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Old Dec 20, 2014, 03:16 PM
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United Kingdom, Wales
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motor/watts question

Hi all, newbie electric question. When e.g. a 300w motor is classed to be used by a 2cell or a 3cell lipo, as the volts are different, does the current change up/down to maintain the 300watts stated. Hope I have put this question correctly. I realize the rpm of prop will change as the kv changes. Thanks
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Old Dec 20, 2014, 03:26 PM
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United States, MN, Eden Prairie
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The watts will only be realized by the load put on the motor. Don’t expect to see 300 watts just because the motor is labeled such. The 300 watts spec. is a maximum limit you should allow with the voltage and prop size you select.

Is there something more behind this question; do you have a concern that you may be running a motor too hard?

David
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Old Dec 20, 2014, 03:32 PM
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Originally Posted by Fly Boy3 View Post
Hi all, newbie electric question. When e.g. a 300w motor is classed to be used by a 2cell or a 3cell lipo, as the volts are different, does the current change up/down to maintain the 300watts stated. Hope I have put this question correctly. I realize the rpm of prop will change as the kv changes. Thanks
Assuming the prop is not changed the current will go up when the battery is changed from a 2 cell to a 3 cell.

The watts will also go up when the battery voltage is increased. BTW motor watts is not a fixed value. It is a value that should not be exceeded. Watts will vary depending on the battery voltage and the prop size.

In order to keep from exceeding the wattage limit of a motor when the battery voltage is increased the prop size must be reduced.

The kv will not change. The kv is a fixed value. It is set when the motor is built.

Glen
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Old Dec 20, 2014, 03:38 PM
yank and bank!!
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I'll try to answer your question based on what I think you're asking.... if you find props that result in 300 watts input in 2 and 3 cell operation for the same motor, the 2 cell CURRENT will be significantly higher.... in fact, if the setup were perfectly efficient, the 2 cell setup would draw 50% more current.
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Old Dec 20, 2014, 03:57 PM
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Thanks all, every answer gave me a better insight to the meaning of the word Watts.
Rhea, one of my concerns was the size of my ESC. Bit worried if I may cook it by using 2s instead of 3s. Thanks to all again.
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Old Dec 20, 2014, 03:58 PM
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Originally Posted by failboat View Post
I'll try to answer your question based on what I think you're asking.... if you find props that result in 300 watts input in 2 and 3 cell operation for the same motor, the 2 cell CURRENT will be significantly higher.... in fact, if the setup were perfectly efficient, the 2 cell setup would draw 50% more current.
For a given motor and prop you cannot realize 300 watts from both a 2S and a 3S battery.

Glen
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Old Dec 20, 2014, 04:07 PM
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I'll try to answer your question based on what I think you're asking.... if you find props that result in 300 watts input in 2 and 3 cell operation for the same motor, the 2 cell CURRENT will be significantly higher.... in fact, if the setup were perfectly efficient, the 2 cell setup would draw 50% more current.
Quote:
Originally Posted by ggcrandall1 View Post
For a given motor and prop you cannot realize 300 watts from both a 2S and a 3S battery.

Glen
Obviously, the prop for 2s would be different than the prop for 3s.

Sorry for nit-picking.
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Old Dec 20, 2014, 04:30 PM
yank and bank!!
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...
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Old Dec 20, 2014, 04:32 PM
yank and bank!!
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Originally Posted by ggcrandall1 View Post
For a given motor and prop you cannot realize 300 watts from both a 2S and a 3S battery.

Glen
With the right gearbox it could be done
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Old Dec 20, 2014, 04:48 PM
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just one more from a newbie if you don't mind. If I change from a 3s to a 2s without changeing the prop size, will I have less revs. ? Thanks
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Old Dec 20, 2014, 05:28 PM
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Yes
Pete
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Old Dec 20, 2014, 05:30 PM
yank and bank!!
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just one more from a newbie if you don't mind. If I change from a 3s to a 2s without changeing the prop size, will I have less revs. ? Thanks
Significantly.... study thrust charts for motors at headsuprc.com and for the cobra rc motors....
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Old Dec 20, 2014, 07:49 PM
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The Netherlands, GE, Nijmegen
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... newbie electric ...
Quote:
Originally Posted by Fly Boy3 View Post
... from a newbie
Do yourself, your equipment and your wallet a big favour ...

For a rainy day(s) ...

E-flight 101 by RCG member Ken Myers, will at least save you a ruined LiPo (or worse!), a burnt motor and a fried ESC:
The Ampeer
-> Electric Power Basics

followed by
The Ampeer
-> Everything youw wanted to know about e-flight (Ed Anderson)
Table of Contents

  • What You Need To Know About Receivers
  • What function goes on which stick?
  • Battery Basics
  • Amps Versus Volts Versus C
  • Lithium Battery Chargers
  • Balance chargers versus external balancers
  • Extending Flight Times While Maintaining Balance (proper CG)
  • Understanding the Electronic Speed Control (ESC)
  • the Low Voltage Cutoff Feature (LVC) Of Your ESC
  • The Role of the Battery Eliminator Circuit in Your ESC (BEC)
  • Sizing Power Systems for Electric Airplanes
  • Prop Versus Amps
  • What Do the Kv Numbers On Motors Mean?
  • Who Needs a Wattmeter/Power Meter? (everyone!, RvS)



www.scriptasylum.com
-> RC calculations
-> electrical
-> system wiring
respectively ...
-> LiPo pack wiring
respectively
-> chargers (2 menu items)
The other menu items and demos are also veeeeery handy/instructive for e-folk, add site to your favourites?

www.ezonemag.com
-> FAQ

Vriendelijke groeten Ron
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Old Dec 21, 2014, 01:11 AM
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just one more from a newbie if you don't mind. If I change from a 3s to a 2s without changeing the prop size, will I have less revs. ? Thanks
With the same motor: Just a little bit more than 2/3 the revs, and only 30% of the power. Power required to turn a prop varies with the cube of rpm. rpm varies proportionally to voltage, although losses mean it's not quite linear
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Old Dec 24, 2014, 04:10 PM
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Thanks all, looks if I have some reading to do. LoL. Never mind, the weather is terrible. Cheers
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