Lengthening Wires

[Dave Potter]

Hi Folks, will it be ok to lengthen the wires on my speed controller by about 3 inches ? it's the 2 that go to the lipo.

Thanks friends…

[Dave Hopkin]

a 3" extra should not make any practical difference as long as the extension wires are as thick or thinker than the originals

Yes there will be a theoretical tiny increase in resistance in the circuit but it doesnt matter

[Engine Doctor]

Why not replace the wires completely with wires of correct length and thickness as any joins can be a source of failure or resistance.

[Frank Skilbeck]

Resistance is only part of it, it's the inductance in the system, if the wires between the battery and ESC are too long then the surge current may overload the input capacitors (Castle Creations sell an additional capacitor pack just for this)

But as Dave says 3" shouldn't be an issue especially if the ESC is being used well within it's rating.

[Simon Chaddock]

Dave P

As Dave H says speed controller to motor wire length is only limited by the effect of any additional resistance which is very small provided your wire of the appropriate cross section and has good electrical joints.

I have models that have the ESC and motor separated by nearly 1000mm!

The Speed Controller to battery wire length is much more critical and should not be extended, not because of resistance but due to the increased risk of electrical voltage spikes damaging the ESC's components.

[ken anderson.]

hello dave......................i've added 3"..no problems at all ..

ken anderson...ne...1...... 3" dept

[Kevin Wilson]

To my understanding the ESC relies on the (low) impedance of the battery to clamp voltage spikes generated by rapidly switching the FETs.

Lengthening the cable between battery and ESC increases the effective circuit impedance allowing larger voltages spikes to appear across the electronics in the ESC. I can only surmise the back emf generated in the motors is not such a problem.

The common advice is therefore to extend motor connections by preference, alternatively to add additional smoothing capacitors or oversized battery conductors.

In power engineering we see similar problems on sensitive equipment where the earth path is more than adequate for circuit disconnection times but has to be massively over coppered to reduce the earth impedance to clamp mains born interference.

[Martin Harris - Moderator]

I haven't seen that explanation before but perhaps that's why I've never seen advice to separate the individual wires on longer input leads to reduce the increase in inductance - the reason that usually seems to be quoted?

As an example, this is what one manufacturer (Castle Creations) states:

Larger electric power systems lead to larger electric powered aircraft, and sometimes the distance between the battery pack and the ESC grows beyond the length of the wires installed on the components. Adding wire also adds inductance, which can increase the ripple voltage in a system. CC CapPack essentially negates the ripple caused by the addition of up to 8 inches of length to the battery wires.

CC CapPack is designed for quick and easy installation. Users simply cut the soft silicone insulation on the battery wires – do this as close to the ESC as possible – and place the exposed wires in the channels on CC CapPack's posts. Check for proper polarity and then solder the wires to the posts. Use the included heat shrink to finish it off for a professional looking install.

Note: All Castle controllers with data logging capabilities can report the ripple voltage they encounter during use. Ripple voltage peaks should always be less than 10% of the total pack voltage; the smaller the ripple voltage the better. If an application exhibits more than 10% ripple voltage under peak loads, the user should consider using higher discharge (C Rating) batteries, shorter wires, higher current connectors, better gearing or a smaller load.

Multiple CapPacks may be used in an application. They should all be installed as close to the ESC as possible.

[Dave Hopkin]

Technically increasing the length of the Batt-ESC leads will increase resistance (on this side of the circuit we are using DC so it's resistance not Impedance which is only for AC circuits) it will also increase Inductance - BUT the OP was talking about 3" - the increase due to this will be almost un-measurable without Lab equipment

Separation of the DC leads would theoretically reduce Impedance but again you would need Lab equipment to measure it

I would actually be surprised if you could actually measure any effect of adding 12" of lead in real life

[Simon Chaddock]

Dave

I think you are under estimating the effect of inductance on the ESC input leads which are carrying a current that is being pulsed at a high frequency.

