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Old Feb 22, 2010, 12:08 AM
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Alaska
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Help!
Oops..I think I messed up

Hi all,
I am new to electric flight and I believe I have all ready messed up my first endeavor/purchase into electric flight:

25A ESC
Motor recomending 25A ESC

From what I can tell, you can pair an ESC with a higher Amp rating than the motor? If this is true, what is the rule behind pairing an ESC to a motor?

Thanks in advance!

Cheers,
Williwaw
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Old Feb 22, 2010, 01:45 AM
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Alberta, Canada
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Id say you would want the max amps your motor can draw with the prop u are using to be around 80% of the max amps of your esc. There's no harm in over doing ur esc as far as i know.
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Old Feb 22, 2010, 01:53 AM
Plane and multirotor's - FPV
DrEvil's Avatar
Moss, Norway
Joined Jun 2007
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propeller

It's the propeller that determins the amp draw , so pairing motor and propeller tell's you what Esc to get.

Kai
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Old Feb 22, 2010, 01:53 AM
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Oh no....
Now I am more confused.

I thought that the ESC prevented the motor from drawing more amperage than it could handle? Hmmm, I wonder how I would pair a motor, propeller and esc when trying to order them. I guess I need to do some more reading on this Thank you for your help all! Any tips on good reading material on this matter?

Cheers,
Williwaw
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Last edited by Williwaw; Feb 22, 2010 at 02:00 AM.
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Old Feb 22, 2010, 02:16 AM
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Armidale, NSW
Joined Feb 2007
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I dont have much to offer in the way of reading but there are plenty of calculators:

http://www.rcgroups.com/forums/showthread.php?t=606703

I use WebOCalc most of the time:

http://flbeagle.rchomepage.com/

He has a tutorial on using WebOCalc

If you provide some details on the plane you are wishing to equip with an electric power system im sure sombody will provide some assistance.

The rating of the ESC is the maximum current it can be expected to handle without failing. There is a rule of thumb of not pushing very cheap ESC's to that maximum as the rating is optimistic.
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Old Feb 22, 2010, 05:52 AM
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Staffs, UK
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Quote:
Originally Posted by Williwaw View Post
Oh no....
I thought that the ESC prevented the motor from drawing more amperage than it could handle? Hmmm, I wonder how I would pair a motor, propeller and esc when trying to order them.
You have it the wrong way round. The combination of motor and propeller decide how much current they will draw (bigger prop takes more current). The ESC (and battery) have to be able to deal with AT LEAST that much current. If the ESC or battery can handle more current than you're actually using that's a good thing.

Ideally you measure the current. Failing that you read the motor spec. If the motor says it's good for 20A you get an ESC that will handle at least 20A (I'd go for 25A or 30A). And then you only use the recommended props.

Steve
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Old Feb 22, 2010, 08:43 AM
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Quote:
Originally Posted by Williwaw View Post
Oh no....
Now I am more confused.

I thought that the ESC prevented the motor from drawing more amperage than it could handle? Hmmm, I wonder how I would pair a motor, propeller and esc when trying to order them. I guess I need to do some more reading on this Thank you for your help all! Any tips on good reading material on this matter?

Cheers,
Williwaw
It's not too difficult, once you get your head around a few basics

The motor you've linked shows 35A as being its "Max Load", but shows a 25A ESC required Normally the maximum amps can only be used for, say, 30 seconds without overheating the motor, and with some other reputable brands they also quote a "normal" maximum load (i.e. the load you can keep up all day) at somewhere around 2/3 of the absolute max. So, in this case, 25A or so is probably safe for prolonged running.

That being the case, I would choose an ESC that's specced for at least 35A, probably 40A. Going bigger is no harm, except it's more expensive and will weigh more.

Next thing to note is that motor specifies a voltage range of 6~12v. So, if you use a 3S LiPo (11.1v nominal) that'll give you a potential of 11.1 x 25 = 277.5 watts. A general rule of thumb is that you need anything between 50 and 200 watts for each pound weight of your model (including motor and battery), depending on the type of model and your flying style.

Lastly (though many people might say it's the first consideration), you need to determine what prop size is needed to use that power with that motor and battery. If you put too big a prop on (diameter and/or pitch) it will force the motor to draw more amps than it can handle, so it's a case of starting with the manufacturer's recommendation (if there is one) and then using a wattmeter to determine by trial-and-error what size will give the maximum amps you want when the transmitter throttle stick is at WOT. The rating of the ESC will normally not prevent the motor from drawing too many amps, and you could even put a prop on that'll draw more amps than the ESC can handle which, in the brief time before the motor expires, would also fry the ESC

The fact that the linked motor has a high kv number indicates that it's going to need quite a small prop, or a gearbox with a larger prop, or even a ducted fan, to keep it within its amp spec. If I were looking for something to direct-drive a reasonable size prop I would be looking for a motor with a kv of somewhere around 1,000 or so

