Constant current

[Simon Kirk]

I'm trying to get my head around choosing motors etc for a few projects iv got lined up.
I'm a complete newbie when it comes to the whole picking motors props and esc.
I understand you need to pick an esc with 10 to 20 percent higher amp rating than the motors maximum amp, however iv also read about constant current playing a big part as obviously you dint want to be running your motor at its max current constantly.
And iv been using webocalc to try dial in correct props and motors for my super cub (already got a power set lined up from someone in my club but just using it as a theoretical practoce)

The trouble is all the motors I'm looking at on HK only list the max current, not constant.
Is there some way of working this out using maths?

Sorry I'm a complete novice when it comes to all this and iv read the tutorial on rcgroups but it doesn't mention finding constant current, just that you need to know it!

Thanks

Edited By Simon Kirk on 15/09/2016 08:29:54

[Frank Skilbeck]

It's the cooling that is important, if the motor is really well cooled then running it at the max constant amps wouldn't be a problem, but if it's tightly cowled with a large non-vented spinner then anything other than short bursts would see it overheating.

Plus with you Cub you won't be flying it round at full throttle so the continuous amps will be much less.

So use the same rules as ESC i.e. 10 to 20% headroom and you won't go wrong.

[Denis Watkins]

Just to move you on Simon, about current, this starts with your lipo. For example, a 2000 ma 3 cell lipo, maybe rated at 40C bursts, that is 40 x 2000ma = 80 amps, bursts, so that's 80 amp max.

The ESC then should be chosen to embrace that current, yet that current should seldom be reached with the correct prop

This is just a starter advice

[Frank Skilbeck]

Dennis I've never done that, I always size my ESC on the load rather than the capability of the Lipo, if the prop/motor combination can only pull 30amps then I'll fit a 40 amp ESC regardless of the lipo size and C rating. On your basis my 8s model using 5,000 mah 40C batteries should be using a 200 amp ESC rather than the 75 amp one it has to deliver the required 65 amps at full throttle.

The benefit in using higher C rated batteries is the max current, but the fact that the voltage doesn't sag as much under load due to the lower internal resistance.

[Andy48]

1. There will be no such thing as constant current. As soon as the motor draws any significant current, the battery voltage will drop which will in turn reduce the current draw.

2. The actual current draw depends on the prop size being used, the only way to check is to use a watt meter.

3. Electric systems don't fare well on full power. The throttle stick position is probably the only way you'll know how fast the plane is going, as the motor is quiet so its very easy to throw the plane about on full power all the time. However there are a number of factors which work against this. Firstly drag is proportional to the square of speed, so drag increases rapidly. Secondly the motor will try and overcome this drag by putting more power in, however the electric motor prop combination is less efficient at full power. The higher current draw will reduce the battery life considerably. Thus running flat out will give about a third the flying time of half throttle with little noticeable speed difference. Put a bigger battery in, then more weight to lug around, and with it more drag..... its all a compromise.

[Simon Kirk]

So basically when doing the maths for probably size etc I should Base it on around 80% of the motors max amps?

[PatMc]

I usualy choose the a motor based on the weight that will help achieve the correct cg, then the Kv that will suit the size prop for the model. For most models this means that the motor will be running well within it's current limit at the required power level.

For sports models I estimate the battery capacity to give about 6 minutes at WOT. With normal throttle management this usually equates to around 10+minutes flight duration without draining the battery overmuch.
Models, such as E-glider that don't need such long power on periods I go for a lower battery capacity - up to around 4 mins WOT. That's enough for 6 - 8 of 30sec power bursts between glide phases per charge.

[Geoff S]

An older member a our club (even older than I am!) has been having some difficulties with his flying and some of us have persuaded him to go electric as it's far less hassle. With a little help he flew his new Wot4e last Tuesday and really enjoyed stooging around for 7 or 8 minutes quite slowly and steadily. I checked his battery (2200 mAH 3S) afterwards and he'd used about 25% of the capacity. It just demonstrated how energy consumption is so variable and tied completely to how hard you fly (after 7 or 8 minute I have about 15% left in my similar models).

Dennis:, esc capability is based on load not the potential capability of the energy source. The new 6S pack for my Mew Gull could deliver well over 100 amps but as my motor max current is only 60 I've settled for an 80 amp esc.

Simon: You've got it the wrong way round. The max motor current should be about 80% of the max esc current spec, as I indicate above.

Time and cooling of the components is also a factor (eg you could draw infinite current for an infintessimally short time and still not damage anything) but assuming you exercise normal throttle control, the maximum current will only be drawn for very short times. A lot of specs state time for the peak current should not exceed 60 seconds; that's a very long time and even a full throttle vertical will be for just a few seconds. Most models would be out of sight in a minute!

Geoff

[Simon Kirk]

I understand 80 percent max of the esc.

But what I mean is how do I know the constant current?

There's so much technical talk when it comes to motors and not much idiot proof newbie information.

