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Amp Draw?

Old 09-08-2008, 12:11 PM
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greigmckean1
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Default Amp Draw?

Right guys need your help!

Im really confused. Im new to the whole electric thing and I have a question.

Say I have an aircraft that requires 40 amp maximum power. So I have a Motor and ESC that can withstand 40 amps maximum. The problem I have is my LiPo. For talking sake say I have a 3000 Mah with a 20C discharge. That LiPo can produce a maximum of 60 amps. This is too powerful for my system and could damage my Motor and ESC. How do I limit my power so that I dont damage my gear? The maxiumum discharge I want to draw from the LiPo is just over 13C which will give me around 40 amps (3x13=39amps). How do I do that so when Im flying I know that I wont go over 13C? Can you limit the power to this discharge? Can you set up your Transmitter (throttle stick) so you dont use too many amps? As you can probally tell I know nothing about electrics so there is probally a very easy answer. Go easy on me guys!
Old 09-08-2008, 01:28 PM
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dignlivn
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Default RE: Amp Draw?



You may want to get a Watts Up Meter to help
determine how many Amps you are pulling at
WOT. A larger than needed ESC is what I use.
Larger as in Amps capability. The Motor determines
how much Amperage is needed at full throttle.
Your Lipo will supply the Amperage that the motor
draws according to your Stick position.

My Electric trainer drew 29 amps at WOT. I had
a 45 amp ESC and a 4s 3200 mAh lipo,which could
provided up to 96 Amps continous. With more Amps
at 5 second burst. Which my motor could not draw,
since it drew 29 at full throttle. Hope that make sence,

Bob
Old 09-08-2008, 01:37 PM
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JohnW
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Default RE: Amp Draw?

You are fine. It is better to have a battery that can safely deliver more current than the motor requires.... the opposite would be bad. The 20C figure on the battery is the maximum safe discharge current. The battery will only deliver the 20c (60amps) if your motor and speed controller try to pull that much. You won’t ever pull 60amps as long as your battery is the recommended voltage and you have the recommended prop. You can double check the current draw with a current meter when running the motor if you want, but as long as your battery voltage and prop are within spec, everything will be fine.
Old 09-08-2008, 03:26 PM
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spiral_72
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Default RE: Amp Draw?

If you hammer the throttle and your motor pulls a maximum of 40A with a properly sized prop:

You want the ESC to be able to deliver AT LEAST 40A without damage to the ESC. SO, you pick a 45A ESC or a 50A ESC

Since the motor and ESC are now paired, you still know your maximum current draw is 40A regarless of the ESC..... You choose a battery that can deliver AT LEAST that much without damaging itself. So you pick AT LEAST a 13C 3000mA Lipo. A 15C is the next larger "standard" so you can go with a 15C or a 20C.

Keep this in mind: You match your equipment to the motor's current draw. The above battery and ESC will happily deliver 0.5A from a tiny motor all day long. They only deliver what the motor demands. You can run that motor off a ESC and battery the size of a house and it'll only pull 40A max.
Old 09-09-2008, 06:45 AM
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greigmckean1
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Default RE: Amp Draw?

Ahhh!! Right guys I get it now! So the motor will determine the amps drawn from the battery. So my 40 amp motor can only draw 40 amps and no more from the battery. Even though the battery has potential to deliver more. I knew there would be a simple answer to it!! Ha!
Old 09-09-2008, 08:37 AM
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Default RE: Amp Draw?

Yup, you basically got it. The only thing I'd stress is that even if the motor is rated for 40amps max, it is possible the motor can draw more than the 40amps if it is "abused." If you use a battery with too high of a voltage or put too large of a prop on the motor, you could draw more than the maximum safe 40amps listed for the motor.
Old 09-09-2008, 12:10 PM
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Rodney
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Default RE: Amp Draw?

You still must choose the proper prop. If you overprop (to much pitch or to large a diameter) you can exceed the recommended limits on either the ESC and/or the motor. That is why you should use a meter to measure the max current at WOT and adjust prop size to get the desired value.
Old 09-09-2008, 01:17 PM
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greigmckean1
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Default RE: Amp Draw?

