The great Ohm's Law debate

[scott cuppello]

As a Professional in electric flight, I think all of this is, to the average flyer....meaningless.

It's all very well to publish these theories for those who are interested, but does it help somebody get a model in the air and flying the way they want?.....no generally, it doesn't.

I'm not knocking those who are interested in such things, after all, their opinion and expertise is much needed from time to time, but, it's about time something was written in layman's terms that simply says, "fit A, fit B.....it will do this", and it really can be summed up in pretty simplistic terms, believe me....I do it on a daily basis.

[Phil Winks]

hear hear scott  I do find the necc calculations for matching motors speed controllers and batteries some what tiresome even confussing. At the risk of being corrected (and believe me I welcome it if I'm wrong) the way I've simplfied things is to assume a 3s bty at 10 volts then make sure that on say a 130w motor I'm drawing no more than 13A keeping my chosen bty in this case no less than 1300mAh within 10c discharge and that the 130W is adequate for the model AUW Still looks confusing to me but to make sure a set up isn't working beyound the limits of any one of its components I do a test run checking power consumption at full throttle when static. knowing full well that the prop will unload a little once airbourne. all the above seems to work my 2 current rides seem to be comfortably turning back 20 min flights one a 14Oz westwings toucan on a 1300s1p with a motor thats running at 90 watts (static)and an own design low winger weighing in at 1.1lbs with a 13amp 130w motor pulling 120w (static) on an 18003s1p bty. both bty's could keep their resp[ective rides up longer as on charging I'm only putting about 60% of the bty's rated capacity back in but better safe than sorry eh. should also mention all my esc's are rated at twice what I expect to draw through them for safety.

so assuming I'm not wrong the simple way is

Model AUW = Watts req       ie: 1.3lbs = 130w

Watts / volts = amps req by the esc   ie: 130/say 10v =13a so x 2 for safety = a 26a esc

to match bty capability a/10 = 1.3ah or 1300Mah

so to power said aircraft you would need a motor capable of providing 130w while drawing no more than 13amps and a 13003s1p 10c or bigger bty and a 26amp esc

Now bring on the experts AM I RIGHT????????????????????????????????????????????

Hope so because that is more or less exactly what is flying my low winger and rather well to

[scott cuppello]

Sounds good to me.....problem with all these articles seem to be the same, too much theory, not enough practice, there needs to be more balance......you see, looking at your specs above, for instance, did you look at Lipo C ratings? Because very few seem to, and they are the key to airframe AUW/model balance/flight times....and put simple, the ability of the Lipo to deliver adequate power......other than that, you seem to be spot on.

[Pete Rieden]

I would disagree with that last bit. Lipo C-ratings are a useful indication of the quality of the particular battery (in general the higher the C-rating the better the cells), but I size ALL my lipos for a full-throttle draw of 10C or at the most about 12C. Anything less just gives rediculously short flights. A typical aerobatic model with the pack sized for 10C at full throttle gives 10-15 minutes of general aerobatic flying on a charge, but using a 20C pack at 20C only gives around 5 minutes.

PDR

[Phil Winks]

Good point scott in fact I usually try to source bty's at 20c then run them as close to 10c as i can to give, as peter points out, reasonable flight times. one point peter surely any bty run at  a true 10c will only give its power for 6 mins surely the mah rating of a bty = the amount that can be discharged from that bty in 1 hour therefore times  that by 10 and the hour has to be divided by 10 ie: 6 mins so if like me your getting app 20 mins the average draw from the bty must be in the region of 3.3c. obviously it will vary imensely during the flight. much more on take of for instance and virtually nil during any sect that you've got the throttle closed so that would suggest that flying style should be taken into account when calculating flight times. I simply try to make sure I'm not overloading during take off and then take as much as I safely can in the way of flight times. This would suggest that some form of low bty level warnig system is more than just a good idea.