[Dave Hopkin]

Its a long time since I did my Radio and Line Transmission but assuming the Lipo wires and not coiled then the formula is

The most significant variable is the length but for frequencies under the 1Mhz the frequency and wire radii make very little change to the outcome and the result will be in the range of 60 to 150 nano Henrys when the cable length is kept to less than 150mm or so, above that it starts to increase rapidly

[Martin Whybrow]

Dave, the formula for impedance assumes a sine wave; ESCs are switching the waveforms very fast, with modern FETs that can be in the range of ns, so you get very high harmonic frequencies generated as a result, so the effect is much more pronounced than the first look would indicate.

[Former Member]

[This posting has been removed]

[Allan Bennett]

There's a long thread in another RC forum, entitled something like "Too long battery wires will damage ESC over time". I'm not qualified to comment on the electronic theory and formulae so I just follow the advice in that thread. It recommends extending motor wires rather than battery wires, but suggests capacitor values for when you can't avoid extending the battery wires.

[Peter Beeney]

As I see it, the fact that because the power input into the ESC is actually rapidly pulsing DC this is going to result in a reactance. Reactance is the resistance of a circuit caused by the action of an inductor or a capacitor; an inductor can be just a straight piece of wire, but it’s almost invariably in the form of a coil. These react to moving or changing voltages and currents. Impedance is simply the sum of adding the resistance and the reactance of a circuit together, so I guess this will now have to be classed as an impedance, but fortunately the units of measurement are still ohms.

When a current starts to travel along a wire it causes a magnetic field to surround the wire. During the time the voltage is building up this magnetic is also building up and is in fact also moving; and when a magnetic field is moving past a conductor, or a conductor is moving through a magnetic field, a voltage is created in that conductor. So our moving magnetic field is creating a voltage, or electromotive force, known as an e.m.f., which is in opposition to our applied voltage and is thus slightly slowing it down. This is known as a back e.m.f.
When the current is flowing in a steady state there is no back e.m.f. developed, but when the current stops the magnetic field collapses, it’s now moving again and a voltage appears in the conductor, albeit if only for a short time.

Our brushless motors can demand relatively large currents at times, and as Martin W said, the FETs can turn off very quickly, both of these situations will contribute to the strength of the voltage that’s then generated; a hefty magnetic field as a result of the high current flow and it’s fast moving too because of the quick switch off; this voltage can be now high enough to seriously affect other parts of the circuit. This is not at all desirable, so these ‘spikes’ are absorbed by the capacitor(s) on the power input to the ESC.

Generally speaking, I would suppose that the manufacturer would need to make the ESC as small, light and as cheaply as possible. So to that end maybe he only provides just enough capacitance to cope with the inductance produced by the length of battery leads he provides. One of the benchmarks of a good capacitor is it’s lack of an E.S.R. (Equivalent Series Resistance); a perfect capacitor has none. Therefore I suspect a low e.s.r. is most likely going to be related to the cost, so as with everything else, it has to be a bit of a compromise. Is is possible that the cheaper the capacitor is, the greater the e.s.r. is, and that the hotter it gets, for the same set of circumstances?

Ohm’s Law says that the resistance of a piece of wire is proportional to it’s length and inversely proportional to it’s cross sectional area. So if we double it’s length we double it’s resistance; if we double it’s sectional area we halve it’s resistance. Adding extra capacitance to the circuit as has been advised by others, including the manufacturer, seems to be a good workable answer if it’s essential to lengthen the battery leads by any significant amount.

Once upon a time I had a hotliner powered by a sizeable 2.3 to 1 geared Aveox inrunner, with maybe up to 2 kilowatt output on a good day. Back in the day the Aveox was considered to be quality kit, the ESC was never a problem, but the large exposed capacitor on the power input got hot enough in normal use to turn dark brown and totally discoloured. I guess it got pretty hot at times; but it never gave up. The motor had a rotor position sensor and I occasionally had to repair this, the thin wires tended to break. Also I was asked to look at an ESC that was running erratically and I found that one leg of one of the pair of smoothing capacitors had come adrift. The repair instantly cured it.

