PTP Racing

iTrader: (2)

i was also told electricity flows from neg to pos in a class i took some time ago

both wires are part of 1 circuit and since 1 has to be longer it doesnt matter which

and try to find the resistance of a piece of wire i dont know how expensive a meter you have to get before you would actually get a reading but its going to cost a whole lot

my xxx-s has a short pos wire why? because thats how i assembled my battery packs

and im pretty sure when wiring a car amp you try for a short ground because the wire picks up interference compromising sound quality

dpaton

Eelctron charge does, yes. Conventional current, the kind you measure with a meter, and the kind that most folks think of, goes from + to -. It's mentally convenient to think of current flowing from 'more' (positive voltage source) to 'less' (ground).

Yup.
And since the change in total loop resistance is so small compared to the loop components (motor arm, ESC) it's not worth worrying about.

The one I used was a $12,000 Tektronix meter with a lamba (4-wire) test probe configuration, specially calibrated for measurements below 1 ohm. For those of you playing along at home, that's about 1200 times more accurate than the handheld Wavetek or Fluke you probably have, and about 120,000 times more accurate than the one you bought at Radio Shack.

The ground in a car is a sticky issue I'd rather not delve into here, but the main issues are that so many other things share the same ground, and the ground has resistance (not anywhere near ideal). The combination of those two things are what can make low noise amp installs problematic.

-dave

DynoMoHum

Put 50 amps though 6 inches of tyical RC power wire, and measure the voltage drop... You can easily measure it, with any digital volt meter.


I've done tests on a dyno, where I added about 6 inches of wire in line with the motor... I forget exacly how much power loss there was, but it is deffintely noticable, say like 3 to 5 watts if memory serves me...

The resistance of the wire, is a factor, and often it's as much or more then your ESC's internal resistance...


I'll try and do some more tests soon and give actual numbers... but trust me... reducing wire length to a minimum is the cheapest way to increase power you'll ever find in RC.

brians11

iTrader: (40)

now that we can calc how many watts 6 inches of wire is worth, how much is that same amount of power worth down the backstretch in seconds? that should help a lot of people understand exactly how much you lose by adding a couple inches of wire....

DynoMoHum

I just recently obtained a Eagle Tree RC car data recorder, with a electric expander. I can now measure current pretty accuratly as a result.... It should be a realitvely simple thing to measure the voltage drop across a known lenght of wire. Knowing the voltage drop, and the current, we can then caluclate power loss due to wire length.

Stock motors draw less current then, 19 turn motors, and sub 10 turn draw even more current... so, power loss due to voltage loss in wiring, will effect racers differntly depending on what class they run.

Anyway... I'll try to do some testing soon and give some actual numbers from testing.

dpaton

I did this math for the kids in another thread. 12AWG wire (Deans in this case) has a resistance of about 1.6R per 1000ft. That's 16 thousandths (0.0016R) of an ohm per foot, or 0.000134R per inch. If I have 6 inches of that wire (0.0008R), at 50A, my total voltage loss will be 0.04V (40mV). I^R gives us a grand total of 2W, not 5 or 6.

Your average stock motor will pull somewhere around 25A during a race. If we figure that motor is at an average of 3/4 throttle (~5.3V) throughout the race, we get an average power to the motor of 132.5W being consumed by the motor. Your 6" of wire in that case (0.0008R, 25A) will be dissipating about 0.5W, a grand total of 0.377% of the total fed to the motor. RC motors are arount 50-60% efficient once they warm up. You can see where I'm going with this I think.

As to the best way to go faster, it's to drive better. Those 40mV and 2W are lost in the noise once you sideswipe another car, or tag a board, or tak the wrong line.

As far as testing for losses, attempting to measure the current via voltage drop in an RC car is a longshot at best. I've been through this before. I spent quite a while instrumenting my XXX-S with current and voltage probes, and the current measurements via volatge drop were a disaster. There's way too much noise (ESC and motor induced) to do it successfully and repeatably. I ended up using some very nice (custom) hall effect current sesors from Allegro to get linear, precise, repeatable, accurate measurements of current, and some very nice differential ADCs to grab the volatge data (to two or three decimal places on each). I was working on a stock motor powered car, and the current was well below 50A, more like 25-30 over time.

Losses attributable to wiring (tiny), motor inefficiency (huge), and setup (gear choice, tires, etc, also huge) were all miniscule compared to the variations in the power profile caused by imperfect driving. When you crash, the current draw can exceed 150A for a split second as your motor stalls at full speed. The batteries take a while to recover form that kind of draw, as it messes with their internal chemistry. When you tag a board or another car, the power you spend getting back onto your racing line is significant, as it usually takes more than a second or two of recovery to make it happen. The numbers for my car are probably the most accurate taken outside the test labs at places like Novak and LRP.

Yes, there are losses in wire.
No, they don't matter like people say.

Unless you can drive a perfect line for an entire race, never passing, avoiding, changing lines, hitting anything, or slowing down, you have other things to fix first.

