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Old Jan 25, 2009, 06:13 AM
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East sussex
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ESC question.

Hi guys,

just wondering if an esc with a 2amp bec will be able to take 4 servos? ive heard that it could possibly not. what are your opinions?
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Old Jan 25, 2009, 06:50 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,192 Posts
Yes, but.

As long as you don't intend using all four servos at the same time, and they are heavily loaded.

By heavily loaded I mean, aerodynamic load from big surfaces, stiff hinging on the control surfaces, stiff or dragging control push rod, snakes etc.

Also the quality of the servos, some draw more current even when stationary that other, and more when driving. Digital servos can draw more than analogue ones.

It comes down to what the load is likely to be on the servos, the greater the load the higher the current drawn.

I have flow models with lowish bec outputs, but I don't fly 3D type maneuvers.

So, back to the first statement. Yes, but........
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Old Jan 25, 2009, 07:14 AM
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East sussex
Joined Jul 2005
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ok thanks. its for the great planes DVII, so no 3s maneuvers there!! good point on the load. Now the esc is 15 amp,the suggested motor pulls 15 amps max. i would prop it to around 14 amps. Does this change anything with regards to the esc's ability to handel anything?
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Old Jan 25, 2009, 09:11 AM
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East Anglia, UK
Joined Sep 2002
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That's the wrong info.

Nearly all BECS will power 4 decent servos on a 2s pack, most will do it on a 3s pack, but heat will get you on 4s and above.

Irrespective of the amp rating. Its the power rating you need to worry about.

If its a smalish plane and using smallish servos and 3s or less, just arrange a bit of cooling and be happy.

If it isn't, don't mess around: get a Switching BEC from e.g. dimension engineering. They cost a lot less than a smashed plane.
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Old Jan 25, 2009, 09:34 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,925 Posts
You don't mention the battery type and cells or identify the ESC by name.
Since those kits can be completed with any number of motors and ESCs it is hard to provide advice.

Assuming the ESC is one of the cheap imports with a linear BEC and you are using a 3S battery, with four servos (even using them lightly), and is running the motor at a 14 Amp draw, all of those combine to put a 15 Amp ESC as working at or maybe even over it's rated maximums.

A 18 or 25 Amp ESC would be a better choice for use with a 15 Amp motor as it will give more power for the servos, have some reserve, and will not be working at it maximum ratings.

The thing that kills ESCs is basically heat causing electronic components to fail. A linear BEC has to drop the battery voltage to the 5V BEC voltage and it does it by converting the extra voltage into heat. So the BEC will get hotter with a 3S battery than it will with a 2S battery.

But if your ESC is well ventilated or, even better, mounted exposed to the prop blast and air stream, it may survive OK.

To do a seat of the pants test try the following (I'm assuming you do not have a watt meter):

Tie the plane down. Get the ESC between your fingers and run the motor up to about 1/3rd or 1/2 throtte. If you cannot continue to hold on to it at that throttle setting, it is getting over 130F. Get it exposed to the airstream and touch it occasionally to see if it still too hot to ouch.

If it stays too hot for anything but a momentary touch, I'd really want to try to figure out how hot it is getting. You can do that with a thermistor or by holding a quick reacting probe on a digital LCD thermometer (you can find them in the cooking section of stores) against the ESC.

Or, worst case, you can simply fly it and see if it survives. If other have run the same components and they survived, you should be able to do it too.

Most ESCs should be able to, and do, survive at temps up around 200F. But if you can get them to run cooler it is better of course.

I'd do a little testing first, the ESC may have overload protection in it and that may shut down the motor and prevent ESC failure if you fly it. So I'd try to be prepared for anything until you got some experience with it.

Jack
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Old Jan 25, 2009, 10:06 AM
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East sussex
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its a scorpion 15 amp esc with a 2amp linear bec using 4 micro or sub micro servos. its intended for a electrifly DVII. I know i should go with something larger but was hoping to get away with it as i dont want to buy another esc! oh well guess il have to. basically my wings folded for the second time on my e-flite se5a during a lovely flight yesterday. i had that esc in there so want to use that and the s60's in another bipe. one on which the wings wont fold. i even reinforced them with carbon after the first failure and was getting to enjoy the plane and then they go again! oh wel, thats the hobby we are in so...
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Old Jan 25, 2009, 05:51 PM
Jack
USA, ME, Ellsworth
Joined May 2008
16,925 Posts
And you might be able to get away with it. You don't necessarily have to pull 14 Amps from the ESC unless it is absolutely necessary to get the plane to fly.

It is answering questions like this that makes makes a watt meter so useful. If you found it was pulling less than 14 Amps at full throttle or that the plane could be flown easily at a 10 amps it makes a big difference.

And if you are the least bit curious or anal about details, some of the answers generate more questions. After a life long interest in numerous things I happen to a nice collection of tools and can measure many things (amperages, voltages, temperatures, etc.) that tell me more about what is going on and that is all just part of the enjoyment to me. But I have bought a Watt's Up watt meter and some other things just because they were more suitable for RC use.

Jack
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