[Ron van Sommeren]

Scott Cuppello wrote (see)

As a Professional in electric flight ...

... it's about time something was written in layman's terms that simply says, "fit A, fit B.....it will do this", and it really can be summed up in pretty simplistic terms, believe me....I do it on a daily basis.

The floor is all yours Scott, go ahead ...

Vriendelijke groeten Ron

ps. BTW, what are FAQs?

[Ron van Sommeren]

Very useful FAQs

The stickies in the RCGroups power-sytems subforum

Understanding Electric Power Systems
http://www.rcgroups.com/forums/showthread.php?t=333326

www.ezonemag.com
-> FAQ

http://www.wattflyer.com/forums/forumdisplay.php?f=3
-> ¨Everything You Wanted To Know About Electric Powered Flight¨

Selecting power system, several links:
http://www.wattflyer.com/forums/showthread.php?t=18521

Some electrical knowledge, the basics
http://www.ampaviators.com/
-> Beginners guide
-> About Ampère and Volt

Wiring diagrams:
http://scriptasylum.com/rc_speed/_wiring.html

Vriendelijke groeten Ron

[Phil Winks]

Peter I'm not sure what has upset you but let me say that no offence was ever intended and as for keeping discussions down to infant school levels I think what is being tried rather unsuccessfully is to distill the information to as simple a form as possible so those off us who haven't been able to devote our lives to understanding electrical theory in any depth can have a near foolproof system for choosing the correct power-train for our models.

[scott cuppello]

Very constructive Peter.....thanks for that......I'm sure that you think you have a good reason to be personal, I just don't happen to see one, in fact, I'm going to cross swords with you no  more than to say that there is NO justification for slinging insults like that, and a man of your alleged intelligence should perhaps know better?

I can say that you have mis-read, and mis-understood my point totally, I was pointing out that is MOST circumstances, people do NOT pay enough attention to C ratings, and purchase a battery with too LOW a rating for the job..so, we are actually agreeing, in most circumstances....however.

In high power set up's [EDF, Hotliners, etc], where space and AUW is more critical, then C ratings have to be pushed to their limit, and therefore it is essential for best performance overall to work out the Minimum you can get away with......referring to C ratings as an indication of quality is way wide of the mark....ask anybody who has purchased a so-called 20c pack, only to find it is nothing of the sort, in high power set up's, flight time is NOT affected that much due to the fact that you are rarely using all the power available, only for short bursts....unless of course you fit an oversize Lipo...then AUW goes up....then you have marginalised the whole point of sticking in a high power set up.....it's all about balance, flying quality of the airframe/power/duration.

There are plenty of circumstances where lower C rated packs are a good choice, perfect example is a Ripmax Spitfire, for instance, where 300w [30A] is all that is needed for good performance, the model will take a 3300mAh 3s pack, in fact, for C of G, you really NEED  pack that size.....so, what exactly would be the point of fitting a 25c pack, capable of 82A constant? Fact is, a 15c [49A Constant] pack is going to perform perfectly.....so save yourself some money, and WEIGHT.

This is an example of what I'm talking about, understanding Ohms law is all very well......provided the theories are carried throughout the choice of power train.......and they rarely are. And at the moment, nobody on this thread can even agree on what ohms law actually is!!!!!!!

[scott cuppello]

As for the idea of me "taking the floor"......on a forum? Nope, no chance, and the above should be a good example of why not.

[Phil Winks]

I don't think there is any argument on what Ohms law is Scott just how to simplify its application for the shall we say layperson. but it would seem that simplification is something that this subject doesn't lend its self to

[scott cuppello]

Well Phil, I'm into flying...not electronic's...like most flyers, the basic rules are simple enough;

100w per 1lb for sport scale flyers MINIMUM

150-200w per lb for 3d

150+w per lb for EDF

And may I stress, these are minimum figures, few people seem to appreciate the true versatility of E-Flight over IC.....which is a shame.

