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Selecting Electric Power Systems -

Old 09-18-2009, 04:02 PM
  #151  
kyleservicetech
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Originally Posted by PaperAirplane View Post
Question:
If my motor draws 14A and has 175 input watts the voltage would have to be 12.5V.
I know this is somehow not correct because I have never seen a 12.5V battery and a 4S battery would not be necessary for a 175 watt motor.

Can someone explain what I did wrong?

Thanks
Your battery is a three cell LiPo, so it should be around 11.1 volts or so, not 12.5. Did you measure your setup with an Astroflight Whattmeter, or similar device? Or did you use separate readings of current and voltage with a volt/ammeter or multimeter?

If you measured the setup with a separate meter, keep in mind that the voltage on the battery should be measured while the motor is running at full power. (The battery voltage will drop under load)

Also, this is rather important. If you are using a multimeter to measure current, typical multimeters have wires way to small to handle the currents pulled by our electric motors. It would not be unusual for these multimeter probe wires to drop a volt or three in the wires themselves when pulling over 10 Amps.

What did you use to measure the voltage and current? Could be you've got a problem with your measuring equipment? Just a guess.
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Old 09-18-2009, 08:34 PM
  #152  
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Originally Posted by AEAJR View Post
Your math is right but your approach is wrong.

If you want to know how many watts your current system is producing you have to measure it. A watt meter is good for that.

OR

You can estimate it based on published information from the motor mfg for that battery/prop combo.


If you have done your watts/pound calculation and detemine you want a power system that produces 175 watts then you work from that.

What voltage battery do you plan to use?

2 cell lip0 = 7.4V
3 cell lipo = 11.1V


To get to 175 watts you need a motor/prop combo that will produce 175 watts on that voltage

175/7.4 = 23.6 amps
175/11.1 = 15.7 amps

So, you look for a motor/prop combo that will draw 23.6 amps on a 2 cell lipo, or 15.7 amps from a 3 cell lipo.

From here you look at things like desired performance. Is this a sailplane where climb is king and top speed does not matter? Or is this a pylon racer where top level speed is important and climb is less of a concern. That will help you decide what kind of prop you want.

You can have two motor/prop combos, both of which produce 175 watts, but produce very different performance. This is where kV comes in to play.



I have been trying to get you to read the first post. This is copied from the first post:

If you are going to design your own systems you will need some help and some
knowledge. What I have compiled is a list of links that take you to resources
to help you. First I recommend you read this article for background
information. It will help you understand and use the rest of the resources
listed below.

Sizing Electric Power Systems -
http://www.ampaviators.com/index.php?option=com_content&task=view&id=41&Itemid=27

Remember that the system operates as a whole. If you have the right motor
with the right battery and put on the wrong prop, things can get pretty ugly
pretty fast


Thanks.

Ive read the article, and am still understanding it more and more. It has really helped. I selected a motor that has the input watts rating that i need for my 25-30 oz plane (175 watts) and I was just trying to work the battery in.

I will post the combo I get for double checking.

Thanks

EDIT: wait.

would this combo still work?
this is the battery:
http://hobbyking.com/hobbycity/store...00mAh_3S1P_20C
this it the motor
http://www.hobbypartz.com/alpha-450-...ess-motor.html

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Old 09-18-2009, 10:11 PM
  #153  
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As it happens a fully charged 3 cell (11.1V) lipo will read about 12.6V. But the specification for that motor doesn't really mean very much. It is not clear if the 175W is continous (i.e. at 14A) or burst (at 18A).

In reality you probably don't want to use that motor at more than about 12A so around 130W. That's not very high for a plane which may weigh almost 2 pounds. I'd go for a slightly larger motor. If you particularly like those Alpha motors the 480 might work better and is more of a match with the battery you've chosen which is a bit big and heavy to suit the original motor.

