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

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MattyB19/04/2017 01:36:57
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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

MattyB19/04/2017 01:49:15
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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

Erfolg19/04/2017 11:37:33
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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.

Dickw19/04/2017 13:29:17
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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 smiley.

Dick

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

Martin Harris19/04/2017 13:38:05
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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

Nigel R19/04/2017 15:24:54
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Martin, the figures I have seen (and it varies depending on whose analysis you look at) suggest prolonged contact (large fraction of a second) with over 40V DC to be "problematic". How problematic, depends of course on conditions and where the contact is.

40V is "only" a 10S setup.

Denis Watkins19/04/2017 17:11:08
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My comments often cause a furoar on electrics, but here we go

In a previous life before retirement, my local authority only allowed students to use a maximum of a PP3 battery in their work and the room sockets were set to 100mAh "trip" during experiments

This was due to the sliding scale of conditions for electrocution, as has already been said, the conditions do alter the outcome vastly.

In the correct conditions 100 to 200mah sustained supply can cause death

Erfolg19/04/2017 17:20:03
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Guys

I was not really suggesting 1000V. It is a technique picked up during my acdemic studies. Identify those components which are significant. Particularly if there is a power invovled ie x^2 or maybe x^1/2. and consider where the argument takes you. In some cases it becomes apparent that there is a limit, beyond which you cannot go.smiley I was not serious, honest.

PatMc19/04/2017 17:53:13
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Denis & Nigel, telephone engineers have routinely handled non-insulated 50V DC without taking any precautions for probably more than a century. They've also often been in contact with AC ring current (IIRC 80v) on top of the 50v DC.

Switchboard operators would also have had regular contact with 50 DC unless they were operating cordless systems.

Andy4819/04/2017 18:16:51
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Erfolg, it may help if you give us a guide as to how heavy your plane is going to be . Saying it is for a "40 glo" is not a great deal of help, especially for those of us that only fly electric. I would also suggest the characteristics of each are very different in actual use. You've already said you're looking for a decent turn of speed but not super fast.

You also need to consider balance. An electric motor is generally lighter, but with the ESC and battery the weight should be reasonably similar. You want a battery that basically will give a decent flight time, but balance the plane without adding any weights in the nose or tail. Its no point in having a super large capacity battery with a church roof in the tail when the battery is only half discharged at the end of the flight. Usually balancing the plane involves having the motor and battery as far forward as possible, and generally placing the battery where the fuel tank was leads to a rather inelegant design where the battery is fiddly to get in and out.

Erfolg21/04/2017 17:46:36
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Andy

What I was looking to obtain was a better understanding of the balance of all the factors in the design of the electric power train. MattyB, DickW, and others has helpt a lot with respect to the motor voltage and a few other factors.

DickW, highlighted one of the aspects that has bothered me for some time. That is motor winds/turns, often I have seen, for example, an 8 turn winding. Yet an examination of the armature, provides the visual impression that there are considerable more winds than the stated 8. Now I know that it is often because each winding can contain multiple strands. It is almost embarrassing, because in my youth i often saw, coils, motors being wound, yet never enquired enough to really understand what was being done and why.

Any way, back to my decision making. Yesterday evening i found the time to do a quick calc, on the basis that I would use a 4s battery of approx 14v. A standard, commonly available prop with a 6" pitch. Using the relationship of

  • Speed =revs * pitch, which can be better expressed as S = rev per volt *V * Pitch/1056, assuming a 1000kv motor on 4s is approx 70 mph max.
  • Just picking a typical speed of say 60 mph, again using the same expression which now looks like Kv = S *1056 / pitch * 14, provides a value pretty much 750 Kv.

You have convinced me that 4S is the way to go and that a motor of 750 to 1000 Kv will provide the necessary speed. Having decided that the model needs something in the region of +500w. The search can now tentatively begin.

Thank you all for your inputs.smiley

Now if I insulate myself, I could perhaps do over 400 Kv, just like BICC jointers used to do, on live circuits.angel 2

kc21/04/2017 19:18:34
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Erfolg perhaps a setup that works well would help your choice. I am using a Turnigy 3536/9 910kv with a 4S3000 and an APC Electric 11 by 5.5 this produces a measured 500 watts at 33amps. 60 ampESC Gives 6 minutes of full throttle flying with an Avicraft Moronic ( think of it as similar to a balsa 52inch Wot4) with good vertical performance on this model whih weighs 4 pounds 1 ounce. So far this has done 1060 flights in just over 3 years. A proven formula I reckon which would fly any similar model too. Note that I was careful to extend the nose of the model enough that the lipo went well forward so no lead was used. Hence weight of 4 pounds 1 oz.

Edited By kc on 21/04/2017 19:19:30

Edited By kc on 21/04/2017 19:19:57

Geoff Sleath21/04/2017 20:49:42
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During and just after the war my parents had a family business charging 2v lead acid cells used for battery radios (a lot of properties in Eastwood at the time had mains gas but not mains electricity). I can remember as a pre-school age child seeing this huge bench covered in nominally discharged cells being connected together in series/ parallel (I assume) to be recharged over night (I can still recall the acid smell). My mother was able to withstand the voltage as she connected bare copper wire to the terminals much better than my father because she had dry skin. My dad told me that he used to get quite a sharp shock when he took over when she left to put me to bed and put it down to her naturally dry skin. I would think the voltage would be about 100v - DC of course.

When I was a teenager I worked in the service department at Murphy radio where I regularly got shocks at 7 kv from TV eht supplies. The cathode ray tube eht was derived from a high frequency oscillator and had a very high source resistance thus it dropped very quickly. Still made you jump though.

Dennis Watkins: There's an old saying that it's the volts that jolts but it's the mils that kills.

When I think we used to use things we actually called death traps to connect bare mains leads (ie without a mains plug) to the mains. They were lengths of mains lead with plug at one end and croc clips insulated with rubber gas tubing at the other Plus the fact that a lot of radio aluminium chassis were live (ie connected directly to the mains on one side - ideally neutral) which meant that if you had two receivers on the bench at the same time you could have full mains potential between two aluminium chassis, it's surprising there weren't more accidents - and that applied at college where were being taught how to mend things as well as work.

I've used 4S packs in the 1.5 metre ws and up glow to electric conversions I've done and find it works well. The 72" ws (1/4 scale) Percival Mew Gull will have a 6S pack. I like higher voltage low current set ups because I feel things are happier that way. In any case the voltages are still quite modest and the lower current means the esc MOSFETs are working less.

Geoff

PatMc21/04/2017 20:49:54
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Erf, your calculations seem to be based on the applied voltage x Kv. The Kv is the rpm per volt only when the motor is unloaded. In practice a the rpm is about 80% of V x Kv when the motor is under normal load.

Your first calculation using a Kv of 1000 would give a pitch speed of about 63mph.

Your second looking for 60mph pitch speed would need a motor of about 940Kv

Erfolg21/04/2017 23:01:13
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Patmac

I consider all of these type of calcs as indicative and very optimistic.

i know for instance that the prop is much less efficient than the pitch suggests and so on.

Its all about ballparks.

kc22/04/2017 18:00:32
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Re shocks from DC. Anyone watching the last F1 Grand prix might have seen a car up on stands with fences all around and a flashing amber warning light all to keep their own mechanics away whilst the hybrid system of the car was "live" and not yet safe to work on! I don't think they say what voltages are used or give much technical info away so we don't know why this should be so dangerous that they need to keep mechanics from even touching the bodywork. In a sport where the mechnics get right next cars driving through the pitlane whilst in the race this seemed either extreme 'elf& safety' precautions or perhaps there is some greater danger from batteries.

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