Then.....choose motor kv to suit, simple analogy for IC lfyers, High kv motors are 2 strokes, low kv motors are 4 strokes.....this is a total over-simplification, but for your average prop drive, fixed wing aircraft, it will do.

Choose your ESC to match the motor, based on power [here we go...ohms law] quoted.

Remember that in practical terms, Lipo/motor voltage range can be narrowed to;

7.4v, 100w is pretty well as much as is practical

11.1v, 500w is as much as is practical

14.8v, 700w is as much as is practical

18.5v, 850w is as much as is practical

22.2v, 1300w is as much is is practical, after which, you consider 7,8,9,10s, dpending on the application.

I don't really care who agree's with this gross simplification or not, this is what a model shop broadly deals with on a practical day to day basis, and to be blunt....it works, so theories are all very nice, but do they help the average flyer as I said at the start? No, in short......and I think I'll leave it there to be honest, no offence mate.

[David Ashby - Moderator]

Post by RVS removed - Ran, let's not provoke matters.


Steady there gents, please exercise restraint when posting.

[Ultymate]

From a petrol headed layman can I just say that Scott's post at the head of this thread is right on the money as far as I'm concerned yes there is a need for you technically minded guys to help us lesser mortals out when it comes to electric flight, I myself having received a lot of help locally from Timbo, but I'm sure that sometimes that all the "techno geek speak" that goes on could be off putting to would be electric fliers.

[Phil Winks]

Just so right Ultymate that is exactly what I'm trying to put together here as a layman so to speak I've tried to distill the info I've got from experts like Tim and articles from the likes of Mr Hawes and I think I've got the basics just got to be careful that it doesn't get over complicated. I've been forced by circumstances to go electric over the last 9 months and its been a steep learning curve so I can appreciate the need for a simple guide for the novice to electrics. I'm sure that there must be a good book out there somewhere that does just that if not perhaps with the help of my experienced club mates I could write one  

[scott cuppello]

peter rieden wrote (see)

Scott Cuppello wrote (see)

7.4v, 100w is pretty well as much as is practical

Oh come ON Scott! This isn't even VAGUELY true. I run a Graupner Micro Show Flier on an Axi 2814/10 on 2s and an 11-7 APCe. This draws around 35A and happily delivers 250w (input) and around 200W output power - and more power can be had at low volts if you wish to do so. It would certainly be more EFFICENT to do it with a higher wind motor and more volts/fewer amps, but it doesn't make the 2s setup "impractical"!

PDR

So, it's inefficient......so it's not very practical......it's a set up that is pretty useless for 99.9% of applications....so it's impractical.....are you just one of those forum types that just likes to rule the roost?....I already have used words like "simplification", and it was made lear that it was a generalisation.....if I reccomended a set up like the one you used to a customer, I wouldn't be doing my job properly unless the application was very unusual.....forums.....this is certainly the downside of them.

[scott cuppello]

It isn't expunged from customer choice, it's simply an unusual application, we sell very, very little 2s applications [compared to 3s applications], hardly anybody does, and when you do, they are 99% for sub-100w [under 1lb AUW] applications, if a model needed 200w....and this is the point...needed, then 2s makes no sense, you are drawing 35A, when you would be drawing around 20A on 3s......and if you shop around, chances are, you are going to find a 3s pack to suit.....35A current draw for most 2s [small model applications]  is......IMPRACTICAL.

Peter, it doesn't make any difference just because you have done this for a certain application, in the world of the average electric flyer, it's not a set up for a model that that majority would be interested in, my job is to deal with things on a day to day practical level.....not to sit experimenting for days on end with odd-ball set up's just in case somebody just might come along one day and need that set up.

We test all the motors we sell, and they are all supplied with prop data sheets, IF somebody comes along with something out of the ordinary, then we talk about it and nail a set up for it, and believe me, we deal with everything from vintage, to EDF, to aerobatics to scale, to seaplanes to multi's.....the list goes on.

