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WarehouseCrew
03-26-2011, 09:26 AM
What are the pros and cons of using stranded versus solid core wire for the electrical components? I'm not an electrical guy, but am trying to help our student's clean up the rats nest of wiring on our practice robot (may make same changes on competition robot when at the Regional).

We have been using stranded wire, but I just read that solid core wire allows you to make 90 degree bends.

Does your recommendation change based upon the wire size (eg. 12 AWG vs 14 AWG or 18 AWG)?

I'm assuming if we have a motor mounted on an arm that moves (wire needs to have some flexibility) that stranded is preferred. Is that a correct assumption?

Any issues with crimping on ends (eg. eyelet for attaching to Jaguars)?

Thanks.

Jon236
03-26-2011, 10:06 AM
I would recommend that only stranded wire be used on a robot. If a solid wire breaks, you lose all continuity.

sanddrag
03-26-2011, 10:07 AM
I cannot think of a single instance in an FRC robot where solid wire would be preferred, unless it was wire-wrap-wire on a hand-built prototype sensor circuit or something of the sort.

MrForbes
03-26-2011, 10:39 AM
Solid wire is commonly used in houses. Houses don't usually move, so it works fine. But robots move, and the flexing and vibration could lead to the wire breaking eventually.

Tanis
03-26-2011, 10:45 AM
I would highly recommend sticking with stranded.

A parent kindly donated some solid stranded wire left over from a housing project last year. It is very difficult to work with, especially if you are at the stage during which you are moving electronics around.

Our crimps (unsoldered) didn't stay on very well either.

kstl99
03-26-2011, 11:53 AM
I am glad to see that all are agreeing with stranded. Solid can break with vibration, metal fatigue I believe it is called. Also, screw and crimp terminations work much better with stranded, which is why you should not tin wires before crimping or using in a screw terminal.

Lastly, 90 degree bends with wire are a bad thing. They can cause the wire to crack inside. Most wire has a minimum bend radius as part of its specs. Better to loop into a connection if possible, avoiding stress on the connection.

nuttle
03-26-2011, 12:29 PM
The only thing to watch out for with standed is that is you cut short amounts from the ends, you create a bunch of short, fine individual wires that are as bad as metal shavings in terms of getting into electronics and causing shorts. Also, it is fairly easy to have individual strands break and cause the same problem. It is good practice to 'tin' the ends of stranded wire before you trim the ends and insert them into things like the power distribution board or a Wago connector. You should also twist the ends, but it is still a good idea to tin them. This should prevent these problems. We also solder crimp connectors, splices, etc. The increased reliability is worth the little bit of extra time and tiny amount of added weight.

Stranded wire comes with various stand counts, even for the same wire gauge. Finer strands are more flexible, but tend to make the above problems worse.

DonRotolo
03-26-2011, 02:20 PM
But to reiterate: Solid wire has no place on an FRC robot.

BitTwiddler
03-26-2011, 05:58 PM
. It is good practice to 'tin' the ends of stranded wire before you trim the ends and insert them into things like the power distribution board or a Wago connector.

Tinning of the wires to be inserted into Wago connectors is NOT recommended by Wago. The Wago's are designed to make a solid connection by actually compressing the strands in a predictable and reliable manner. Tinning the wires would prevent them from compressing properly thus reducing the contact area. There has been a discussion on this topic at
http://www.chiefdelphi.com/forums/showthread.php?t=69355

Teched3
03-26-2011, 07:48 PM
Stranded wire comes in a variety of different # of strands (more strands = higher cost) for the same gauge wire. The greater the number of strands, the more flexible the wire. In addition, the insulation type will also affect wire flexibility. Use crimp connectors on ends that are placed on screw terminals. Aways twist the strands together, use proper wire strippers to avoid cutting any of the strands, and test your crimp connection by pulling on the wire/crimp connection to ensure a soild connection. If you can't get a small enough connector for the smaller gauges of wire, strip more insulation and double or triple back the bare wire into the connector before crimping.

Solid wire is highly prone to breaking where you strip the insulation, as it can be easily nicked, which will lead to failure if you don't use a wire stripper. Use small zip ties and Velcro wire ties to bundle your wiring paths as much as possible. :) :)

MagiChau
03-26-2011, 09:20 PM
Solid wire is highly prone to breaking where you strip the insulation, as it can be easily nicked, which will lead to failure if you don't use a wire stripper. Use small zip ties and Velcro wire ties to bundle your wiring paths as much as possible. :) :)

My team placed the bundlles of wires for our claw's roller motors inside of cables resembling curly telephone cables. Worked pretty well.

bookworm0422
03-27-2011, 02:46 AM
Also let us not overlook that the Game Manual does not allow us to use solid core wire any where on our robots no matter the use for it.

