Why 3s or 4s, Lipo for a particular power output.

[Erfolg]

I am starting a new build.

Normally I have gone for a 3s battery, and then sized the motor to provide the power output wanted from this power source.

However i do now have a number of models which are running 4s batteries.

It is obvious that you can use a smaller frame motor, for a given power output when using a 4s lipo source, as against a 3s.

It is also apparent that the ESC requirement is lower.

Yet I am not clear other than these obvious differences that a higher voltage provides. Other than the propeller size seems to have to reduce.

What is the obvious method to adopt to determine the most appropriate Motor, Lipo, ESC package.

[Martin Harris - Moderator]

A higher cell count, as you say, drops the current required for a given power output which allows more manageable sized cables as well as lower losses due to resistances (of a few milliohms) in each connection. The prop size is more a function of selecting the right kV figure for the model although a simple change in cell count will require a prop change in almost all cases.

With so many variables, EP component selection requires more thought than simply bolting the right prop on a suitable sized engine and there may be several alternative solutions which are equally valid. A good computer application such as MotoCalc is probably the best way forward when designing your own installation.

Edited By Martin Harris on 17/04/2017 17:35:05

[Frank Skilbeck]

It all depends how many watts you want, I typically move over to a 4s set up at around 400 watts to keep the amps at a reasonable level.

[Erfolg]

I am looking for 600-700 watts Frank.

It just seems easier, to get this level of power on a 4s set up. Although I was contemplating that the combined weight of motor and Lipo could well be very similar, with either a 3 or 4s, at a guess.

I am not wanting to go to 6s, as this is a step change in that i would need a new charger and 6s lipos, whereas i have a few 4s, of very similar capacity.

[kc]

Dave Burton did an article on exactly this - choosing 3S or 4S or 6S for 850 watt  - in RCME November 2016.

Edited By kc on 17/04/2017 19:05:20

[Mannyroad]

Ha! Cheers for that info kc, I recall reading something but thought it was further back and couldn't find it. I'm going down the electric route with a Mirus I'm building and am not over-sure if my thinking / purchases is good. Might take a look at that Motocalc software too, but I'm not really that techno-minded so it will probs just go over my head.

[Andy48]

Posted by Erfolg on 17/04/2017 18:54:36:

I am looking for 600-700 watts Frank.

It just seems easier, to get this level of power on a 4s set up. Although I was contemplating that the combined weight of motor and Lipo could well be very similar, with either a 3 or 4s, at a guess.

I am not wanting to go to 6s, as this is a step change in that i would need a new charger and 6s lipos, whereas i have a few 4s, of very similar capacity.

For 6-700 watts you don't really need more than a 4S. Then actual current drawn at maximum will be about 40amps, and a 60amp ESC will give a good safety margin. Using an 800-1000ish kva motor and something like a 12x6 (not critical) you will probably get a static thrust of around 2-2.2kg with a fresh battery. If current draw is higher, simply use a smaller prop.

Incidentally, at that sort of power rating there isn't much of a range in motors that will handle 6S.

[Erfolg]

The factors which resonate with me are those which have raised by the contributors.

• MH, the cable size issue, in that less copper is easier to solder, particularly when the fittings and cables are quite different in their thermal mass.
• A48, that motors are far more readily available at 4s sizes, that 4s Lipo batteries are cost effective, and that I can obtain good quality, low cost ESCs in the 4s range. I do not see many marketed as suitable for 6s.

As for propeller size, I know that the model is designed for a 40 glo, at present I do not know the typical propeller size in this motor range. My only concern is a prop strike with the ground if the radius is excessive.

I will probably go for the 800 kv using a 4s, although I have not done a calc for theoretical speed at the max revs. I am not looking for all out speed, although lively at max will fit with the model.

My opinion that I was moving from the viable range of model for 3s, into the 4s, seems to have some support. My only reservation is, from a theoretical, rather than a pragmatic judgemental basis, I cannot say. It would be interesting to know BEBs reasons for selection. I will have a look to see if I still have the mag, although to maintain marital harmony, I save very few old issues now. I am going out shortly, so the search will come later.

[Allan Bennett]

My 6lb Flair Magnatilla (heavier than normal because of a sound system) flies on 4S LiPo with a 11x7 prop turned by a 760kV AXI 2826/12 motor. It draws 36A for 560 watts at WOT, and cruises at 50% throttle. It was powered by an OS48FS in a previous life.

That gels nicely with your thoughts about your power system Erfolg.

Edited By Allan Bennett on 18/04/2017 10:29:28

[kc]

Erfolg. If you are an RCME subscriber the article ( and all other recent articles) should be available online here.

If you just support your local newsagent like many of us then it's not! Very short sighted that because if we all became subscribers and dont buy from a newsagent it means eventually newsagents won't display RCME and there won't ever be any new readers who find the magazine!

[Chris Walby]

I just like to keep it simple and generally Power = Volts x Amps so if you go from 3S to 4S that's an increase of 25% in voltage, prop speed and power, so you can either prop it down to keep the same power and use less amps or use the increase in power for something else....