[Simon Kirk]

OK assuming iv worked it all out right this is what webocalc has told me



Flies Like: Park Flyer/ Basic Glow Trainer.
Power Level:
(with white highlighted prop) Excellent. Unlimited vertical. Can hover.
Thrust/weight about 1.3:1
Minimum Pilot Skill Needed: Basic Intermediate.
Minimum Flying Field Size: 810 x 580 feet.
Minimum Battery Capacity: 650 mAh, 20 C, lithium polymer.
Suggested ESC Rating: 14 A to 16 A.
Power Into / Out of Motor: 131.8 watts in / 98.8 watts out.
Power To Weight Ratio: 84.33 watts/pound.
Estimated Stall Speed: 15.8 mph.
Wing Loading: 10.43 oz/square foot.
Cubic Wing Loading: 5.35 oz/cubic foot.


With an apc sf 10x7 prop

Could anyone link me to a motor that would fit this description?

Or even tell me what I'm looking for in a motor to fit the bill?

I'm starting to understand it better from playing with webocalc but imy still not sure what I'm looking for other than the kv

[Simon Kirk]

Also il be using my 1300mah 20c 3s lipos to begin with

[Attilio Rausse]

Hi Simon - Your question about constant current refers to the maximum current the motor can stand for an indefinite time - in practice the current varies with the position of your throttle stick, the max current refers to the max current the motor will stand for 20~30 secs before the wires melt etc.

If you let us know what the model flying weight is we can advise better.

Til

[Simon Kirk]

The weight is 25 ounce mate. It's a hobbyzone super cub

[Simon Chaddock]

Simon

My advice is don't skimp on the battery capacity.

Never mind the batteries maximum discharge rate, most tend to be optimistic anyway. Also any LiPo has the capability to "fry" almost anything (including itself!) if you short circuit it.

What matters is the rate at with the motor/prop combination draws the current from it. The 'calculators' give you a good idea of what the figure is likely to be but only way of knowing for sure is with a Watt meter.

For a 25 oz (1.5 lbs) plane you ought to looking at 100 W/lb (but it will cruise on a lot less than that) so you need 150 W.

For a 3s LiPo that means you will be drawing close to 15 A.

As an example

A 1500 mAh battery can provide 15 A (a 10C rate) for 6 minutes if you take out all the capacity, which not recommended, but you should get 10 minutes 'gentle' flying.

So I would suggest a 1500 mAh battery for your Super Cub - Hobbyzone suggest a 1300 mAh.

I hope this helps.

Edited By Simon Chaddock on 15/09/2016 15:24:17

[Peter Beeney]

Simon,

You may care to check out this for size, actually this motor does come with a few different kV rating so you may be able to fine tune if you wish. This result was the result of a quick Google and a spot of substitution, took all of five minutes.

I’d absolutely agree with PatMc too, as far as starting from scratch is concerned. Very straightforward, the result of a lot of practical experience, I reckon.

Why not make up a few hypothetical model examples and then find suitable power trains? Checking out some of the advertised planes and comparing which various items, batteries, ESCs and motors etc., are used might be a starting point.

Good Luck!

PB

Edited By Peter Beeney on 15/09/2016 15:25:19

[Attilio Rausse]

Hi Simon - lots of choice really look at this one, use 10x5 prop and measure current - good for 20A use 25A ESC you need to buy/borrow a wattmeter to ensure that you do not exceed 20A.

http://www.hobbyking.com/hobbyking/store/__28675__Turnigy_D2830_11_1000kv_Brushless_Motor_UK_Warehouse_.html

[Frank Skilbeck]

Posted by Peter Beeney on 15/09/2016 15:24:12:

Simon,

You may care to check out this for size, actually this motor does come with a few different kV rating so you may be able to fine tune if you wish. This result was the result of a quick Google and a spot of substitution, took all of five minutes.

Edited By Peter Beeney on 15/09/2016 15:25:19

Peter that motor spec has probably answered Simons original question as it quotes max current for 15 secs (28A) and max current (22A), so max current for this motor is the max continuous current it can take. What Simon wanted to know was if he's sizing a motor for a plane and the estimated amps is say 20A then is a motor rated for 20 amps OK or should he get one with a slightly higher amp rating like you would with an ESC.

[Simon Kirk]

No what I meant was frank, on hobbyking they only list the MAX amps, but not continuous.
However when using webocalc it was saying what rating I needed for the motor.

So am I right in thinking I should get a motor slightly above the amps that webocalc states, and then obviously an esc with 20% high amps?

And then obviously bench tested to make sure it's well within my batteries limits?

[MattyB]

I never worry about the constant current rating of the motor TBH, as motor stats on websites are notoriously unreliable. Instead with cheap motors in a standard tractor prop installation with reasonable cooling I just work on the 3W/g rule of thumb. That means if you want to generate 300W go for a 100g or higher motor, 600W = 200g etc. Once you have chosen your motor calculate the rough current draw in an online calculator using the prop and motor Kv. Add 20% or 10A (whichever is lower) for your ESC rating and you should be golden.