It would appear there is no doubt that these little caps really are essential items……..

As always, just a few random ramblings.

PB

[Dave Potter]

Thanks guys, some very interesting answers. I am just going to solder the 3 inches on and see how it goes.

Thanks Friends...

[John Roberts 9]

Posted by Tom Sharp 2 on 04/05/2016 19:59:50:

Some brands of lipo have much longer wires than others, or to put it another way some are much shorter than others.

Therefor any calculations fall at the first fence if you take several brands of the same size lipo to the flying field.

Whilst I am not qualified to comment on the technical issues arising from lengthening the wires I have to say that my thoughts were exactly the same as those articulated above by 'Tom Sharp 2' in his posting.

Batteries and ESCs from different manufacturers will, in the absence of industry standards, all presumably have different length wires and this would lead me to conclude that relatively modest adjustments in length are unlikely to have any significant real-world effect.

[Kevin Wesley]

Hi, Just my humble practical opinion. Ive been flying a Blackhorse Mosquito for at least 3 seasons now. (over fifty flights) both speed controllers are in the nacelles with approx 400mm lead to them from the battery which is in the nose of the aircraft. I have had no problems with this set up At all. I also have 4mm bullet connectors in mid length, so i can remove the wings when i need too. Right or wrong, its worked for me without a hiccup

[Kevin Wesley]

[Peter Beeney]

I’ve always had a small problem with some of the general perceived ideas and procedures concerned with aeromodelling, and now maybe this is another. I’ve never really given this much thought, but I’m now beginning to ask myself is it possible that there is actually a real problem here anyway? I’d easily be convinced that increasing the lead length will increase the impedance, but for the amount of added length in general that modellers are doing will it make that much difference? I have noticed that sometimes the advice not to extend the battery leads is almost at the point of death, indeed very serious stuff, but is this because there is proof positive, or is it just because somebody’s mate at the patch overhead a rumour at his friend’s flying field…

Certainly there is some indication that it ain’t always necessarily so, Kevin Wesley’s very nice looking Mossie is one such very topical example, although I’m sure he always operates it well within the limits set by the manufacturer, such as prop sizes and battery cell counts etc. I’m sure there are other instances that can be quoted, too. It’s perhaps not until you start to wander outside the parameters, (like me), that it might make a difference. And in which case, I can’t really then complain when it all goes belly up…

Just to illustrate what I’m trying to say in all of this, another old favourite chestnut of mine is the two, (or more), BECs connected together with both of the infamous red wires still in place. In my opinion, I’ve aways thought that it doesn’t matter at all if they’re separated or not; the system will still work perfectly ok either way. Although I would consider that for a couple of reasons two in parallel together would be slightly more secure than one.

Just a few idle thoughts, but if I have the occasion to ever want to extend the battery leads by a considerable amount I’ll certainly keep a close eye on it for a while, but I think I’d be unlikely to make any other alterations, at least in the first instance.

PB

[Martin Harris - Moderator]

I think there may be more to it than idle speculation. Several of the higher end ESC manufacturers sell (and say they are required) capacitor packs for extended input leads. Although they might have a vested interest in selling the additional equipment, they aren't exactly somebody’s mate at the patch overhearing a rumour at his friend’s flying field…

[Robert Parker]

Hi All,

I have been reading this thread with great interest as I'm building a Catalina at present and just building the wing and thinking what to do with the esc's, motor and battery wires. I have to say I'm quite new to electric flight and I started at the deep end.

I have built two models prior to the Catalina these are a TN Lancaster and my own design Stirling.

I built the Lancaster as per plan with the battery in the fuselage and the esc's in the necelles. I believe that there have been many of these built over the years and I don't know of any problems regarding the wiring, With the Stirling I replicated the wiring again the battery is under the cockpit. The cable to the motors has been extended by around 900mm to get to the outer esc's. Both aircraft are 72" span.

