Motors ...for beginners - please keep it simple

[Geoff Harris]

Hi everyone, has anyone come up with a simple table where you can cross referance weight of model against such things as KV,Amps,RPM etc etc.

[Bruce Richards]

  Unfortunately there are far too many variables to make any kind of table. Each model has its own requirements. The best way I have found is to set some fixed points. For instance you can say I do not want to pull more than 40A from my batteries. From this you can decide how many cells you will need based on model weight and the performance you want so if my model weighs 3lb and I want very high performance then I will need 150W per lb so thats 3 X 150= 450W. 450 / 40A = 11. Therefore a 12V or 3 cell Lipo battery will be fine. I need a 50-60A speed controller. Now to the difficult part of prop selection. If its a fast model then a small high rpm prop is needed if its a slow flier them a larger prop is better. So choose a prop size that suits your model then look at how fast you feel it should be spun. So with reference to IC experience props in the 11-12 in diameter and 4-8 pitch range are spun at around 8-12000 rpm. So you want a motor that will spin around this speed. I always use 10V for 3s pack under load so  so that give a motor Kv of 800-1200. the only real way to make the final prop choice is to try a range of props with a watt meter.

 

Simple as that really

Edited By Bruce Richards - Moderator on 05/12/2009 13:00:30

[Erfolg]

Geoff

 

The nearest fit to your suggested table approach are the spread sheet type calculators. These use the idea of a set of calculations which are hidden from view. What you can see are a set of input fields where you supply your known variables. Supply enough info and out pops an answer.

 

From my limited experience with these pieces of software, is that if you have identified a motor and know enough about it "all is well".  If yo do not have the data then tough.

 

What these pieces of software will not tell you is what motor to buy.

 

In many ways electric motor selection is more difficult than IC, as they are relatively inflexible, will only use a small range of propeller dia. Pretty much fixed power output, that is quite narrow.

 

Yet the apparent disadvantage in motor selection, can be viewed very differently, it is an advantage. Electric motors are very much more flexible, dependant on , voltage supplied, amps together with the propeller selection will determine watts drawn. So the propeller becomes very important. The range of propellers that a motor will successfully work with can be quite wide.

[Geoff Harris]

Thanks Erfolg,

I can see the advantage of that method, I suppose I had better try to remember how those damn spreadsheets work, inputting formulae etc etc , I expect others have gone down that route. But it will be a nice project for the winter in between building. 

Of course if anyone else has done any work on same already I would be happy to receive some advice etc. 

[Hamish]

Kv = RPM/volt    -  How?

 

Trying to get my head round how voltage can alter the speed of a brushless motor.  Back to my earlier training a 3ph motors speed was determined by the frequency of the supply and the number of poles in the winding, voltage provided current to give torque.

 

Whilst motors can be wound with different poles to give speed variations the speed changes in models are provided by the changing the frequency by the ESC.

 

Am I correct in assuming that the ESC detects the voltage and therefore ups the frequency to give higher rpm?

 

If this assumption is correct why do motors have so many Kv ratings or is this just a figure if exceeded they will disintegrate?

[Myron Beaumont]

Hamish

I too have been battling with trying to understand it as well & agree with your points made . As a sort of "inverse analogy" I think of a wind generator in reverse where the required output is one of constant voltage -the current being produced being dependant on the wind strength which in turn necessitates the blade pitch to be variable to maintain constant rpm

OR have I got it all wrong & maybe electronic wizardry &/or gearboxes (like a sort of inverse ESC ) control the power output in a different form to be converted into "mains" power for consumers . Do you get my drift ?'I can't see how an increase in voltage produces more speed either in our set ups .'like your self

I'm in no way any sort of expert (up to HNC engineering level only) & maybe there is more going on in an ESC than my brain cell can understand -eg phase changing & that sort of thing that requires quite a lot of know-how to comprehend fully .

Hopefully someone can come up with a full explanation OR is it all secretive I wonder -patents & the like

[Myron Beaumont]

PS

Just dawned on me that the thread is called "keep it simple"   WHOOPS  Sorry

[Tim Mackey]

Yes indeed...simple for beginners please.

I will post something in another thread.

[Bob Black]

Hi to you all

 

A very simple one.............is that the question or me ...............Doh?

 

Am about to start to build a depron   "indoor park flyer "with a micro 5g motor, the said motor has just arrived, but I notice all 3 wires are black.

