Amp ratings

Quick question, how close can you take your power train to the lowest amp rated component in the chain?
I want to run a max 37amp rated motor at close to 34 amps. I presume this will get pretty warm but will it significantly reduce the life of the motor?
Is there a safety margin I should be observing?

A lot depends on the quality of the gear - a cheapo chines motor rated at say 40 A will probably be pretty stressed at 35A, whereas a quality Mega or similar will happily run at its rated figure all day.
Magnets tend to lose efficiency at high temperatures, and have been known to let go when really hot - again the better motors will have provided for this by wrapping them in kevlar and other such luxury.
34A should be OK..... and TBH they are so relatively cheap these days, its no biggy to replace if and when the time comes.
Also, is this sustained 34A or "burst" ?

The amp ratings are usually pretty vague and unreliable in terms of model components.

Wire: limits are pretty easy to check, just look up the current rating of the wire on common tables on the net (compare conductor dia, against current handling). The easy check is to see if it feels warm. If it is, then you are pushing your luck. Wire should be cool (don't forget that some heat may be conducted from other hot bits, not the wire iheating because of the current flowing through it.

Batteries: Stick to the limits on the packs. The C rating thing is usually a big joke, with numbers plucked form thin air. I try to keep below about 20C regardless of the packs C rating.

ESC: This depends. Cheap chinese rubbish is usually advertised so that the current rating is the absolute maximum limit. European/USA stuff is usally quite conservatively rated. I wouldn't want to get anywhere near the rated current of a chinese ESC, either intermittently, or continuous. I treat the rated currents on euro/USA escs as a continous limit, and the intermittent limit to be double that. For example, I used to run an 80A esc in my Graphite, but was frequently running that at over 100A for 10 seconds or so.

Motor: The current limits here are usually safe maximums. However, motors can be run at any current you want PROVIDED you can keep them cool. It is the heat build up that kills motors. More efficient motors convert less energy to heat, so are easier to keep cool (and therefore generate more power, last longer etc). As an example, the motor in my Graphite2 was rated at 750Watts maximum (continuous). I was running it at 2.2kW (in 10 second bursts). I could get away with this, because I was able to keep the motor cool by having a good airflow over the motor, and only running it intermittently.

For your motor, I would be looking to ensure that there is good airflow over it, and try to manage your throttle wisely. Don't fly around on full beans all the time, shut the motor down for downlines etc etc. Doing that should mean it works fine. But, the thing to do, is to check the motor temperature often on the first few flights, every couple of minutes is good.

My usual rule of thumb for leccy bits, is if I can hold my thumb on the component indefinitely, then it is okay. If it hurts, or burns me, then things are too hot, and are going to break sooner rather than later.

Andy

Cheers for that chaps - working out the components for my Unicorn rebuild. Because the motor tube was melted off first time out, its thrown up all these electrical questions!
The repair is going to consist of an inrunner mounted externally on stick mounts and a self made aluminium bracket. Plenty of cooling airflow this time round!

To answer your question Tim, the set up pulls 34 amps pretty much full beans, and as its a 3500kv motor turning a 6x4 prop - it should go like stink!, I cant see it being laid flat out for any more than 15 - 20 second bursts to be honest, but as you say, the components are cheap enough!
Just dont fancy trying to land a screaming fireball thats all!

Hmmm, Are you sure about that 34A reading for that motor/prop combo? 3500kV should pull a fair bit more current than 34Amps.

The 2200Kv motor in mine is pulling 24 amps on a 6x4 (3s), and the 2650kV motor in my Green thing pulls about 30Amps on a 6x4 prop (3s). a 3500kV motor would be up around the 45-50amps if I have done my sums right?

Andy

Its what my new fandangled watt meter says - but granted the reading was taken 7/8ths of full throttle!
Will test again when the bracket is built and the motor is mounted properly, rather than hand held - I wasnt overly happy holding it back

To be honest even with the extra cooling 34 amps is a bit close to the max limits for my liking (once bitten twice shy and all that!) so will probably drop the prop size anyway!

Will keep you posted - hoping the aluminium will arrive tomorrow so can get the bracket built!

Lots of other bits but No bloody Aluminium this morning Grrrrrrrr..........

how many lipo's you usin ?

what aluminium do you need ? I have several sorts here.

Gary wrote:how many lipo's you usin ?

2 x 3s 2300mah wired in parallel!

Tim wrote:what aluminium do you need ? I have several sorts here.

Wanted some 10mm box section but couldnt get any anywhere - got some 10mm U channel coming instead, but if you have about 12 - 18inches of 10mm box section I will snatch your hand off!

Andy -clamped motor down and fired it up again this evening - definately pulling over 40 amps towards the top end of the throttle,so will have to drop the prop size big style or fit a lower kv motor! Hmmmmmmm.....

I've got quite a selection of small props for high rpm use if you want to try some out. Ranging from 4.1x4.1 to 6x6 and load in between. I'll have them with me tomorrow night at the field

Andy

Sorry, U channel , flat strip, round tube and bar.... but no box section.

Hmmm Box section. You tried B&Q? they have a section with a load of (rather overpriced) metal and plastic sections. I'm sure they have box section in there. If I get chance tomorrow afternoon, I'll have a quick look in B&Q in Chester...

Andy

Cheers for the thought Tim!

None in B&Q Rhyl Andy (although I wasnt really expecting any) - hadnt thought of Chester though! Im going there on Saturday so will have a look if I cant get this U channel to work - dont go out of your way bud.

Will take you up on the loan of your props for testing though cheers! will save me a small fortune till I find the right one!

See you later!

U channel has worked a treat! Second maiden on the cards!