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Old 06-07-2004, 11:10 AM   #1
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Thumbs up voltage drop calculator

having some suspicious problems at the track lately motivated me to do some research. it's very elemental, but appears to be very substantial as well.

it seems to me that there has not been much discussion about the importance in wire gauge for the power transmission wires. perhaps it is not that critical. that may, indeed, be your opinion, but i found this page that indicates otherwise.

http://www.currentsolutions.com/knowledge/vdrop.htm

i have seen statements that approve of higher gauge wire on 12th scale cars to save weight or to keep the rear flexible. so far, the tradeoff of voltage seems to be a questionable recommendation.

the page does, however, not account for material of wire being used. i would assume copper?

have at it. please discuss your conclusions and indicate any errors in thinking along the way.
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Old 06-07-2004, 11:51 AM   #2
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As indicated by the web site, voltage drop is dependant on the wire, the load, and voltage. You may also note that if you ask it to use greater than 19A on 16 AWG wire it isn't impresed too much as you exceed the thermal characteristcs of whatever wire it's programmed for. Now there are a few other factors to take into account. The ampacity of the wire is dependant on more than just the wire material but also the the insulation. The more insulation the less the ampacity as the insulation will increase the temperature (or decrease the ability of the wire to conduct heat away into the air) which increases the resistance which increases the temperature etc. However on the other side the spikes in current for RC cars are temporary which would mean the temperature would not have the opportunity to build and cause a problem of thermal breakdown. Another issue which I have heard nothing about in the RC industry is the effect of "skinning" on the wires due to the high frequencies used by our speed controllers. This would mean that the electricity would be carried mainly on the outside "skin" of the wires which further limits the ampacity of the wires. This is coroborated somewhat if you have read Big Jim's theory on brush serration, he says that the current likes to flow at the edges of the serations to the commutator which is why serrating the brushes seems to be so effective.

I use 16AWG wires on my 12th car, I have taken to feeling the power wires to see how warm they are after a race, I have had no problems with excessive heat so far. This I think is a very good indicator of potential problems in the wires, either ampacity or voltage drop.
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Old 06-07-2004, 12:59 PM   #3
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wow, that's good stuff. even w/o heat generation, the smaller cross sectional area of the 16 awg is inferior to larger wires in terms of energy transfer. is that not a substantial concern?

also, when you consider the frequency that our esc's operate at, is there any consideration taken to the shape of the waves? my understanding is that our esc's use a pulsed dc square wave that oscilates from 0 to battery voltage. does the skinning phenonemon occur with this type of signal? or must the signal be a sinusoidal wave symetric about zero volts much like a/c?
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Old 06-07-2004, 01:36 PM   #4
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Seaball,

For the dia of the wire generally it's the cross sectional area which dictates the current carrying capacity (ampacity) of the wire so generally the larger the wire the better the current carrying capacity and the lower the volt drop. However there comes a point of diminishing returns. The savings in wieght compared to the better voltage by the bigger wire. The obvious next question is where is the best tradeoff point for this. With the wires being so short I'm inclined more to worry about the Ampacity in the wires rather than the drop in volts.

For the sake of the discussion lets assume that the voltage drop is 0.05V (which I think is high). Now how much better acceleration do you think you would get if you could have say .03v of that drop back. Then how much would it slow you down by having the extra wieght of the wire to accelerate. It's possible that at a certain point these things may cancel out.

As I understand the waveform is not much of a consideration in the skinning effect of high frequency electrical supply though I am not an expert. I did work for a company which manufactured Inverter transformers for Telecom power supplies, we were always having to do exotic winding techniques to get the best power out of a given size of transformer. These power supplies all had weird waveform outputs and depending on the frequency they all had issue with current Skinning.
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Old 06-07-2004, 01:49 PM   #5
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In regards to the crossection of a wire, and a larger wire carrying more current, consider that the power wires we use for rc are multi-strand, and there are a bunch! So, I think that eliminates the possible problems with skinning, because there are so many wires for the current to travel over.

