your set up's bec can't handle the load, you can disconnect it. And install a UBEC, all it is, is a stand alone bec, just like the one in your esc, but it's a much heavier duty version. It will handle more servos and run everything except the motor, your esc still runs it.

As for your question about not worrying about what the pack is, no you need to throw that into the mix also. If it's too high of a cell count like a 4S pack, and your esc is only rated to 3 more magic smoke will fly. You'll either have to add a ubec, if your esc will handle the voltage with it's bec disconnected, or switch to a higher voltage rated esc.
Also just cause the voltage is correct, like your 3S, you also need to consider the other part of the battery ID, like I said about before, it has to be able to handle the amp draw from the motor, and provide it for the length of time you wish to fly before it hits it's cut off point.

Lastly, just because your motor has a max amp draw listed, doesn't mean that's where it stops at. Your prop size will affect it. So will the amount of cells you power it with.
Too big of a prop and motor will pull more then the max rating and the smoke will leak out of it.
You need a meter, it's the most important piece of equipment you can own. Like an Astro, or Watt meter. It's plugged between your pack and airplane, when you run up your motor it's going to give you all you need to know about your system. How many amps it's pulling at WOT, how much the packs voltage is dropping, how many watts it producing, and so on.
With out these figures your are flying blind and something bad is going to happen.

And after all that you'll find out specs are sometimes way off the mark. Mfgs built in a safety factor sometimes. And sometimes you can exceed them and get away with it, sometimes not.

Heat is the enemy to electronics, eliminate heat and you'll be surprised at how far you can go above what the specs say you can.
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Old 04-13-2007, 08:27 PM
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Default RE: how the electric system works?

WOW!! Thanks for all the info! I just came from the hobby shop where i picked up a eflite power meter, just read your reply and am wondering if the eflite is a good option to the astro watt meter? Dude at the shop said its a "good one for the $". Thanks again for all the info i'll have to spend some time researching and decifering!!


Also thanks CitaFly for the tip on the web site!!!
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Old 04-14-2007, 05:16 PM
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Default RE: how the electric system works?

ORIGINAL: spunkler
... am wondering if the eflite is a good option to the astro watt meter?
If the E-flite meter you purchased is [link=http://www.horizonhobby.com/Products/Default.aspx?ProdID=EFLA110]this one[/link], it has one disadvantage (in my opinion) compared to some other power meters. The E-flite displays only one electrical value (volts, amps, watts, etc.) at a time. To see all of the readings, you must switch the display from one value to the next. That sounds easy enough, but it can be challenging when you are trying to write down the values, switch the display from one screen to the next, and do all of of this while your motor is running at full throttle.

Personally, I prefer a meter that displays all of the parameters at the same time, such as the [link=http://www.astroflight.com/store/store-type-tem.html?item=products:af-101n]Astro[/link] or the [link=http://www.bphobbies.com/view.asp?id=A0320107&pid=U054250&img=l]Watt's Up[/link]. I find this type of meter much more convenient to use. As for cost, they are about the same as the E-flite. My own meter is an Astro.

- Jeff
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