 

What is the easiest and safest way to work out which wire is which?

 

Thanks and Regards

 

Bob

 

[Tim Mackey]

Simple answer too Bob

Just connect any of them, in any order, to the 3 wires from the ESC output. Power up the system and see which way the motor turns - if your a lucky chap, then it will be spinning the correct way, if not, dead simple again, simply swop any two of them with each other and bingo, the motor is now reversed.

[Bob Black]

Hi

Thanks for reply

My reason for wanting to know which is which is because I plan to use 3wire servo type block  connecters ( as opposed to single bullet type ) so I would not be able to juggle them about later

Thanks again

Regards

Bob

[Ed Anderson]

Many of the manufacturers now produce charts, or on-line calculators that help with motor selection.  However a new pilot might be confused about the meaing of the inputs that are required.  Reading Timbo's first post can help a newbie understand how these power system selection programs work, the inputs they requests and the results they provide.

 

Nice job Timbo.

[Chris Sessions]

Firstly I must say that this forum has been a real gold mine of information - thanks to everyone!  I'm coming back to the hobby after a 10 year break and all this electric stuff has really taken off!!

 

Anyway, I have a real simple 'motor' question...

 

When bolting a motor onto an airplane, is there any harm in having a more powerful motor than is actually required?  I ask this because if I buy a 500W motor for a 2 - 2.7Ib trainer then it would equate to 185 to 250W per Ib.  This is much more than I need and I wouldn't plan to use this much power but in buying a larger motor it could be used later on in a bigger/heavier model.

 

I'm aware that there would be negatives to this choice, such as higher motor weight and cost, but is there any other considerations I have missed.  BTW the Kv of the motor I have in mind is just over 900Kv which would suit a trainer type model.

 

 

[Tim Mackey]

Hi Chris and welcome aboard.

The simple answer is no, there is no real reason not to do this. This of course assumes the motor Kv -and therefore the RPM of a suitable propellor- is correct for the model, and this can be "altered" by using different cell counts on the battery ( more volts = more RPM etc )

[Chris Sessions]

Hi Tim and thanks for the welcome (and speedy reply).

 

Now I don't want to go off-topic, but just to clear things up, could I just ask your opinion here.

I plan buying a Decathlon ep with the advertied weight mentioned eariler.

A 3s LiPo and a 920Kv out-runner motor would mean 9200rpm (max). 

1. Would this rpm be enough for a trainer style model fitted with a suitable prop?

2. Am I correct is thinking that prop size primarily affects current drawn and has minimal affect on rpm?

3. What range of props would I need to consider and/or try?

 

Sorry if I'm asking the impossible here but any guidance would be greatly appreciated.

Thanks,

Chris

 

[Tim Mackey]

Well we are straying off topic really, but as you are new I'll let you off

The revs you mention will be more than enough - providing the prop is correct, and herein lays the rub. To get the power needed ( say 100Watt per pound ) you will need to experiment with different prop sizes, remembering that diameter and pitch will both affect the power ( and current draw as you already stated ) yet pitch will affect speed, and diameter will affect thrust /climb. You will need around 300 watts I reckon minimum, which is fine on 3s as the current will be a manageable 30A or so.

As to which props.... well given the factors above, you also have other factors.

I like big slow revving props -cos they are more efficient and quieter too - but you may prefer faster revving small ones. That motor on 3s will likely need something around the size range 11 - 12 " diameter, and perhaps 7 or 8" pitch. ofcourse, if you go to a 4s or higher, then you can "prop down" for the same power, at higher revs.

You really should get wattmeter to check the vital statistics of these props.

On 3s , you are probaly gonna need the 12", or at least, a high pitch version.

IMO only by actually flying a few different props will you find the one that best suits you and the model.

HTH

Edited By Tim Mackey - Administrator on 19/06/2010 00:10:28

[Chris Sessions]

Thanks Tim that has been a great help.  There does seem to be a vast array of variables to play about with.  From what I've read here, I think I've a better idea from where to start.  Large, slow props and slow(ish) flying speeds suits my style!

 

I can also see the value in getting a watt meter - taking some of the guess work out of the whole process.  Promise to stay on topic in future.