Great thanks guys!
Old 09-10-2008, 12:53 AM
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Default RE: Amp Draw?

For any given voltage the motor will try and spin at a given RPM. The bigger the prop the more power (and hence current) it will take to get to this RPM.

Bigger prop - bigger current.

There are other factors also, but this rule is one of the basics


Terry
Old 09-10-2008, 06:32 AM
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greigmckean1
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Default RE: Amp Draw?

How much in amp will a prop draw? are we talking 10 - 20 amps or more? Or 2 - 4 amps? I realise the bigger it is the more amps it draws. I just want to have an rough idea of what we are talking about here.
Old 09-10-2008, 09:14 AM
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Flypaper 2
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Default RE: Amp Draw?

This is where you need a wattmeter to tell what's going on. Sort of like a car with no gas gauge. How much do you have left? I use an Astro Wattmeter. Tells you the amps, volts and watts, while the motor is running. If the motor specs call for 30 amps max, then try around a 12 in. prop. As long as it's under 30 amps your good to go. Although if you use to small a prop, say that only draws 20 amps, you arn't getting the full potential out of the motor.
Old 09-10-2008, 11:07 AM
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greigmckean1
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Default RE: Amp Draw?

So lets say I need a 30 amp max system. Do I spec a motor that is bigger than this? Let say around a 40 amp. So by the time I put a prop on it brings tha amperage down to 30? Or do I stick with a motor that is closer to 30 amps?
Old 09-10-2008, 11:54 AM
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Flypaper 2
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Default RE: Amp Draw?

Hi Greig. Most motors are not chosen by the amp draw. More likely watts or equivalent glow. The reason being, amps times volt equals watts. If you change the voltage, you have to change the prop to get the same watts. Stick to the motor closer to the amps. A good place to check things out is the Himax or Eflite websites where they show the equivalent glow motors. Specs show what speed controller and recomended prop sizes for that motor. Also whether it's for sport, 3D or heavy warbirds. Click where it say (additional info)
http://www.maxxprod.com/mpi/mpi-26.html Lots of usefull info Any more questions, just yell.
PS do you have a particular plane in mind. If so we can go about picking a motor speed controller and batt, and why.
Old 09-10-2008, 01:55 PM
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Default RE: Amp Draw?

Flypaper is correct in that when selecting a motor, one is generally interested in the power, whcih would be watts, not amps. Electric motors are differnet creatures than a IC motor in that if you add more prop load, the motor will put out more and more power, until the motor or soem system componet finally smokes. This is why electric motors and thier subcomponets will have a maximum safe rating. As long as all the componets in the system are not pushed beyond thier maximum ratings, you will be fine. They of course can be run at levels much below thier maximum ratings.
Old 09-10-2008, 03:28 PM
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spiral_72
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Default RE: Amp Draw?

Yea, and if you're getting confused by all the motor specs, wait till you start comparing motors of different manufactures......

Try not to overthink it too much. Unless you are flying competition or something, there's no need to dig that deep. Planes usually have a recommended motor. Goto the high end of their recommendation if you want to dazzle, or the lower end if you just putt around. 3D usually requires a larger motor BUT planes have different purposes. If you buy a 3D plane, their recommendation is probably OK...... If you buy a Cub and expect to 3D it, ya might consider going ALOT bigger motor than recommended.
Old 09-12-2008, 10:26 AM
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geefish1
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Default RE: Amp Draw?

Hi Guys,
I have been flying two different planes with an E Flite power 46 outrunner on 4s lipos. I have been pulling about 60 amps wot and 1000watts. I am going to 5s and wanted to know if anyone is using this motor and what the maximum safe amp draw is. My esc is a hacker 70 amp. My bench readings with a smaller 12x8 prop seemed still a little high at wot: 67amps and 1275 watts. Considerably less at 3/4 and 1/2 throttle. Any ideas on the right prop size? I wanted more speed from my mustang and pulse 40 xt.

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