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Somebody please explain this amp rating thing to me! Empty2019-07-12, 18:53 by Rich

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Saturday 7/7/18

2018-07-07, 21:07 by Gary M Jones

I was at the field today between 14:00 & 15:00 all on my own , good flying too. There is a dead sheep along the fence line towards the gate from the pits, I saw the farmer so reported this to her. I hope no one had plans for a BBQ Smile .

Farmer …

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Somebody please explain this amp rating thing to me!

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Somebody please explain this amp rating thing to me! Empty Somebody please explain this amp rating thing to me!

Post by Stubbsy on 2011-09-03, 13:29

Ok - I thought components amp ratings were definative. If a motor or esc is rated at 20amp for example, at a 20+ amp draw the component gets super hot, super fast and goes bang- end of story!
Problem is im reading of electrical set ups on other forums which by my calculations are pulling far more amps than the components are rated at and yet people are reporting all being well! scratch even suggesting the set up for other people to use!

One example I refer to, is a 40amp esc running a 3550KV motor spinning a 5x5 prop and a 4000mah 3s lipo. If im right, then this set up should be pulling around about the 45amp mark study

Okay, the esc is rated at 40 amps so you might get away with this with loads of cooling, my problem though is that in this set up, the align motor being used is only rated at 28 amps! affraid

What the hell is going on? how is this possibly still flying? The set up in question is being used in a pusher prop configuration aswell, so I cant see the motor getting mega amounts of cooling from airflow? This isnt isolated either, reading of loads of over-amped set ups flying very well!

Basically this brings into question my unicorn set up - 4600mah 3s lipo, 50amp esc, 3800kv motor spinning 6x4 pusher prop. Motor is rated at 37amp - is this one going to go bang? scratch
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Post by Rich on 2011-09-03, 14:41

I'm not 100% sure here correct me if I am wrong but I don't think the motor rating is something I would consider as IMO this is the maximum current the windings of the motor will stand for any period of time, the amount of current a motor draws can only alter by changing the prop size or supplied voltage 2s, 3s etc.

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Post by Tim on 2011-09-03, 22:55

Well its true that its usually Amps that kills motors /escs/ etc.
A motor will fail for one of the usual reasons.
1) Too much current, and the windings overheat and "break"
2) Too high RPM - bearings or magnets let go
3) Magnets weakening...again often through excessive heat.

If we look at 1) in particular, then the ratings are often stated as constant, or burst. Constant means just that - but is also dependant on the honesty or otherwise of the manufacturer /supplier, as well as proper cooling.
Burst is often for just 30 seconds or so...maybe less.
The tolerance to excess power is VERY dependant on the overall quality of the item - cheapy chinese stuff is usually intolerant, and the better quality gear from the likes of Mega, Axi, MVVS, and Hacker and so on can often be pushed beyond the specs.
As Rich says, the current will increase if the prop fitted increases in size or pitch, as the motor has a greater workload, so needs more juice.
Increasing the battery cell count ( Volts ) will increase the current ( and RPM ) a LOT, and may well push it beyond its comfort zone.

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Post by Andy Sayle on 2011-09-05, 16:47

Ben. Stop buggering about with the numbers, and worrying. If you put a 2200kV motor in there with an APC 6x4 prop and a 3s lipo, it will pull around 24Amps or so, and have pleeenty of power for mucking around with it at the field.

If you are dead set on more power, a 2800kV motor and an APX6x4 prop on 3s will pull around 30-35A, and be enough to climb vertically (and make an even bigger hole on the way down).

If you go with the former, don't worry about the cooling, it isn't an issue (mine has over 50 hours on the TX now). If you go with the latter, put a couple of little scoops on the top surface of the motor tube. If you have a mount where the motor is mounted externally, even better.

If you are dead set on going with a motor that would normally be used for EDF or Gearboxes (which a 3800kV motor is by the way) then you are doomed to failure if you use a 6x4 prop. The motor is just going to burn up sooner rather than later (unless the motor is a fecking big heavy one, and you limit full throttle usage to once a month).

If you want to save money in the long run, save up a few quid and buy a Mega 16/15/4, and use that. You will not fit another motor to that Unicorn for as long as you fly it.

Cheers Smile
Andy

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Post by PDQ on 2011-09-05, 16:50

Wise words indeed from Tim and now Andy.

I still have and use Motocalc to check out electric setups.
(Althought some tell me it's a load of buttocks)

However, I did try Motocalc with your proposed set up and it showed it to be unuseable. Far too much current stuffed through such a relatively small motor. Effectively - - - the wrong combination of bits.