Steve
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Old 09-18-2009, 11:06 PM
  #154  
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How would you only use it at 12 amps, even though i will get the bigger motor. Do you set it somehow?
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Old 09-19-2009, 02:02 AM
  #155  
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The propeller places the load on the motor. The wider or the deeper the prop the greater the load on the motor. By matching the propeller to the motor and the load you want to place on the motor you can control how much the motor draws.

For example, I have this motor in one of my 2M planes:

Rimfire 35-30-1250 Brushless Outrunner Electric Motor
http://www3.towerhobbies.com/cgi-bin...?&I=LXLWU0&P=7

Aircraft Application:
75-100 Watts Per Pound
Trainer & Sport Performance Weight Range: 3.3-4.4lb
100-150 Watts Per Pound
Aerobatic & High Speed Performance Weight Range: 2.2-3.3lb
Over 150 Watts Per Pound
3D & Extreme Performance Weight Under 2.2lb


Motor Diameter: 1.38" (35mm)
Motor Length: 1.18" (30mm)
Shaft Diameter: .16" (4mm)
Shaft Length: .65" (16.5mm)
Lead Length: 3" (76mm)
Connectors: Bullet type, .14" (3.5mm) diameter
Max. Constant Current: 30A
Max. Surge Current: 35A
Max. Constant Watts: 333W
Max. Surge Watts: 390W
No Load Current: 1.2A
Input Voltage: 11.1V
RPM/V (kV Rating): 1250
Weight: 2.5oz (71g)
Suggested Propeller Size: 10x7E


If I run it on a 10X7 prop with a 3S lip, it crankd about 290 watts at about 28 amps.

If I put an 11X8 on it it will go to over 400 watts which is too much for this motor. It would burn it out.

However if I go to a 2S lip on that same 11X8 prop it will run at about 150 watts, around 20 amps.

I fly it with the 3S 10X7 and it will go almost straight up. I use that for contests but I only have one 3S pack that can handle the amp load.

I also fly it with the 2S 11X8. I get about a 45 degree climb. I have 4 packs that I can use with this prop.

I plane to try the 2S prop with a 12X8 to see if I can get it up to about 27 amps, or about 200 watts. That is about as much load as I care to put on the 2S packs.

So, as you can see, I can tune the load to the kind of flying I want to do and the kind of battery pack I want to use. All work and all work with the same motor.

How do I know all this? I put a wattmeter on the motor and measure what it is doing with the different props and different battery packs.

How was that.
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Old 09-19-2009, 02:37 AM
  #156  
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Thanks; I got it!

I guess I need a wattmeter then:-).
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Old 09-19-2009, 02:40 AM
  #157  
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Originally Posted by PaperAirplane View Post
How would you only use it at 12 amps, even though i will get the bigger motor. Do you set it somehow?
As AEAJR points out in posting #155, even a small change in propeller can make a MAJOR difference in the Amps and Watts pulled by a motor. What could seem to be a small change in propeller can make the difference between an underpowered model, to one that is so overpowered that it overloads the motor/ESC/Battery, or all of them.

That's why a number of postings in this and other threads in Wattflyer indicate that some sort of metering such as Astroflights Whattmeter are almost required for flying electric models.
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Old 09-19-2009, 03:34 AM
  #158  
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If the motor mfg provides good documentation, then you can go with that.

For example Maxx Pro provides wonderful documentation with their motors.
http://www.maxxprod.com/pdf/HC3510-1540.pdf

You can read right off the chart what each prop draws with what voltage. So you can plan your set-up. You don't need a Wattmeter.

However many of the cheaper motors provide little or no documentation. Now you are flying blind. Better get a watt meter. It pays for itself in a very short time as you avoid destroying a motor, ESC or battery pack. And you can tune and try with confidence.

If you are going to be a designer, pulling together your own power system, you need the tools of the trade. A wattmeter is one of those tools.

Ed
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Old 09-19-2009, 12:45 PM
  #159  
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Ok then.