Also, if you figure the max current spike carried is nearly 35 amps, at 7 volts, thats 245 watts. Now, think about car audio and the diameter of the speaker leads. Much more watts, in many cases, are transmitted over wire with no larger diameter than that used in RC.
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Old 06-07-2004, 02:55 PM   #6
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Quote:
Originally posted by PitCrew
In regards to the crossection of a wire, and a larger wire carrying more current, consider that the power wires we use for rc are multi-strand, and there are a bunch! So, I think that eliminates the possible problems with skinning, because there are so many wires for the current to travel over.
I would say that this is a great point.

Depending on the motor, the peak amps can be as high as 120 amps.

As for the frequency thing, a square signal like this switching on and off, in an ideal world where the FET's dont have a rise time, the vertical part of the signal, equals the highst possible frequency. But there's a rise time of the FET's, so the frequency is limited. These things affect the phases, but I'm not good at that part of electronics. The phases mainly matter in the motor coil, but there's always a parasite inductance and capasitor in a wire.
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Old 06-07-2004, 04:03 PM   #7
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Pit Crew,

The multiple wire theory was what I thought at first but this does not affect the skinning issue, the current still moves towards the ouside of the bundle and of individual wires. I discussed this with an electronics engineer a couple of years ago. Also the wires we were using were individually insulated against each other and this effect was still pronounced in the bundle and in the individual wires. Our solution was to weave the wires in such a way so that each wire dipped into the bundle and then rose to the surface again. This also only worked when each wire in the bundle was individually insulated against the other. (similar to multi wire winds in a motor).

Also the current carrying capacity of the wire is exactly that, the current in Amps (ampacity). Watts and voltage are irrelevant.
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Old 06-07-2004, 04:18 PM   #8
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personally, when looking at the tables, i see marked improvement in ampacity when going from 16awg to 14 to 12. and it seems that the industry has already come to that conclusion, as 12 awg seems the largest commonly available. i am sure the calculations can be done for added mass to accellerate vs. added potential to do so.

for stock on road sedan say... ave current draw = approx 22a(conservative). maybe the average total path length for the electrons to travel is about a foot. that yields:

for 12 awg a vdrop of .035v/pack, = .006v/cell
for 14 awg a vdrop of .055v/pack, = .009v/cell
for 16 awg a vdrop of .080v/pack, = .015v/cell

the difference of 12 to 16 gauge wire is .009/cell

so to me, running 1.170 cells with 16 gauge wire is like running 1.161 cells with 12 gauge. is my math or my estimating off?

it just seems wierd to me that we spend all this energy finding and funding "team" cells only to let some of that power be dissipated as heat in an effort to save weight. maybe i'm being too dramatic, but no more so than those who insist on running packs that are unavailable to the general masses.

what do you think?
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Old 06-07-2004, 04:34 PM   #9
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crimson eagle has hit the nail on the head on this topic. The main concern is the ampacity of the wire since all of the losses are related to current, actually current squared times the resistance equals the power lost. Or. Voltage drop squared divided by the resistance OR The current times the voltage dropped. These are all the same by way of Ohms law. There is a point of diminishing returns when it comes to wire size, but I don't know what it is. 12 awg is good for pretty much everything we do in rc sedans. I do know boat racers that will use 8 or 10 awg, but they draw more current than a sedan ever could. More cells, high power brushless motors.... etc. The other thing that needs to be considered is how much current can be carried by the wire used to wrap the motor. 120 Amps?!?! Probably not for more than a second, probably less. This is especially true for a stock motor where there is 1 piece of 24 awg wire wrapped 27 times. Also, if a battery pack has .006 ohms of resistance (6 milliohms) the battery voltage at 100 A would be 8.0 V (fully charged batteries have more than 7.2 volts) minus (100 * .006) , or 8.0 - 0.6 = 7.4V. Compared to this the wire for the connections is tiny. The soldered connections to the battery and motor will have more resistance than the wire itself.
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Old 06-07-2004, 05:28 PM   #10
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Kufman,

All very good points, I was thinking earlier on about the wires not being the limiting factor for ampacity you nailed it very well. This same engineer I speaking to also told me that a solder connection solder joint has 14 times more resistance than as compared to piece of wire without a solder joint. Imagine how much resistance a mechanical connection is like we use with our battery connectors!