Therefore, in the broad scheme of things.......it is generally impractical.

[Tim Mackey]

Well I said it over on the original thread - its extraordinary the way things have "developed" from this magazine piece.

It was myself who made the "original" reference ( albeit simplistically...which is the way I try to write ) in response to the editor's claim that "using 6V battery packs gave him complete piece of mind"  - or words to that effect.

I was merely trying to clarify that this in itself will NOT guarantee peace of mind, and that due to Ohms law, ( and gosh do I regret qouting him now ) - using a higher voltage on the exact same circuit will cause a higher current flow, higher wattage and possibly make things worse through SHORTER duration, quite aside from the ability / inability of the cells to deliver said current. As most people will now realise, I was perfectly correct in this statement, but quite why the debate has digressed as far as it has into motors and props etc I dont quite know - sure, the "law" applies here too, but the piece was about the merits / problems associated with 6V radio supply.

[John Cole]

Timbo: I don't think you were necessarily correct as I said in my original posting.  Most of the posts which related to the original subject focussed on a simple resistive load, and in that simple case (as Ohm said) if you increase the voltage you'll increase the current - and thereby exhaust the battery quicker if it's of the same mAH capacity.

But you were talking about feeding a Rx and servo system.  The bulk of the current draw for a model helicopter will be to drive the servos, as they work a lot in a helicopter.  The bulk of the current draw for the servos is to drive the servo motors to move the blades.  If you increase the voltage, these motors will draw more current.  BUT they will move more quickly to the target position.  So they will draw more current for a shorter period.  So they won't necessarily exhaust the battery more quickly.

If the servo motor efficiency does not change much between 4.8V and 6V then we can ignore the ELECTRICAL issues and look at the WORK the servos do per unit of flight time (the average WORK rate).  If that increases at 6V then you need a battery with more ENERGY stored in it.  Whether the WORK rate increases (averaged over the flight, not just when the servos are moving) depends not on electrical issues but on what "friction" they are working against.  If it is an aerodynamic-drag  (i.e. viscous) load then this will increase with servo speed.  If it's straight friction it will not.

To take the extreme case of straight friction (i.e. no increase in mechanical load at higher servo speed) with no change in servo motor efficiency at 6V, and ignoring the "electronic" current for the Rx and the servo electronics then the energy capacity of the battery needed for a given flight time at 6V is the same as 4.8V, as the increased current draw will be exactly offset by the reduced movement time.  BUT if you add a cell (from 4 to 5, sticking at the same mAH capacity) you have INCREASED the energy / work capacity by 25% already!  So maybe a smaller cell-capacity would do!

[Tim Mackey]

PDR - by "original letter" I presume you really mean the "follow up letter" by K.A.

I still maintain ( given the acceptable over simplistic approach I used ) that in general terms my "original letter" was correct - a "normal" R/C radio system will likely consume a little more current from a 5 cell pack than a 4 cell, and it would be wise to increase the capacity of said pack slightly.

[William Halgarth]

You all seem very confused about what stays constant and what is variable.

1 .a battery has a fixed voltage and a varible current flow. ( But a max Current flow ) 

 2. a motor has a fixed resistance but the voltage and current can be varied to inrease power output of the motor.( but it also has a max current flow)

3. The esc varies the resistance of the circuit allowing greater/less current to flow. therefore adjusting the motor output. ( but yes this also has max current  flow ).

If the max current rating of any of the above is exceeded it will get hot .

SIMPLE 

ps Mr A made one mistake later in his reply when he said bigger motors have greater resistance, this is WRONG they have less, therefore they allow greater current to flow and thus increasing power output and thus battery drain is quicker with a bigger engine.

[Tim Mackey]

Motor - not engine, William - and I think to state that "you all seem very confused about what stays constant and what is variable" is a rather sweeping and inaccurate summation.

