I’m planning our general electrical system and I have two quick questions.
First: Ive been playing with running cable along a frame and I have had great success running cable in our frame (Its non metal and has proper stress releases). So my first question is “realistically what effect does excess wire have?” Ignoring weight gain and assuming you had it properly protected from being yanked, does the excess of cable have any real (not in theory) loss to the entire system and has anyone actually experienced such a negative effect. I want to run cable along our frame or in it because we had trouble with cable getting pulled.
Second: Since i wrap everything in a nylon sleeving i can’t easily trace wires (nor do i feel the inclination to). I would like to mark the connectors in a way that i can tell what type of component its powering and a abbreviation for the component (ex mtr1 for motor 1). “What do you recommend and what have you seen that is particularly cool?” Ive already seen all the pictures of various systems on CD and i was thinking a colored power connector (if they make some that are aesthetic and not indicative of gauge). of use some type of tape to connect wrap around the wires. Ive heard something about the Inspectors making you remove such tape though and I want to be sure its ok.
I’m planning our general electrical system and I have two quick questions.
Well, as resistance is a function of length, among other variables, and voltage drop is a function of resistance, longer wires are not a good thing. However, only take this into consideration when thinking about this, as I wouldn’t say it eliminates the possibility of doing this. And yes, this does amount to a measurable (not theoretical) difference, epically on 50-70 A going through 6 ga wire for an extra 2 feet or so. You can do some searches to find actual calculations, or google for wire resistance calculations to do the math yourself.
As far as knowing what wires are what, you can get these books of # stickers for wires. We use them on all our PWM cables.
You can get Anderson PowerPoles with different colored housings as well, which would be a good way to differentiate between circuits, as well as providing a better connection than the standard F disconnects.
One of the main lessons learned over the years for team 45 has been wire length. In general, the bigger the wire is, the shorter it needs to be. For instance, we try to keep our wires on the battery and to the distribution panel very short. Also, we try to keep the wires to the motors as short as possible. There are a few reasons:
As wire length increases, so does its resistance. Therefore, longer wires transmit less power. For instance, a 6 gauge wire has 0.00047ohms of resistance for every foot referenced here. During peak operating times, this wire may get 300 amps ran through it. If this wire is 3 foot long, then there is a 4% loss in voltage. If the wire is only 12 inches long, then the loss is 1%. This slight edge in power may give you the extra edge in a match.
- Weight savings
Longer wires weigh more, as mentioned by mechanicalbrain.
- Lower Center of gravity
If designers make the effort of keeping their wires short, then this usually leads to motors being closer to the base of the robot. Lower-placed motors means lower Cg. As we all know, low Cg is a beautiful thing. I’d rather have our robot not fall over.
As for labelling wires, that is good practice. Panduit has great products for this. Many professional electricians wire their labels with 1 and 2 letter codes. Some teams color code their labels also, for quick reference. If the labels are small and do not lead to question the safety of the wiring, a FIRST inspector should not make any team remove these labels.
As Andy has pointed out, extra wire can be a problem for many reasons. In most cases when this question comes up, teams are asking about motor wiring. Although the rules allow for #12, I don’t recommend it. #10 is a nice wire to use and has about .001 ohm per foot. Most of our motors are running close to 100 amps stall current and so a quick calculation will tell you a #10 will drop 0.1 volt per foot under these conditions. Make the wiring two feet longer than needed adds 4 feet of wire (red and black both are part of the circuit, remember.) or drops 0.4 volts in addition to all the other losses. When motor ops are important (when are they not?) I would rather have the 0.4 volts supplied to the motor instead of heating up the wire.
As to coding the wiring, this is what we do. Digikey sells a 3M color tape dispenser. (It is expensive for what it is but it has 10 colors on it.) We color code everything on our robot. A single circuit will have a colored tape on each wire, circuit breaker, position on the power panel, speed controller, PMW cables, PWM output on the RC, motor and motor position. That way if the “yellow” motor is out of control, you pull the yellow circuit breaker and it stops. You can tell by looking that the “yellow” circuit breaker goes back into position 2 because there is a “yellow” tape marking the breaker panel. You know it was the “yellow” motor because there is tape on it and there is yellow tape next to it’s mounting position on the robot so when it is pulled out, you know which motor is missing. When the motor isn’t running, you look at the “yellow” speed controller to see if the fan is running. (We tape the fan so it’s spins a pretty color.) If the fan is running you look to the RC to see if the “yellow” PWM has pulled out.