With high KV motors dropping prop size (pitch or diameter) is generally not an issue, but extracting high powers out of low KV motors leads into all sorts if problems....don't ask about trying to get 1200w out of a 370KV motor as the prop looks like its fallen off a wind turbine!

Not needed for your application....but two 3S packs in series will give you 6S if its performance you are after.

If you have some 4S packs, work with them, pick a 500 to 1000 KV motor & follow the advice from the others about prop size if the motor manufacture does not give a guide size.... Lastly use a power meter just to check current/power at full throttle to make sure you are not exceeding motor/ESC maximums

Last simple rule...you can always prop up another day, but the smoke of failure is terminal.

[Frank Skilbeck]

Posted by kc on 18/04/2017 12:02:19:

Erfolg. If you are an RCME subscriber the article ( and all other recent articles) should be available online here.

If you just support your local newsagent like many of us then it's not! Very short sighted that because if we all became subscribers and dont buy from a newsagent it means eventually newsagents won't display RCME and there won't ever be any new readers who find the magazine!

kc, you only get the digital back issues if you are a digital subscriber, if you subscribe to the paper copy you can't access the digital issues, been like this for a couple of years now.

Chris - if you want to get 1200W out of a 370kv motor without resorting to a wind turbine, you can always increase the number of cells and up the rpm.

[kc]

Frank, that's what I said! We cannot access the online issues.

[Frank Skilbeck]

Posted by kc on 18/04/2017 14:13:44:

Frank, that's what I said! We cannot access the online issues.

Ah I thought you were referring to those who bought it over the counter and had assumed that those who subscribe to the paper copy got the digital access.

[MattyB]

Posted by Chris Walby on 18/04/2017 12:46:11:

...With high KV motors dropping prop size (pitch or diameter) is generally not an issue, but extracting high powers out of low KV motors leads into all sorts if problems....don't ask about trying to get 1200w out of a 370KV motor as the prop looks like its fallen off a wind turbine!

Not needed for your application....but two 3S packs in series will give you 6S if its performance you are after.

If you have a motor of Kv of lower than ~600 you are going to be looking at 5S at least, more normally 6S (IMO 5S is never a good choice as there is a smaller selection of batteries available, and to use smaller packs in series will require a mix of 2S and 3S batteries). For example on my Miss Wind I use the stock setup to generate 1100-1200W - a 470Kv motor on 6S and a 16x10 or 16x12, pulling (from memory) 55-60A. Try to generate that much power on 4S and you would require a ginormous prop, as well as pulling currents that would require a much larger ESC and be generate much higher heat in your batteries, shortening their life.

Posted by Chris Walby on 18/04/2017 12:46:11:

If you have some 4S packs, work with them, pick a 500 to 1000 KV motor & follow the advice from the others about prop size if the motor manufacture does not give a guide size.... Lastly use a power meter just to check current/power at full throttle to make sure you are not exceeding motor/ESC maximums

This is very good advice - go with it!

[Chris Walby]

Hi MattyB, slightly off topic, but looking back at my test data, the motor is actually 320KV on 6S swinging a 3 blade 16 x 10 prop (just to add load!)....at 48A (max rating 80A). The motor is rated at max 6S so can't go higher

....Now what I really need is a project that is light & slow and I can use my "bargain at the time" motor, suggestions (or is that for another thread!)?

[kc]

Sorry Frank - you are right! I had not noticed the change. You have to be a digital or a Paper Plus Digital subscriber to see online archives. ( 56 pound year rather than 47 pounds paper only but still a better deal than we get from the newsagents at 64.87 for 13 issues )

[Erfolg]

Intuitively we all seem to favour a $s set up for this project.

As I have written previously, others have put forward sound practical reasons, some based on availability of equipment, some relating to specific apects of current flow.

Yet other than a universal gut feel, non of us has put forward a technical reason relating to motor design. It does seem that a 3s motor could produce something like the power i am looking for, albeit with a rather bulky, hence heavy motor. Although the all up weight of a 3s motor and Lipo could be similar to a 4s set up.

We all would say a 6s set up on an indoor model, will not work very well. We all will say that a 2s set up on a 20kg model does not compute.

All I am looking to establish, what is it about motor design and voltage, that pushes our decision making one way or the other.

I am not doubting that for my model that it is in the 4s region. I can see that there will be grey areas which are transitional from say 2s to 3s and to 4s. I want to look beyond my present approach of comparing a lot of motor sizes and performances, to being more targeted and knowing why i am looking for certain criteria. For at present it is all experience based, knowing that if i have got my 3s set up wrong, I just go to 4s, and hey presto, I have moved up a power category, with the same motor.

[PatMc]

Posted by Chris Walby on 18/04/2017 18:33:46:

Hi MattyB, slightly off topic, but looking back at my test data, the motor is actually 320KV on 6S swinging a 3 blade 16 x 10 prop (just to add load!)....at 48A (max rating 80A). The motor is rated at max 6S so can't go higher

....Now what I really need is a project that is light & slow and I can use my "bargain at the time" motor, suggestions (or is that for another thread!)?