Nirvash
03-27-2011, 06:11 AM
Also let us not overlook that the Game Manual does not allow us to use solid core wire any where on our robots no matter the use for it.

What rule is that?

Teched3
03-27-2011, 06:28 AM
There is no rule I can find governing use of solid wire. The rules cover gauge and color coding. As the say in Missourie, "Show Me". In the end, stranded is the preferred wire of choice.:) :)

Al Skierkiewicz
03-27-2011, 07:08 AM
Let's start out with the first post. Either stranded wire or solid wire can be used on the robot. Either can be bent at 90 degrees, solid will hold that bend better than stranded. The Wago terminations are better suited for stranded, untinned wire insertion of one wire only. As the wire is inserted and the terminal is allowed to compress it, more surface area is created between the wire and the terminal giving a lower contact resistance. Tinned, stranded wire often reduces contact surface area to a fraction of either wire terminations raising the contact resistance and therefore heating the contact. Although the PD can accept solid wire termination, stranded is better in my opinion. Solid wire on moving and vibrating objects is asking for wire failure due to flexing of a single conductor. Stranded wire handles vibration with far less metal fatigue.

Teched3
03-27-2011, 02:11 PM
Al,
Excellent point made on stranded wire compression at a terminal connection.:) :)

MikeE
03-27-2011, 06:36 PM
There is no reason to use solid core for any of the "large wires" i.e. 22 AWG or larger, but it's fairly common to see some of the bundled data wires (RJ12 telephone cable for CAN bus or ethernet cabling) using solid core rather than stranded. I'd still prefer to use stranded for these applications but IMHO it's less critical.

MagiChau
03-27-2011, 08:25 PM
If my memory serves me correctly the required power convertor for 12V to 5V solely for the bridge's power supply has solid wires for connecting it to the power supply & bridge.

Al Skierkiewicz
03-28-2011, 04:54 AM
Chau,
Those are pre-tinned stranded wire. You insert one side in the connector for the PD and solder the output to the power cable for the radio. Pre-tinned seems to indicate the primary or intended customer for this regulator would solder it to a circuit board.

wireties
03-28-2011, 10:25 AM
FYI - When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle. This effect is more pronounced on high frequency AC than it is on DC or low frequency AC. This means that a "wire" of a given size that made up of many smaller strands can carry more power than a solid wire - simply because the stranded wire has more surface area. Plus flexibilty is key on our robots and all the termination mechanisms we are given work best on stranded wire.

So... my professional opinion is there are no compelling reasons to use solid-core wire on a FRC robot (even if it was donated).

HTH

Al Skierkiewicz
03-28-2011, 11:39 AM
FYI - When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle.

Not exactly, the "skin effect" is more pronounced by higher voltages and higher frequencies. While the Jaguars may introduce this effect, I am guessing it is very small compared to the other losses in the system. There is very little effect at DC.

Jon Stratis
03-28-2011, 12:06 PM
Also keep in mind that there are different kinds of stranded wire. The number of strands and the insulation material can bake it hold a bend more or less easily. On our robot this year, we've got two different types of power wire. One is very stiff, and holds a bend very nicely. We use that for a vast majority of our wiring, so we can get nice, clean runs that aren't going to flop loose over time. The second type is much, much more flexible. It won't hold a bend at all. We use that for our elevator, since we require that wire to be constantly moving. Stiffer wire in this situation would be more prone to breaking over time.

Al Skierkiewicz
03-28-2011, 12:14 PM
The second type is much, much more flexible. It won't hold a bend at all.
This is sometimes marketed as "rope lay" or "superflex".

wireties
03-29-2011, 10:26 PM
Not exactly, the "skin effect" is more pronounced by higher voltages and higher frequencies. While the Jaguars may introduce this effect, I am guessing it is very small compared to the other losses in the system. There is very little effect at DC.

Jeez Al - did you miss the "This effect is more pronounced on high frequency AC than it is on DC or low frequency AC" caveat or the "flexibility is key" line in the original post? And "Very little" is not accurate. The robot is NOT a static load. The load is variable with components (of the load) related to the rotation of the motors etc. People confuse this all the time, DC means Direct Current, not Direct Voltage. And direct current does not always imply constant current. Nor is the robot's ground reference an infinite sink. Just because the source is a battery does not mean the load can't vary, perhaps with some periodicity. This effect in the robot is important enough to be aware of.