-dave

DynoMoHum

Well I don't really disagree with what you've said dpaton...


I did a quick test, the quickest I could easily proform.... Using my T35 discharging at 30 amps, I added a 9" peice of 12G Wet Noodle wire. Using my Fluke multi meter, I measured 40 mV across the 9" of wire while the 30 amps was flowing though it...

Using Ohm's law and all that, the numers come out pretty darn close to those you've given. I get .000148 Ohms per inch for my 12 guage Wet Noodle wire..


The numbers I gave previously were from memory of a test I did about 2 or 3 years ago.... Sorry if I mislead anyone with the numbers I gave in my previous post.

To put all these numbers and comments in a differnt perspective...

Consider that the published specs of a Quantum Competition ESC give .00044 Ohms... 6 inchs of 12G wire has about twice the resistance of a high quality ESC. ESC cost $150... Reducing wire length $0.


Typical 1.165- 1.169 six sell pack of GP-3300 $50, typical 1.170+ six cell pack $70. Price differance of $20. This gives you roughly the same increase in voltage as you'd see from eliminating 6 inches of 12 Gauge wire. Cost of 4 packs per seaon, $80... Cost of reducing your wire length $0.

Cost of all your RC equipment? $1000+ .... running the perfect race, and winning by .1 second over second place because you had six inches less wire in your car... Priceless...


Leave your wires as long as you want.... I don't care.

dpaton

Sorta. The published resistances of the ESCs you see are the theoretical on resistances of the FETs in question. Unfotunately, the on resistance (denoted Rdson in the datasheets) of the FETs will increase as they warm up, and I don't know of anyone that can keep things in their car under 25C during a race. Also, those results are for when the FETs are all the way on. When you switch a FET, there is a period when it is neither on nor off, when it is in it's linear region. This is the same part of the transfer curve that is sued for high quality audio amplifiers, where the FET outputs a fixed voltage less than the one applied to it's drain and wastes the rest as heat. In a PWM implementation like that in modern ESCs, the microseconds spent in the linear region every time the output is cycled add up to soem real live power bring lost. It's not a lot, but it is significant. My envelope calcs say it's a couple of watts at 2/3 power, which is the worst case scenario. Not a ton, but something to think about. It's also a nice explanation on why speed controls still get warm

Regarding your battery upgrade comparison, that's really not a fair thing to do, since the higher voltage batteries typically also have lower IR, making you faster for a combination of reasons. Picking the same volatge cells with lower IR will cancel out the wire, but unpredictably. There's so much that's dependent on cell characteristics that I kept it out of this discussion on purpose. A little bit here or there can totally negate these last few posts.

With the reserve power available in today's batteries (I stuff ~4700mAh into my GP3300 practice packs, and can't get them to dump until >10 minutes in stock, my 4 cell packs are the same way) I honestly don't think people need to worry nearly as much about fractional percentages now as they did when I started racing, when 1700SCRs were brand new. Really. There's so much reserve energy left, and the races finish so far up on the discharge curve, it's a non issue.

Yes, there are situations where that fraction of a percent will help you win, but those situations are only faced by theoretical racers at the very highest echelons of competition, where every car makes all of it's laps without ever touching another car or a board, or crashing, or getting tweaked, or anything else. In engineering, it would be said that the losses from the wiring are "sufficiently small they can be ignored." That's the official way of saying that there are bigger and better things to worry about, like the internal resistance of your cells (yes, it can make a huge difference) or the shape your motor is in (brush bounce will kill you in a race).

If it makes people sleep better at night knowing they have half an inch less wire on their car than the guy next door, so be it. I'll be the guy at the track with enough wire to wrench on his car comfortably, and I'll be just as fast (driving ability notwithstanding).

Like I said before, yes, there are losses. No, they aren't big enough to worry about. I think both of our numbers bear this out.

-dave

DynoMoHum

I just keep my wires long enough to work with... I typcaly don't use 12 gauge myself, at least not on anything shorter then about 4 inches, because I don't like how stiff it is...

All of that wonderfull knowelge asside... it's still pretty darn inexpensive to reduce wire length... and I beleive it's a good habbit to get in.

My next test... is to measure the voltage drop across several of my ESCs and/or come up with a way to test them at heavy load... say 50 or 60 amps. I'll let you all know what I find....

dpaton

Indeed it is. I dont' advocate leaving wires long either, it's messy. But I do think if an extra inch or two makes the car easier to work on, or the batteries easier to change, etc, it shouldn't be left out because it's less efficient.

Make sure you also let it run for a while to let it heat up, as that will change the resistance of the FETs. I don't remember what my work said, since it was a while back (in the days of the 410-5) but I do recall temperature beign more or less directly proportional to the change in resistance. I'll see if I can dig up my notes for you. If you have the capability to check the voltage directly at the ESC PCB it will help too (keepign the wires and associated solder joints out of the equation). I'm looking forward to hearing what you come up with.

-dave