If you let us know what model you have in mind for that power setup, someone should be able to give you more-specific advice
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Old Feb 22, 2010, 08:52 AM
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Looks like the basic aftermarket brushless setup for an Easy Star. Should be OK with a 6 x 4 APC Thin Electric. Quite a few people run this setup on their EasyStars and are pretty pleased with it. (It's what I'm using. LOL)
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Old Feb 22, 2010, 10:49 AM
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Originally Posted by Pete Miller View Post
Looks like the basic aftermarket brushless setup for an Easy Star. Should be OK with a 6 x 4 APC Thin Electric. Quite a few people run this setup on their EasyStars and are pretty pleased with it. (It's what I'm using. LOL)
No, that motor is a bit too high Kv for a prop like that. If this is indeed for something like an EasyStar where prop size is limited, I would go for this motor: http://www.hobbyking.com/hobbycity/s...idProduct=5203

If it's for a more conventional airplane, Abenn is correct and you would be better off with an outrunner motor of around 1000 Kv.
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Old Feb 22, 2010, 11:47 AM
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Wow, thanks for the help everyone! I haven't flown in over 20 years and I am completely new to electric flight. It was just way to expensive back in the day and FPV was only a dream and not option. So, yes...it is a set up for an Easy Star with the childhood dreams of FPV someday Thanks again for all your help.
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Old Feb 22, 2010, 01:39 PM
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If you want a 6x4 then 2200Kv is a good choice (probably ~25A on 3s Lipo). As mentioned 2700Kv with a 6x4 may well be more than the motor can cope with... maybe 35A-40A??
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Old Feb 22, 2010, 02:13 PM
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Quote:
Originally Posted by Williwaw View Post
B2835-2700 Test Data

A 25A ESC is recommended because this motor can't really take more than 25A ('Max Load: 35A' is a joke!). For the Easystar I recommend using a 5.7x3 prop, as it provides a good combination of thrust and speed without overloading the motor.
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Old Feb 22, 2010, 02:26 PM
c/f
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Will,
While very BASIC............

It gets more complicated, "IF" you want to have an E-setup out perform a nitro in every way important such as lighter wing loadings and thrust to weight ratio's.

While the above info is relevant, I find setups all the time where a ESC was done overkill and the battery used was only able to push 50% of the ESC rating.

Or converesly someone wants to buy a battery twice the ESC rating and I ask you like to fly for 20 minutes????

To get best wingloading and thrust to weight it all starts out with the battery. The battery is the available energy density required to get the required thrust needed to get the job done.

Now the motor as a shell becomes an empty gearbox, thus why so confusing, by inputing, the prop, the KV, and the cell count you are basically appling the gears to obtain the needed thrust your battery energy density can produce.

So when I outfit a E setup I guesstimate my all up weight, then a watts per pound target, then start with the battery that can produce that amount of wattage for 5 minutes 100% duty cycle.

Then I shop for a motor that can withstand that amount of wattage, Then I select the prop diammeter, then cell count/KV to insure.


Consider this just to make a point.

To get 100watts of power for 5 minute flight.

It doesnt matter if its a:

1s 1400MAH 20C

2s 700MAH 20C

3s 500MAH 20C

4s 350MAH 20C

5s 110MAH 20C

6s 80MAH 20C

All these batteries will give you 100watts for 5 minutes flight and the funny thing is they all weigh the same in grams.........And the funny thing is they let you refigure the KV, because KV is a function of input volts to produce a particular RPM.

As a 30 year rc modeler taking the advice of the Ecalc wizards in the beginning and you end up saying to yourself at the end of the day, "Hmm not bad" a bit pricey to go e-power.

After figuring it out, and applying as I described you say "WOW", I'll never fly liquid fuel again AT ANY COST..............

I will always have the battery as the weakest link, I rather have better wingloading and change batteries than repair models from poor flight envelopes. And I get a kick out of comments, when most all my sub 20oz models are running 2s on high KV motors.

In the case of your Easy Star, if going to brushless Lipo, your going to need nose weight ballast so battery weight is your starting point.

Figure all up weight now knowing your battery weight penalty, 50gram for a motor and 25 grams for an ESC, then say 75-100watts per pount thrust.

using 3 lbs as a guestimate, 3 X 75 = 225watt motor needed.

If you have batteries you may input their voltage under load ie 3s is 3.54VDC per cell X 3 = 10.62 VDC, so now we get to 225W / 10.62VDC = 21 amps. To go full circle of watts needed into a given prop and KV to stay within motor wattage range...........

Good luck and enjoy E power, nothing even compares.................
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Old Feb 22, 2010, 03:04 PM
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Originally Posted by machinate View Post
No, that motor is a bit too high Kv for a prop like that.
My bad, saw 2700, read and thought 2200.

The Easy Star was just a guess, we still don't know what plane he is putting it in.
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Old Feb 22, 2010, 04:21 PM
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Armidale, NSW
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Quote:
Originally Posted by c/f View Post
It doesnt matter if its a:

1s 1400MAH 20C
2s 700MAH 20C
3s 500MAH 20C
4s 350MAH 20C
5s 110MAH 20C
6s 80MAH 20C
Those last 2 configurations are only about 2 Watt Hours each compared to a little over 5 of the first 4. At 20C that is about 40 Watts.

Assuming the power system unloads in flight I would expect after 5 minutes the batteries will be almost 100% empty. Not such a good think for LiPo's.
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