I will get a wattmeter.:-)

Last edited by PaperAirplane; 09-20-2009 at 12:06 AM.
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Old 09-20-2009, 12:14 AM
  #160  
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I found this wattmeter:
http://hobbyking.com/hobbycity/store...att_Meter_75A#

It is more in my price range and it has good reviews. My only question would be what connectors I would need. Bullet connectors?

I will use this motor (different from the first one i posted: its a bit bigger):
http://www.hobbypartz.com/alpha-480-...ess-motor.html

and this ESC:
http://www.hobbypartz.com/30a-electr...o-series-.html

and this battery (same one as before):
http://hobbyking.com/hobbycity/store...00mAh_3S1P_20C.


Thanks for all of the help so far,
PA

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Old 09-21-2009, 12:27 AM
  #161  
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.......
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Old 09-21-2009, 01:24 AM
  #162  
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Default Wattmeter Hook-up

Hello Paperairplane,
I'm not familiar with that particular power meter, but most hook up in line between the battery and the ESC. This means you will need a set of male/female connectors that are the same as your battery/ESC connectors. Here's photos of my meter, using Deans Ultra connectors.
Ron
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Old 09-21-2009, 12:30 PM
  #163  
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Originally Posted by PaperAirplane View Post
I found this wattmeter:
http://hobbyking.com/hobbycity/store...att_Meter_75A#

It is more in my price range and it has good reviews. My only question would be what connectors I would need. Bullet connectors?
Thanks for all of the help so far,
PA

PA,

I have not used or seen that WM, but most of the revews are good. There seems to be some concern about the quality of the soldering but it seems to work for most of the posters.

As Ron said, you connect this between the battery and the ESC. So your connectors will be determined by that connection.

I am standardized on Deans Ultra connectors so that is what I have on my wattmeter.
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Old 09-21-2009, 09:13 PM
  #164  
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Thanks to both of you. I will be placing my order soon.
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Old 09-21-2009, 11:56 PM
  #165  
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With my Medusa Analyzers (Plus and Pro) I fit them with Deans Ultras (since I don't need to carry the Analyzers in a plane). My battery packs, motors and ESCs are all fitted with MP Jet Gold connectors... but I have patch cords to connect packs to Analyzer and Analyzer to ESC.
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Old 09-22-2009, 02:39 AM
  #166  
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All,

Do you still use bullet connectors on the motor? And is it pretty easy to switch connectors?
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Old 09-22-2009, 09:42 AM
  #167  
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Bullet connectors between the ESC and the brushless motor are pretty standard. There is no reason to change these and changing them might even cause problems. Leave this side alone if possible.

What you use between the ESC and the battery is not standard and there are all kinds of options here.

Deans Ultra plugs are pretty popular for battery to ESC connection. This is my standard today. Some people find them hard to separate.
http://www3.towerhobbies.com/cgi-bin...&I=LXHGL9&P=ML

One that has my interest is the EC3 connector. This uses bullet connectors in a case that makes them easy to grip. This is what is used on the Parkzone Radian. I don't know if these are new or have been around a while and I had not noticed them before.
http://www.horizonhobby.com/Products...odID=EFLAEC303

Both will require soldering.

Anderson Power Poles are also well known. Could not find a link. They use a crimper rather than soldering but I find them somewhat large for small planes.
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Old 09-22-2009, 12:15 PM
  #168  
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Now i am sad that I did a scratchbuild for my first airplane. I should have gone with a HZ Super Cub. On the other hand though, I am learning a lot. So, many little things like the soldering iron to worry about. Oh well:-).


Where would you recomend to get the iron and how would you solder the connectors on? Thanks.

PA

Last edited by PaperAirplane; 09-23-2009 at 02:03 AM.
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Old 09-24-2009, 08:38 PM
  #169  
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I edited the above.
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Old 09-25-2009, 01:01 AM
  #170  
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How would this set up work?