Having a great interest in multi engined aircraft, Tony Nijhuis's other multi engined designs have been based on this set up with the battery in the fuselage such as tne B17 and Sunderland for example have all flown well, well then what is the problem with lengthening the battery wires on these 3s designs with so mamy sucessful designs, perhaps it is a different matter for much larger designs with much higher currents and amps to consider

Regards

Robert

[Allan Bennett]

According to what I've read, problems (if any) caused by too-long battery wires will occur over time, not immediately. First, the capacitors on the ESC will be under stress, but they may be able to withstand that stress. Second, if/when they do fail, there'll be nothing to protect the electronics from the "spikes" that are being generated in the wires, so they may then fail. You probably won't notice anything wrong until that happens, unless you check the temperatures of the capacitors immediately after each flight.

[Peter Beeney]

Robert,

Your claim to be new to electric flight would appear to me to be something of an understatement, in fact I’d be extremely tempted to say that you’d be very close to being in the expert class. Particularly as you both designed and built the Stirling after just one model previously. They both look to be magnificent models, I’d reckon that must rate a very well done indeed!

I was just mulling over an answer for Martin, but you seem to have very succinctly done it for me. I think I had been intending to basically agree with him, but you would rather appeared to have reinforced the idea that additional capacitance is not always strictly necessary. And by some margin, too, I’d say. I’d consider that 900mm is not really a trivial amount for an extension, by any standard, and if all these models have flown consistently ok with these long power feeds then this must mean something…

Love the 900mm lead length and 72 inch span remark by the way. Just my language. These figures I can totally understand. Quoting a 2000mm span, say, will always leave me cold. But then I did grow up using proper money…

Maybe to some extent part of the reason lies in the fact that the esc is in the nacelle, I suspect there is a good draught of cooling air though there. If you can keep electrical items cool enough you can probably safely overload them by a considerable amount. I’ve just had a glance at 30A esc, this is in a foamie and it fits in custom made slot cut in the foam, quite tight fitting, but with an air duct straight to the front. It’s labeled 3-4 cell lipo. A pair of 220uF smoothing capacitors, marked 25V and 105 degrees C, (that’s max temp, I assume). 25 volts gives a bit of headroom for spikes and 105 degrees of temperature allows it to raise a bit of a sweat in safety. Because it’s a bit cramped in the battery compartment I did cut the power leads down to short, which can only help. Quite by chance when I was tinkering with it one one occasion I found the maximum demand took it over the 30A level by about 5 amps, and nigh on 40 on a fully charged pack; I use 4S, but as it has never shown any outward signs of distress I leave well alone. The model performs quite nicely, loops as big as you like and a turn of speed in a power dive.

All good luck with your Catalina, would I be right in thinking you are going to use a couple of fairly hefty motors? Also I reckon you will stick to your previous proven route as regards the electrics; it obviously works well! If I were doing one of my own I would tend to err on the generous side with the cable size, to keep the resistance as low as possible, and the escs as well, it seems to me that if an esc is spec’d for 30 amps it will cope much better with only 20 amps. But that’s just me, and maybe I’m just being overly cautious anyway.

Just reflecting back on the ESC manufacturers advice to add extra capacitance for a moment, this can only make good sense really. They can only provide for an average or normal length of lead, and they have no control at all what is connected to to their product so they cover all eventualities with this recommendation. And if you have a bit of experience with a soldering iron and like to have a little dabble with the sparky bits it’s a pretty simple matter to add these anyway; and even if it never does any good, it’s never going to do any harm!

All the very best…

PB

Edited By Peter Beeney on 08/05/2016 22:57:03

[Robert Parker]

Hi Peter,

I am new to electric flight, I have been modelling ic since 1990 sorry for misleading you all.

My Catalina has a span of 54" and a weight of around 54ozs and I've got a pair of 1000kv motors and 30Amp speed controller.

My measuring skills goes back go back to when I was an apprentice carpenter where when in tech we had to use metric and on site everyone used imperial. Nearly 40 later this has not changed a great deal I call it bi-numeracy.

Regards

Robert