This has helped a bunch and thanks. Is their a way to boost a loss of voltage from the cable? Also, if our team doesn’t make some type of panneling is their a good way to prevent cables from being pulled and snagged? Finally ive gotten mixed info about refrigerating batteries. Ive heard that while it helps keep voltage that its still negligable but also it keeps sulfation to a minumum and allows you to store batteries longer betwean charges. Is their any real truth to keeping your battery on ice?
If you use larger than requried gauge sizes, which have less resistance, you will have less drop. Otherwise, there’s no way to keep a longer wire from having a larger voltage drop.
We use self-sticking cabletie squares, with ziptie’s not 100% tightened, so that you can still trace the wires, and the squares allow easy replacement of the cable tie if you need to remove it. Some people recommend against it, but we’ve found it does a pretty good job.
Keeping your batteries cool while they are not being used makes sense.
One thing to keep in mind, lead acid battery capacity goes down with the temp, so you want to make sure your battery is at room temp when you use it in a match, or it will not give you 100%.
Various outlets (car radio suppliers) have a product called spiral wrap. MCM sells it in various sizes and it simply just wraps around the wire bundle.
There is obvious diminishing returns with wire sizing. To halve the loss, you need to go to #6 (.00047 or approx. .0005 ohm/ft.) which becomes a problem with weight (more than twice the weight) and drawing the larger diameter through a structural member. In the above example you would drop 0.2 volts instead of 0.4 volts. There are many other losses in the system so the electrical design must minimize those losses within reason. A warm battery will have higher resistance and a cold battery will not be able to make enough chemical reaction to produce full current. (old northern trick in winter is to turn on headlights for 30 seconds to get battery warmed up before asking it to crank at 0 degrees.) Connector resistance adds up (two for each connector) especially if the crimps are bad or the connectors are loose. Each circuit breaker adds a little and the speed controller is not perfect, it represents about four feet of #10 all by itself.
This is an article about batteries in the White Paper section…
We have not had problems with the power harness wires. We have had problems with sensor wiring and pwm cables. Therefore I would not have a problem with fishing power cables through structural members. Any digital or senor wiring needs to be replaceable fast. We have used 1" PVC wire race way for wiring. Pop the cap off and wires can be replaced easily.
i can see alot of people have answered the resistance and power loss problem…now onto the connectors i’ve herd of many different types of connectors, stickers, etc. All i can recomend is zipties if you have enough. They work great for our team. We just attach small ones around the wire at random increments and at the ends.
If you plan to run lots of wire through the frame, be prepared to err, have fun, if something comes loose and it requires the changing of a wire or removal of the board. Been there, done that, not fun. Also even though you said to not pay attention to weight, wire does add up. We saved nearly 3 pounds in wire last year. (And it wasn’t even thick stuff)
our team saved half a pound by just getting shorter pwm cables and other high gauge wires
Funny how this is called two quick questions yet its anything but that. OK during the clash our power wires got pulled out from the power connectors (the big copper ones that have a screw to hold the wire). My team leader wants me to solder the cable into the connector but I’m 95% (rough estimate) sure that it would be illegal. Yes the screws were tight and yes we used shrink wrap but as i understand it the cable got yanked right out. Can i get some confirmations on the soldering thing and if anyone knows any rules in specific I would appreciate it. Also if i need to make some PWMs run a good distance (4ft) should i just connect one, with a S bend in the wire so the connector can’t be pulled, on to the next? Thanks all.
AFAIK, there’s no rule against soldering the copper lugs, but you will need a pretty good iron to get it hot enough. We did it on all of ours, as I don’t really trust or like those connectors but the rules say we must use them, so we’ll make the best out of what we have. Of course, I don’t think anyone is going to stop you at an offseason from using the non-kit connectors, and who knows what the rules will be next year.
OK, I did a rule check in Section 5 and the only thing I found was that you had to insulate the connection. Nothing about soldering. Just make sure that you can get the connectors off the battery terminals for shipping. Maybe Q&A can tell you if it’s legal.
As for the PWMs, that’s about how you’d do it, unless you made your own to lenght, which it is possible to do. (I don’t know the procedure. Someone else probably will.) If you do several, use electrical tape around the joint.
Of course you can solder these connections. As a lead inspector, I strongly recommend soldering connections especially primary power. A large iron helps, but a trick many people do not use is “wet” the tip of the iron with a lot of solder before placing it against the connetor. Add more solder until you get a large surface area between the tip and the workpiece. This will insure max heat transfer. When you think the connector is hot enough, apply solder away from the tip (and the solder pool) and if it flows then continue soldering. Add solder to the end of the wire and the connector but don’t add so much as to allow the solder to wick up into the wire under the insulation. This will cause a stiff spot in the wire that makes it bad for forming the cable when you need to. Allow the connection to cool before handling or trying to add heatshrink.