ericrw Posted April 3, 2012 Share Posted April 3, 2012 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 ! Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 3, 2012 Share Posted April 3, 2012 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.... Quote Link to comment Share on other sites More sharing options...
ericrw Posted April 3, 2012 Author Share Posted April 3, 2012 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 !!! Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted April 3, 2012 Share Posted April 3, 2012 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. Quote Link to comment Share on other sites More sharing options...
Codename-John Posted April 3, 2012 Share Posted April 3, 2012 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 Quote Link to comment Share on other sites More sharing options...
Allan Bowker Posted April 3, 2012 Share Posted April 3, 2012 I have a few aircraft with extended wires from the battery to the ESC with no issues. Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted April 3, 2012 Share Posted April 3, 2012 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 ? Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 3, 2012 Share Posted April 3, 2012 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.... Quote Link to comment Share on other sites More sharing options...
ericrw Posted April 3, 2012 Author Share Posted April 3, 2012 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. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 3, 2012 Share Posted April 3, 2012 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!!).....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... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 3, 2012 Share Posted April 3, 2012 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 Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted April 3, 2012 Share Posted April 3, 2012 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 ? Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 3, 2012 Share Posted April 3, 2012 No problem Martin...happens to me all the time....... 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..... Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted April 3, 2012 Share Posted April 3, 2012 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 Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted April 3, 2012 Share Posted April 3, 2012 Posted by Steve Hargreaves - Moderator on 03/04/2012 15:02:08: No problem Martin...happens to me all the time....... 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. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 3, 2012 Share Posted April 3, 2012 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...) Quote Link to comment Share on other sites More sharing options...
Andrew767 Posted April 3, 2012 Share Posted April 3, 2012 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...) 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 Quote Link to comment Share on other sites More sharing options...
ericrw Posted April 3, 2012 Author Share Posted April 3, 2012 Talk about opening a can of worms!!!! Quote Link to comment Share on other sites More sharing options...
Andrew767 Posted April 3, 2012 Share Posted April 3, 2012 You started it Eric!!.. Quote Link to comment Share on other sites More sharing options...
Dave W Posted April 3, 2012 Share Posted April 3, 2012 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. Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted April 3, 2012 Share Posted April 3, 2012 Are these capacitors something to do with"smoothing " perhaps ,and why are they not adjustable to wire length if that is so important .Does anyone know the answers to what has become a very interesting subject .By the way I used to repair TV's etc so you can't pull the wool over my eyes .C'mon someone -How about a detailed explanation as to what's going on at these hyper velocity electronic impulses levels? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 3, 2012 Share Posted April 3, 2012 That, I'm told by my tame electric equipment torturer (F5b flyer), is the very reason (apart from the fact there are very few >5KW Wattmeters around) that he uses a DC clamp meter... Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted April 3, 2012 Share Posted April 3, 2012 I’m wondering if in fact there might be another slightly different answer to the lead extension/ESC capacitor conundrum. I think I might consider, rightly or wrongly, it’s maybe because of what is happening on the ESC. When the transistors are switching on and off the voltage on the PCB might be going up and down like a yo-yo, perhaps for a few reasons, one being the the self-inductance of the cables is slowing the current down a trifle. Some of the components on the PCB might not take to kindly to this, and might start complaining by getting all out of harmony. So to avoid this, some capacitance is placed at the input to the ESC, this acts like a little reservoir of power, and as the voltage falls it discharges, so maintaining said voltage and keeping the current flowing. By the same token, if the voltage rises on the wires at all because of the switching effect the capacitance will accept this and prevent it getting on to the PCB. This ‘reservoir’ is topped up, or recharged, every time the transistors are briefly switched off. If the capacitance were perfect there would be no associated resistance, and so there would be no problems, but in fact, for a variety of reasons, possibly size, weight and I’m sure most importantly of all, expense, this capacitance, in many applications at least, is only just about at the legal requirement anyway, and even under normal circumstance is working at it’s top limit. There is always an element of resistance, the ESR, or Equivalent Series Resistance, and it can also happen that in some capacitors, such as those we are talking about, this resistance can be quite high. Therefore with all this current popping in and out of the capacitor, at a rate of knots, the I squared R losses will be considerable, relatively speaking, and this manifests itself as heat. Even in normal circumstances these capacitors, on better quality ESC’s, can get very hot, I’ve seen some examples of this, so when we increase the cable length considerably, and therefore the inductance, the capacitor workload goes up a bit higher, the heat gets a bit hotter, so the capacitor goes right over the top and throws in the towel and very quickly the components on the ESC are suddenly left running around like headless chicken, and then eventually they just lie down, dead! So we just need to add some extra capacitance to cope with the added inductance. I’m sure that it’s quite possible that better class ESCs (more expensive!) are better able to cope with these sorts of situations. I think sometimes it’s all just a reflection on just how close to the working limits some of our toys are! The motor, of course, has no such components, and therefore has no need of continually receiving a smoothed supply. So no capacitance needed. I very much suspect that there are also perhaps one or more smaller capacitors on the PCB, between the neg and pos tracks, for the very same reason, to help keep the voltage on a nice even keel. Some industrial units I remember used a lot of capacitance, multiple rows of capacitors, as big as as, or sometimes bigger than cocoa tins. On one occasion I remember some Continental technical men working in one of their products managed somehow to pop a whole bank of these simultaneously, this resulted in black soot being expelled everywhere, covering everything, including these gentlemen, and we were treated to instant course in protracted European cuss words. This is only my theory, and it’s surely possible it may be wrong. But until I see a better one I’ll stick with this. Don’t I remember the Cox Tee Dee 0.01? glow engine being advertised as running at 27,000 rpm, in times long past? PB Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted April 3, 2012 Share Posted April 3, 2012 Castle Creations actually do capacitor packs to enable you to lengthen the cable between the battery and ESC. Ans some more reading. Edited By Frank Skilbeck on 03/04/2012 23:06:26 Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 3, 2012 Share Posted April 3, 2012 Good stuff Frank. Seems like there's a bit more than an urban myth here if manufacturers such as Shulze and Hacker confirm the problem exists! Edited By Martin Harris on 03/04/2012 23:39:26 Quote Link to comment Share on other sites More sharing options...
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