PS welcome to the forum 

[Pete Rieden]

William Halgarth wrote (see)

You all seem very confused about what stays constant and what is variable.

1 .a battery has a fixed voltage and a varible current flow. ( But a max Current flow ) 

 2. a motor has a fixed resistance but the voltage and current can be varied to inrease power output of the motor.( but it also has a max current flow)

3. The esc varies the resistance of the circuit allowing greater/less current to flow. therefore adjusting the motor output. ( but yes this also has max current  flow ).

If the max current rating of any of the above is exceeded it will get hot .

SIMPLE 

ps Mr A made one mistake later in his reply when he said bigger motors have greater resistance, this is WRONG they have less, therefore they allow greater current to flow and thus increasing power output and thus battery drain is quicker with a bigger engine.

Unfortunately this isn't what is going on.

Firstly a motor does not behave as a simple fixed resistance. Certainly the windings have a resistance, but a few quick sums will show that this isn't the whole story. Let's look art an example - the Axi 2820/10 (very popular "600 size" outrunner). This has a winding resistance of around 40mohms (0.04ohms). So one would expect that if it was connected to a 12v battery the current would be as stated by ohm's law:

I = V / R = 12 / 0.04 = 300 amps!

In reality we expect to use currents in the 25-40A region for this motor, so why? It is due to the fact that motors and generators are the same thing. When a motor starts turning the effect of the magnets on the windings actually generate a voltage, and the voltage generated is always in the oposite direction to the voltage that causes it to move - this is called the "Back EMF". The size of the back-EMF is determined by the speed of rotation; the more speed the higher the volts.

So when you connect a motor to a battery it initially draws a huge current, but as soon as it starts moving the voltage AS SEEN BY THE WINDINGS reduces. So our equation becomes:

I = (Applied Voltage - Back EMF) / R

We can turn this around to tell us the size of the Back EMF: Back EMF = Applied Voltage - (I*R)

So for our Axi 2820/10 running from 12volts at 40amps we can see that the back emf must be:

BE = 12 - (40*.04) = 10.4v

So the windings are ionly seeing 1.6v. If we load the motor for lower currents the back EMF becomes even higher (11.2v at 20A, 11.6v at 10A etc). The back EMF rises until the power developed in the windings is just sufficient to turn the load. It is the rate at which the back EMF rises with rpm that determines the "motor constant" Kv.

Secondlty speed controllers are not, of course, variable resistances. They chop the voltage on and off. At the sort of currents we're talking about the use of resistive current regulators would result in vast amounts of power being dissipated, which would be downright dangerous as well as inefficient! For example if a 12v motor was run at half throttle and 20A the speed controller would be dissipating 6volts and 20A, which works out to 120watts - the power of a jumbo soldering iron! So the controllers use switched volts rather than resistance.

PDR

[Bruce Richards]

Peter, thanks for the explanation of Back EMF I always wondered why out motors took such little current given there low resistance.

[Pete Rieden]

If you follow the logic through it tells you just how a motor works. If you consider an "ideal" motor (with no friction, resistance or magnetic losses) with no load you see that when volts are applied it will spin up until the back-emf is exactly equal to the applied voltage. it will then spin at this speed and draw no current at all. "Real" motors have resistance, magnetic losses and mechanical friction so even with no load they draw a current. The current they actually draw with no load (which is usually given in the specs) actually indicates the build quality, because it is the sum of the friction and magnetic losses.

It also tells you why, unlike IC engines, electric motors won't over-rev even if "shaft run". All they will ever do is reach the speed at which the back-EMF equals the applied voltage. This speed is given by the motor constant (Kv) which, being in rpm/volt, is actually the slope of the back-emf/speed graph.

It also explains why if you want a motor to rev hard (say for high-speed flight) then it must either have high Kv or be driven by a higher voltage.

Lots of information just clicks into place, doesn't it!

 PDR