Chris, there is no such thing as a max voltage for the motors we use.You only need find a suitable prop + voltage that keeps within the motor's max current limit combo. However your ESC will be have a voltage as well as current limit.

[PatMc]

Erf, have I missed something ? You don't seem to have given data on your projected model or what sort of performance you want from it.
You have said that you want to use existing 3s & 4s batteries & 600 - 700 Watt of power but without details of the model it's not really possible to give any definitive advice. The capacity of your existing batteries & required flight duration (assuming power is required for the full flight) are also not stated.

[MattyB]

Posted by Erfolg on 18/04/2017 22:45:01:

Intuitively we all seem to favour a 4S set up for this project.

As I have written previously, others have put forward sound practical reasons, some based on availability of equipment, some relating to specific apects of current flow.

Yet other than a universal gut feel, non of us has put forward a technical reason relating to motor design. It does seem that a 3s motor could produce something like the power i am looking for, albeit with a rather bulky, hence heavy motor. Although the all up weight of a 3s motor and Lipo could be similar to a 4s set up.

We all would say a 6s set up on an indoor model, will not work very well. We all will say that a 2s set up on a 20kg model does not compute.

All I am looking to establish, what is it about motor design and voltage, that pushes our decision making one way or the other.

This article may help you Erf. I think the key thing to understand here is the relationship between the number of winds (turns) on the motor, Kv and how the two affect the ability to carry current because of the wire gauges used...

"In summary, a low KV motor has more winds of thinner wire - it will carry more volts at less amps, produce higher torque and swing a bigger prop.

That may sound confusing, but compare it with a high KV motor which has less winds of thicker wire which will carry more amps at less volts and spin a smaller prop at high revs."

Edited By MattyB on 19/04/2017 01:38:05

[MattyB]

The above explains why you do not see many low Kv small motors - they are harder to make with very thin wire and can't carry enough current to be useful on lower cell counts (no-one wants to run 6S on an indoor F3P model!). The reason bigger motors tend to have lower Kv is I guess more about limiting max currents and RPMs - it's much more efficient to minimise I squared R losses (heat) in your motor and battery by operating at higher voltages and lower currents. An added bonus is that it's easier to engineer a heavier motor to drive a larger prop if it's turning that prop more slowly. This is important to the budget manufacturers who can produce more affordable large motors that don't require the absolute best quality bearings etc.

Edited By MattyB on 19/04/2017 01:50:34

[Erfolg]

Patmac

I am looking particularly for advice, rather gaining a better understanding, particularly why I am gravitating to higher voltages as my model get bigger.

With the increasing usage of electric power in model aircraft, the choice of equipment option has increased. Dependent on your starting point, you can argue there has been a number of step changes.

Matty

Your explanation of the relationships and the practical implications to motor design, helps a lot.

I was thinking about that the wiring option of star or delta is never mentioned these days. I guess because it does not matter to me the buyer. I just need to know about the motors performance characteristics.

I am guessing that the reason we do not go for a 1,000 volt set up, is that the winding conductor becomes a bar rather than a wire, availability of electronic components in the ESC, and a suitable battery.

In general i am starting the hunt to select a suitable package, which will start this evening after a calc, to find the ball park revs required, using a typical propeller size for the mode, assuming a 4s Lipo (as that is where the runes are pointing).

Out flying shortly.

[Dickw]

Posted by Erfolg on 19/04/2017 11:37:33:.......

I was thinking about that the wiring option of star or delta is never mentioned these days. I guess because it does not matter to me the buyer. I just need to know about the motors performance characteristics.

I am guessing that the reason we do not go for a 1,000 volt set up, is that the winding conductor becomes a bar rather than a wire, availability of electronic components in the ESC, and a suitable battery.

.........

You will see star or delta mentioned on some sites, but as you say it has little real relevance to what we are looking for. Have a look at the Neu motor site and compare kV for the different types and numbers of wind - Y = star and D = delta:-

Neu motors

You wouldn't need a "bar" for a winding in a 1000v set up as you could put lots of wires in parallel (that is a common practice instead of one thick conductor) but you might need to think about the safety implications of a 1000v battery .

Dick

Edited By Dickw on 19/04/2017 13:30:24

[Martin Harris - Moderator]

Posted by Erfolg on 19/04/2017 11:37:33:

I am guessing that the reason we do not go for a 1,000 volt set up, is that the winding conductor becomes a bar rather than a wire, availability of electronic components in the ESC, and a suitable battery.

I can think of a far more important reason why we don't go to 1000V systems while messing around in damp fields! Anything over 60V is considered potentially dangerous (42V AC) although there are ways to kill yourself with less.

A model running a 1000V system would have me running for the nearest Faraday Cage!

Edit - you beat me to it Dick...

Edited By Martin Harris on 19/04/2017 13:40:00