Exceed RC Alpha 480
Weight: 3.9 oz
Peak amps: 25A
Constant Amps: 20A
Voltage: 11.1
Max Power Input:277.5 watts
Continuous Power Input: 222 watts
http://www.hobbypartz.com/alpha-480-...ess-motor.html

Battery: ZIPPY Flightmax 1800mAh
Weight: 5.4 oz
Max continuous amps: 36A
Volts: 11.1
Max output watts: 399.6
http://hobbyking.com/hobbycity/store...00mAh_3S1P_20C


ESC: Exceed RC Volcano Series 30A Brushless Speed Controller

Weight: 0.9 oz

Max burst amps: 40A (Continuous: 30A)

Volts: 11.1

Max Power Rating: 444 (Continuous: 333)
http://www.hobbypartz.com/30a-electr...o-series-.html


Dunno why the last one got spaced out.

Can I afford to get a 25A esc, or will that be too little?

PA


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Old 09-25-2009, 02:56 PM
  #171  
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Originally Posted by PaperAirplane View Post
How would this set up work?

Exceed RC Alpha 480

Weight: 3.9 oz

Peak amps: 25A

Constant Amps: 20A

Voltage: 11.1

Max Power Input:277.5 watts

Continuous Power Input: 222 watts

http://www.hobbypartz.com/alpha-480--910kv--outrunner-brushless-motor.html

Battery: ZIPPY Flightmax 1800mAh
Weight: 5.4 oz
Max continuous amps: 36A
Volts: 11.1
Max output watts: 399.6
http://hobbyking.com/hobbycity/store...00mAh_3S1P_20C


ESC: Exceed RC Volcano Series 30A Brushless Speed Controller

Weight: 0.9 oz

Max burst amps: 40A (Continuous: 30A)

Volts: 11.1

Max Power Rating: 444 (Continuous: 333)
http://www.hobbypartz.com/30a-electr...o-series-.html


Dunno why the last one got spaced out.

Can I afford to get a 25A esc, or will that be too little?

PA


WebOCalc
http://flbeagle.rchomepage.com/softw..._imperial.html

Rather than you posteing lots of combinations and asking our opinion. Why don't you tell us why you picked these. What was your reasoning and your thoughts.

The goal is to help you understand how this is done, not to do it for you.
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Old 09-25-2009, 09:38 PM
  #172  
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Oops waz responding but have to go away for weekend.
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Old 09-25-2009, 10:54 PM
  #173  
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Default Thanks, Ed!

Ed, you hit it on the head;

Rather than you posteing lots of combinations and asking our opinion. Why don't you tell us why you picked these. What was your reasoning and your thoughts.

The goal is to help you understand how this is done, not to do it for you.
PaperAirplane, we have tried over several pages to help you learn how to select a power system. Seems like every time you say "I've got it" you offer a new combination for us to decide on. I'm done trying, and I suspect a few others are, too. You're not learning, you're asking.
Ron
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Old 09-27-2009, 09:56 PM
  #174  
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Thanks for all of the help. I learned so much more than I expected too. I guess I got a little lazy and took advantage of your knowledge. You guys have helped me learn so much about electricity in general. Your articles were great and very interesting and informative. I now have the knowledge to select a power system on my own and I have discovered good articles to use as resources.

Thanks again,
PA
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Old 10-04-2009, 06:05 PM
  #175  
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Default Corsair nitro to electric

I'm really confused about this. I'm building the GP Corsair .40 size sport model Converting to electric power.
Here is what I figured on using.
Motor Pulso X2826/10 KV=920 http://www.pulsosystem.com/motor/2826.html
ESC 80A http://www.dealextreme.com/photogallery.dx/sku.13025~seQ.3
Batteries 2ea 3cell/ 2200mah 25C http://www.hobbycity.com/hobbycity/s..._25C_Lipo_Pack
Plane weight approx. 5-7 lbs with 3 servo's
Prop? would like to use 3 or 4 blade
Am I even close to getting this in the air???
Thanks
JT

Last edited by Easy Flier; 10-05-2009 at 02:33 PM.
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