My opinion is that you need to try and take care of all the little pieces as well as you can, if you let them all add up then you may generate some losses that could be significant.
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Old 06-07-2004, 06:57 PM   #11
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yup, all very good points. there are losses everywhere. some of which we can control, and others, such as i.r. in the batteries, and in all of the internals of our components, we cannot.

the idea here is to select the parameters that are in our control, and optimize the quality of both the execution and of our decisions.

with the idea to make sure all the little pieces are taken care of, it would appear to me that one of those things would be selecting the correct wire gauge. or perhaps the maximum practical wire gauge. while the "resistance density", if you will, in a connection is very high, the relative length of the connection is small, so the overall voltage drop across it should still be small when compared to that of the wires themselves. the thickness of a soldered connection is very small (.00X"). because the wire is 1000's of times longer the absolute resistance of it will likely be higher, and therefore have a larger impact on overall numbers.

i am speaking in general, and perhaps, my thoughts are errant. i would just have a hard time trading a .009v/cell drop to keep the wires light and flexible.

why do we pay the $ for top cells, when we could loose the very numbers that we are paying for by selecting a less than optimal wire gauge? i appologize if my statements are superficial, but i am still of the opinion that wire gauge selection is anything but insignificant.
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Old 06-07-2004, 11:33 PM   #12
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Seaball,

I don't, and won't unless I get decent data use 16 awg wire in my sedan. I think the current draw is too high and the ampacity of the 16 awg could be an issue. I do use 16 gauge wire in my 12th. It's a very light car, has way less amp draw under acceleration and if you run mod then you try to manage your throttle through the race anyway.

Do you really think you would be able to tell the difference in acceleration between 4.18v to the motor and 4.144v at the motor? this being a 0.9% difference. I've been racing for 12 years I'm not sure I could tell. Also considering the wieght and handling tradeoff's. In 12th class there may as well not be a wieght limit for the car it's so light.

Over the weekend if I have time I'll measure the volt drop on some wires for a given length at 30A and see what it tells me. I'll post the results when I have them. I can't promise to have the time as I'll be racing on Sunday.
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Old 06-08-2004, 08:33 AM   #13
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nah, i don't know that i could detect that magnitude of difference either, but i do know that it's magnitude is along the same lines of the different stages of batteries being offered. and, for some reason, i have been convinced that good cells are a must. .010v/1.170v is approximately .09%!

see what i mean? i guess if i am going to buy high end cells, i'd be reluctant to use any type of "smaller" gauge wire, if only from a numbers standpoint.
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Old 06-08-2004, 11:09 AM   #14
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The last cells I bought were the cheapest gp3300's average voltage per cell of about 1.166. I used them in a car I have had for 18 months which is 4 yrs old but never actually raced, I managed to qualify 3rd. I didn't see any difference between my speed and my opponents. I personally put more store in run time than voltage to keep further up on the discharge curve. These days I would prefer to buy cells more frequently and have fresher cells as I think the GP's do fall off quite a bit, than to have really good sets and make them last longer. I do have to race on a fairly tight budget though.
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Old 06-14-2004, 07:57 AM   #15
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Voltage drop measurement results: -

I used 3 different wire sizes 12, 14 and 16 awg. The lengths of the wires were all 65mm (2.56 inches).

the amps through the wires was the same for each test and was set to 30 Amps (my discharge rate on my Pro Trak)

Wire volt Drop
12awg 0.011
14awg 0.017
16awg 0.025

These numbers do not change for 4 or 6 cell.

As I said previously, I do not and will not use 16awg for sedan. I do use it for 12th where wieght and handling is a factor and the amp draw is lower.

I think these results do stress the importance of keeping your wires as short as possible.
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