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Extending leads - battery or ESC?

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ericrw03/04/2012 13:28:47
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I'm converting a i/c Piper Cub to electric. I've got be able to either extend the battery leads or the ESC leads. Which should it be and at what length would the maximum be ? Hope someone can advise !

Steve Hargreaves - Moderator03/04/2012 13:36:40
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Aeromodeling lore tells us to extend the motor wires. Apparently the back EMF caused by the rapid switching can damage the input capacitors if the input leads are too long. Personally I haven't had a problem with slightly extended (3-4 inches) input wires.

Maximum length? No idea but remember that wire does have resistance so making them longer will add extra power sapping resistance. How long might you need them? I can't see why you would need to extend them massively in a Cub....

ericrw03/04/2012 13:46:37
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Hi Steve, The lenght will be about 3.5", I will not be able to extend the motor wires ,it will have to be either battery or ESC. The joy of converting !!!

Frank Skilbeck03/04/2012 13:59:27
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As Steve says it's best to extend the wires between the Motor and ESC, but adding 3.5" between the ESC and battery should be an issue either.

I am surprised on converting a Piper Cub though i would have thought the best place for the battery (c of g wise) would be in the tank bay.

Codename-John03/04/2012 13:59:31
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the esc output side steve means by motor wires eric, you can either extend the 3 attached to the motor or the 3 cables from the esc to the motor just not the 2 to the battery, 3.5 inches will be ok as the resistance will be next to nothing as long as the joints are good

Allan Bowker03/04/2012 14:07:36
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I have a few aircraft with extended wires from the battery to the ESC with no issues.

Myron Beaumont03/04/2012 14:17:59
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It might seem like a stupid point but being as electriciy travels at or nearly the speed of light .How come 3" will make a scrap of difference to either of the connections even though the current directional fllow is very rapid .? Havn't worked out the actual figures ,but at 186,000 miles per second & even allowing a few magnetic poles to reverse direction at extemely high rates ,I can't see that 3" could be a problem . Please enlighten me someone! .Being an IC man ,I have difficulty understanding how a cycle of burning fuel can happen in such a minute fraction of time but we all know that it does happen -About 300 times every second & of course electricity travels so much faster than a fuel mixture

One for Erfolg maybe or BEB ?

Steve Hargreaves - Moderator03/04/2012 14:19:28
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Ah..OK slightly at crossed purposes Eric.....it makes no difference if you extend the ESC wires that lead to the battery or the battery wires that lead to the ESC.....the alledgedly critical thing is the overall length of the wire run between battery o/p terminals & where it joins the ESC board.

As I said in my earlier post aeromodelling lore tells us it is preferable to extend the three wires between the ESC & the motor......again it matters not if you extend the wires from the motor or the wires from the ESC

But as the other lads have said 3.5" added to any wires will be just fine I'm sure....make a good job of the solder joint & sleeve with heatshrink for a neat job....thumbs up

ericrw03/04/2012 14:22:36
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I've gone in from the bottom of the Piper Cub to place the battery as it makes it possible to get to the battery without removing the wings.

Steve Hargreaves - Moderator03/04/2012 14:28:27
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Well I'm not an expert Myron but my understanding goes like this.....

The ESC is effectively a high speed switch....(your 300 times a second for an I.C. engine (are you sure? That's 18,000rpm!!!) looks a bit puny compared with the 8,000 times a second a typical ESC will run at!!smile o).....everytime the switch turns off you get a back EMF spike...a bit like water hammer when you turn a tap off too rapidly....to absorb these spikes the ESC has a capacitor or two across the input side.....extending the wires can lead to the voltage of these spikes getting too high for the capacitors to cope with & they can then fail.....

Thats the theory as far as I understand it anyway...

Martin Harris03/04/2012 14:36:09
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Posted by Myron Beaumont on 03/04/2012 14:17:59:

It might seem like a stupid point but being as electriciy travels at or nearly the speed of light .How come 3" will make a scrap of difference to either of the connections even though the current directional fllow is very rapid .? Havn't worked out the actual figures ,but at 186,000 miles per second & even allowing a few magnetic poles to reverse direction at extemely high rates ,I can't see that 3" could be a problem . Please enlighten me someone! .Being an IC man ,I have difficulty understanding how a cycle of burning fuel can happen in such a minute fraction of time but we all know that it does happen -About 300 times every second & of course electricity travels so much faster than a fuel mixture

One for Erfolg maybe or BEB ?