[Tim Mackey]

Youre welcome Chris - I always try to remind people who say that motor choice and batteries etc is complicated because of all the variables, that this is actually an advantage over IC as any given powertrain can be made that much more versatile and flexible in application

[Ed Anderson]

This is a link to one of those calculator tools. It is called WebOCalc.

 

This is not one of the very complete ones.  But it is free!

 

 In fact it is somewhat limited in what it can tell you automatically.  However if you play with it it can teach you a lot.

link

There is a help file that can walk you through the process.

 

It asks for things like wing span and model weight as well as max prop size and target performance.   In the begining, don't worry about being exact about these. Just put in some numbers that are approximate. 

 

Or, use the specs from one of your current planes or one that you are thinking you might like to buy.  Just so you have some starting information.  If you don't know values, guess, or use the tools to give your starting targets for kV and other specs.

 

Pick a motor that you think might work, or one that is recommended by the airplane mfg.  Put in those specs and look a the results.

 

Now vary the battery voltage - see what happens with the prop recommendations, thrust and speed outputs.

 

Vary the kV up and down 25 to 50 points at a time.  See what happens.

 

Now vary the target speed. See how the recommendations change.

 

Set different targets for max amps.

 

Give it a bigger prop size limitation.  Force it into smaller props. See how the recommendations change.

 

Only change one vaiable at a time till you understand how that one impacts the outputs.

 

Look at the projected performance.  These are only approximations, but you do get a feel for how the changes will impact performance of the airplane.

 

The idea is to start to develop an understanding of the relationship between these variables. 

 

Find a motor and put in its kV, max amps and see what the recommendations are. Up the amps, or limit them.  See how this would impact the projected prop choices.    Look at climb rate, whether it will likely hover, etc.

 

As a play and learn tool, this is great.  I use it for working out my motor/prop combos all the time. 

 

 

I have one e-glider that uses a 1250 kV Rimfire motor.  The recommended set-up is for 3S lipo and a 10X7 prop.  But I had a bunch of 2 cell lipos and wanted to see if I could use them.  So I played with the numbers and found an 11X8 prop will work well with the 2 cell lipos.  So I got another prop and now I can use either set-up with the same motor. 

 

Note that I use a wattmeter to confirm my expectations.  I have found WebOcalc to be pretty close in its projections.

 

Edited By Ed Anderson on 21/06/2010 13:56:08

Edited By David Ashby - RCME Administrator on 21/06/2010 14:17:25

[Ed Anderson]

Here is a link to a whole bunch of calculators:

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

 

[trevor grace]

hi Ed

i  scratch build all of my planes (mostly funfly) myself & find your WeboCalc

invaluable

if you could add an apprx. flying time to your min.battery size that would be

brilliant !!!

Trevor Grace

[Ed Anderson]

Trevor,

 

WebOCalc was not written by me.  I am just a user.   However I believe the web site does have a place where you can suggest enhancements.  The link I gave you takes you straight to the calculator.  But there is more to the site.

 

http://flbeagle.rchomepage.com/

 

The fellow who wrote the software is known on the forums as "Flies Like a Begle".

The web address is based on that name. 

 

If is funny that you should ask for that enhancements as I just finished an article for our club newsletter that is titled "Sizing Batteries and Estimating Battery Run Times for Electric Motor Applications".

 

I don't believe it is appropriate for this thread and I have had my hand .. keyboard  slapped for posting articles here.    But if you are interested and would like a copy, send me a PM, with your e-mail address and I will send it to you.  You might find it interesting.

Edited By Ed Anderson on 09/07/2010 14:38:06

Edited By Ed Anderson on 09/07/2010 14:42:51

[trevor grace]

help

I have built a small funfly w/s 36",w/a 342sq" weight  less battery&motor=15.5ozs

After running this through MOTOCALC &WEBOCALC iam getting very conflickting

results. I have used DUALSKY motors in both checks with the following results....

Motocalc =Dualsky XM3542CA-6 =450W  940Kv

Webocalc=    "         XM2834CA-10=260W  870Kv

am i missing something????????????????????????????????

Trevor G

[Tim Mackey]

Well the two motors are different - so why would you expect the results to be anything other than different?

[trevor grace]

Sorry Tim,

Not made myself very clear.

I have made the plane but needed to know which motor to use.

I fed the necessary identical  info. into the two websites and they came up with conflicting answers.

i.e. Motocalc 450w  940kv

      Webocalc 260w 870kv

Why the difference?

Trevor