[Peter Beeney]

Frank, yes, indeed, I only came up with that motor because it’s possibly one candidate as a direct replacement for the 480 brushed lump in the hobbyzone super cub. There are a couple of alternatives, an 850 kV version which has a max current of 28 amps but no time limit on it. Interestingly on 3 cells it gives a thrust figure of 1140 grams at 19.5A on a 11 x 4.7 prop. The 1320 kV flavoured can, max current 28A again, gives a thrust of 1310 grams at 25.2 amps on a 9 x 5 prop. The 1020 doesn’t have any similar data though, just 10 x 7 ~ 12 x 6 prop sizes. !0 x 7 on three cells at a guess. But it does state specifically ‘Suitable for sport and scale airplanes weighing 20 to 30 ounces’. So for general sport flying with a hobbyzone super cub I think I might like to place this as a strong contender.

With the greatest respect, I must admit I didn’t really understand Simon’s OP and even after a second scan I’m afraid I’m still rather unsure. But I tend to tinker about with this stuff a bit sideways anyway, for instance I might check the motor current with a contact thermometer, as in if it’s getting too hot the current is too high; and if the max current is given as xx amps and there is no qualifying time limit then I’d consider that is most likely going to mean xx amps rated for an indefinite time; or to re-phrase perhaps, max amps are constant amps. Maybe the trusty thermometer might come into play here again, too, t’would soon prove the issue. But I’m not going there much anyway, as I’m always looking to reduce the amps if possible; I’ve never been a great believer in ‘the more amps the better’. But still maintaining or bettering the same performance, of course. As I’ve said before, I get the tacho out most frequently*!?; nowadays; this gives me the best clues… but that’s just me.

Lots of ways to get confused on this electrickerty topic…

PB

[Simon Kirk]

OK let me rephrase my original post.

After playing some more with webocalc I have come up with thease results


Flies Like: Park Flyer/ Basic Glow Trainer.
Power Level:
(with white highlighted prop) Excellent. Unlimited vertical. Can hover.
Thrust/weight about 1.3:1
Minimum Pilot Skill Needed: Basic Intermediate.
Minimum Flying Field Size: 810 x 580 feet.
Minimum Battery Capacity: 650 mAh, 20 C, lithium polymer.
Suggested ESC Rating: 14 A to 16 A.
Power Into / Out of Motor: 131.8 watts in / 98.8 watts out.
Power To Weight Ratio: 84.33 watts/pound.
Estimated Stall Speed: 15.8 mph.
Wing Loading: 10.43 oz/square foot.
Cubic Wing Loading: 5.35 oz/cubic foot.

And in the "desire current" area it says 12.6amps
710kv motor.

So let's say for example I find a 710kv motor but it's rated at 20amps, can I use that motor safely? Or will that affect the whole balance?

I figured as long as the amps aren't LOWER then it's OK as the motor will only draw what it needs?

Also obviously I understand it need 20 percent extra on the esc and the relevant discharge rate on the battery....

Edited By Simon Kirk on 16/09/2016 06:20:30

[Peter Beeney]

Simon,

Apologies again, but now I think I’m getting your drift. Certainly in the case you quote the 20 amp motor should be fine. The current will be dictated by the applied voltage, in your case with 3 cells 12.6V fully charged down toward 9 volts when getting flat, and the size of the prop you fit. A bigger prop, both in diameter and pitch will increase the current flow; but not always the required performance.

You said, ‘I figured as long as the amps aren't LOWER then it's OK as the motor will only draw what it needs?’. If I think what you might possibly mean is ‘I figured as long as the motor rating isn’t LOWER then it's OK as the motor will only draw what it needs?’ then that’s exactly so.

An even larger motor would still run ok, but it might soon be a case of overkill, in size, weight and cost.

In general terms, if you keep the maximum current flow of your power train below the maximum current rating of the motor you should be ok.

I suspect that in the past some problems have arisen because the manufacturer might get a bit optimistic with his power output figures, plus the fact that that some modellers have also gone a bit over the top with their expectations of performance.

I’ve never consulted any of the webcalc type info. charts and it has been said by others that although it may be a good starting point the theoretical figures don’t always meet the practical experience right in the middle and some adjustments may/will be needed; I’d go along with that every time.

Be prepared to play around with it until you get the result you want. Makes it easier for the second time.

Happy Landings…

PB

[Simon Kirk]

Thanks Peter it makes much more sense now! I'm going to keep playing round with it and invest in a watt meter and go from there

[Frank Skilbeck]

You should be good to go, note the general point about oversizing an ESC is because some of the manufacturers quote the max current for ESCs when they are operated with optimum cooling, not something we usually have, so we go for a higher rated one which because it's bigger it has a bigger heatsink.

[Simon Kirk]

Yeah I totally understand that bit mate, it just the relevance of the amps..... because webocalc was suggesting 12.6 amps I was looking for a motor with 12.6, however now I know as long as it's not below 12.6amps (preferably a little extra) it'll be fine