Might be worth looking on the overlander web page for their table :-
"Setup guide fo Regular Flying".
Just look up the model weight on the table and there's an answer that will work.

They also show a 3d flyers version you could try if you really want to go mad.
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Post by Andy Sayle on 2011-09-05, 17:02

Motocalc is an excellent tool for having a look-see at what sort of motor/prop/battery works at what sort of currents. It tends to get a bit far from the truth once you start pushing the limits of the motors/cells you are using (just like in this application in fact!)

There is a freebie online version that is cutdown a bit, have a look here:
http://brantuas.com/ezcalc/dma1.asp

Like I said though, that is NOT a mega accurate tool, it is more guidelines.

That overlander guide is pretty useful too, well worth a read.

Oh, and the Mega motor will quite happily take a 7x5 prop on 3s lipo too, which will take the current draw up to around 30A, and give a much increased climbrate (vertical in a Light Unicorn, not quite vertical in my beefcake Unicorn!).


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Post by Stubbsy on 2011-09-05, 17:49

Andy Sayle wrote:Ben. Stop buggering about with the numbers, and worrying. Andy

Just trying to get my head round the whole thing boss thats all!
This whole amp rating and Kv and Watts and motor use and weight and physical dimension balancing is a whole new ball game for me!
Had two costly disasters on the trot and just trying to get my head round the reasons why to make sure I dont make the same mistakes again!

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Post by Andy Sayle on 2011-09-05, 19:29

The best ways to avoid costly mistakes are to listen to those who have made those mistakes before, and buy decent quality stuff in the first place, rather than rubbish that needs replacing often.

If you want to tryy Unicorn on the mega setup, just shout next time you are off down to the field. You are more than welcome so long as you promise not to park it in a tree or bush. Unless that bush belongs to billie piper......


Andy


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Post by Allan Patrick on 2011-09-05, 21:20

The best way of avoiding costly mistakes, in my opinion, is to have a wattmeter and USE IT! Never guess and remember an estimate is a glorified guess! With the plane all installed and ready to go put what would seem a suitable prop on the motor and connect your WATTMETER & battery. Then SLOWLY throttle up watching to see what the readings are. Current is the main one to watch - don't be too bothered about power - current is the killer. As you increase revs do not let the current exceed either the esc or the motor specs. If the current is becoming too great stop and fit a smaller dia or pitch prop. If the current is 80% or more of specs go with flight testing. Less than say 50 or 60% then a larger dia or pitch prop could be called for but if its an overpowered monster anyway then perhaps flight test.
Also! Remember only a few seconds on the bench at full power as the motor & esc will not have cooling.... and will fry. The wash from the prop doesn't count

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Post by Robert Piechulski on 2011-09-05, 21:40

One more thing is that the current when flying will be lower than when testing on the bench, the difference is hard to tell in percentage as it changes along with the prop size, prop pitch and the kV of the motor.
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Post by Stubbsy on 2011-09-06, 05:24

Thanks for the advice chaps!
Incidentally I recently bought a watt meter which clearly showed that the setup of parts I had lying around in the workshop will pull far more amps than the motor is rated for. Im pretty sure that the Chinese special 37amp rated 3500kv motor will go bang turning that prop, so is no good for the set up at all- I just wanted to test the parts I had, before going out and shelling out for more! and Andy, I didnt know that high a kv motor was designed for EDF - now I see where the shed load of cooling would come from!

Andy Sayle wrote:The best ways to avoid costly mistakes are to listen to those who have made those mistakes before, and buy decent quality stuff in the first place, rather than rubbish that needs replacing often.
100% Agreed! Im not questioning the sound advice I have already been given, especially where it comes to motors for this unicorn - im just trying to understand how the whole electrical set up works for my own peace of mind!
My whole issue here (and incidentally the reason for the thread in the first place), is the fact that I see a lot of people running too high a current through their kit and getting away with it-I just dont know how they are doing it thats all! - quality kit or not!
How can anyone run 45+ amps through a 28amp rated piece of kit and be getting away with it?
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Post by Andy Sayle on 2011-09-06, 08:17

If it is a motor, simple, don't let it get hot. Sothat means short duration bursts o full power, and/or really good cooling. If it is an esc, then it is probably luck, or one that is designed and rated very conservatively (eg a Castle one). I it is a battery, then it will be short bursts of power, and good cooling.

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