I suspect it could be to do with capacitance/induction effects between the wires at the very high switching frequencies (if there is actually a problem) causing spikes. I suppose that the thing to avoid, therefore, would be running the extended wires together - separating them as much as possible might be a good precaution?

It may explain why all the ESCs I've come acoss have separate wires on the feed where a neater solution would be a twin cable...

 

Sorry for some duplication Steve - I was interrupted by work during composition!

Edited By Martin Harris on 03/04/2012 14:39:02

Myron Beaumont03/04/2012 14:45:07
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Steve

Yep ,I think you've put me in perspective .!8000 rpm 'ish is correct for a top rev of an ic engine (I did pick the top type of figure on purpose ) .So-Next question about this back EMF Isn't the time interval dependant on the capacitor of which you speak . I know a little about electronics but still cannot convince myself that 3" or so on any wire on any circuit can make that much difference .Interesting subject isn't it ? I suppose the inductance or what ever in any wire connection even in various directions has some sort of effect ?

Steve Hargreaves - Moderator03/04/2012 15:02:08
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No problem Martin...happens to me all the time.......teeth 2

Myron, yes I'm sure it is but I've no idea of values or anything like that. A pair of wires will have a capacitance & an inductance but again what these values might be I haven't a clue.... As I & several other posters have observed a 3" extension is unlikely to be a problem however...

As I mentioned earlier the not extending the battery to ESC wires is part of aeromodelling lore & since I don't know better I follow it as best practice but that said I've never met or heard of someone burning out an ESC because his input wires are too long.....

Myron Beaumont03/04/2012 15:17:59
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At the end of the day then ,an extra 3" doesn't make much difference OOh 'er missus (not my expression-learnt it off the forum  

Frank Skilbeck03/04/2012 15:53:02
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Posted by Steve Hargreaves - Moderator on 03/04/2012 15:02:08:

No problem Martin...happens to me all the time.......teeth 2

Myron, yes I'm sure it is but I've no idea of values or anything like that. A pair of wires will have a capacitance & an inductance but again what these values might be I haven't a clue.... As I & several other posters have observed a 3" extension is unlikely to be a problem however...

As I mentioned earlier the not extending the battery to ESC wires is part of aeromodelling lore & since I don't know better I follow it as best practice but that said I've never met or heard of someone burning out an ESC because his input wires are too long.....

Apparently it's the input capacitors that break down, I have seen some artilces where these have been replaced with larger units where long wires to the battery were required. On the ESC to motor end it's not too critical as the windings in the motor count as part of the overall wire length so changing the length between the ESC and motor has negligible difference.

Steve Hargreaves - Moderator03/04/2012 16:03:02
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I think you are right Frank but what I've never understood is that there is a very very big capacitor on the input side called a battery......I wonder why this can't absorb the spikes!!!

But I'm beyond my level of understanding here so maybe I should shut up (everyone breathes a huge sigh of relief...thinking)

Andrew76703/04/2012 16:14:53
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Posted by Steve Hargreaves - Moderator on 03/04/2012 16:03:02:

I think you are right Frank but what I've never understood is that there is a very very big capacitor on the input side called a battery......I wonder why this can't absorb the spikes!!!

But I'm beyond my level of understanding here so maybe I should shut up (everyone breathes a huge sigh of relief...thinking)

No..No Steve please keep going as this is very interesting....It just struck me, what else could you do other than extend batt. to esc wires in say a brushless conversion of a Twinstar where the esc is in the nacelle and the battery in the fuselage?.

Andrew

ericrw03/04/2012 16:21:00
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Talk about opening a can of worms!!!!

Andrew76703/04/2012 16:22:35
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You started it Eric!!..smile p

Dave W03/04/2012 16:31:53
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I'm going to let another worm out of the can!

If it is considered not a good thing to lengthen the wires between the battery and the ESC because of possible damage to the ESC's input capacitors, what effect will putting a watt meter in this lead have?

This introduces more wire and some circuitry.

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