HTH

Al Skierkiewicz
03-29-2011, 10:42 PM
Keith,
The skin effect caused by AC currents in copper wire is about 8.5mm at 60 Hz or about twice the diameter of #10 wire (~4mm). At the switching frequency of the Victor of 150Hz, skin effect is not much different and at full throttle surely does not exist. At 15kHz, (the switching frequency of Jaguars) the skin effect is about the diameter of #17 wire. The difference between solid and stranded conductors is still minimal.

wireties
03-29-2011, 11:10 PM
Keith,
The skin effect caused by AC currents in copper wire is about 8.5mm at 60 Hz or about twice the diameter of #10 wire (~4mm). At the switching frequency of the Victor of 150Hz, skin effect is not much different and at full throttle surely does not exist. At 15kHz, (the switching frequency of Jaguars) the skin effect is about the diameter of #17 wire. The difference between solid and stranded conductors is still minimal.

Who said anything about the switching frequency of the Jaguars? I don't think I did. I'm talking about the variable load presented by the entire robot. I'll tell you what, bet you a dinner (if we are ever in the same town) that I can vary the load (might cost me a motor or 2) in such a way that there is a nice several volt AC signal modulating the DC supply provided by the battery. And that component (if over 400Hz or so) would travel more on the outside of the #6 AWG wire than the inside, more and more the higher the frequency. I never claimed this is a primary or even secondary factor, just something worth knowing about.

And the skin depth numbers you are talking about refer to the depth at which the current density decays to 1/e (0.37) of the surface density. It is a relative measurement and not a threshold, there is not zero decay under that depth. Indeed there is some decay at lower frequencies and with more constant loads (but not with zero load and/or a perfectly static load). We tend to think of the robot as a static system just because it uses a battery as a power source. That is not strictly true, often times not even close.

HTH

nuttle
03-29-2011, 11:30 PM
Skin effect is comparable between solid and stranded and isn't going to be a practical issue in this context anyway. If this were an issue, you'd likely want to use something called Litz wire. The reason this is so is because the fields and forces that produce the skin effect will operate in much the same way in a bundle of stranded wires that are shorted together along their length as in a single solid conductor.

Al Skierkiewicz
03-30-2011, 08:46 AM
Keith,
Before you get too much more enraged, let's get on the same page. I agree that there is significant modulation of the DC voltage (when measured at the battery terminals) with varying load. I believe most everyone by now knows that is a factor in much of our power related problems. However, that effect is due more to the resistances encountered internal to the battery, at each termination, and within the wire used on the robot rather than skin effect. While there are short duration pulse type loads put on the electrical delivery system, the majority of time, the power demand is at a steady throttle value predominated by the switching frequency of the relative controllers (at less than full throttle). I have witnessed pulses in the 10-100 micro second range particularly when changing directions or with noise induced from cheap joysticks. Anyone with a scope can see those pulses on their own robot.
This thread is discussing the relative merits of solid vs. stranded wire on our robots. While I agree that skin effect is a serious issue in many aspects of electronics, it is not the major contributor to the losses encountered in our application. While stranded wire as you have pointed out, is better at minimizing skin effects, any solid conductor wire that we see is either #14 or #12 house wire that can be purchased at a local home store. When driving a robot at a steady throttle, the ~.003ohms/ft. of #14 (solid or stranded) added to the termination resistance and internal resistance of the speed controller, is going to be the predominant loss.

wireties
03-30-2011, 09:44 AM
Keith,
Before you get too much more enraged, let's get on the same page

Al - over the years you have offered very good advice, saving my team many long hours - and we thank you for it. But your a little off the rails here and I'm not sure where you get the impression I am "enraged". This thread is about stranded vs solid wire on the robots. I pointed out a common reason an engineer might use stranded vs solid wire followed by stating that the effect is diminished at DC and that flexibility and tooling are the primary reasons to use stranded wire on the robot. This is not a very controversial statement. You quoted a single line from the post (leaving out relevant and qualifying conditions), added some skin effect numbers from wikipedia (BTW it is more like 9.8mm for good copper wire at 60Hz), proceeded to correct the post and stated a similar conclusion to my own. I thought it an odd thing to do. Perhaps I am too thin-skinned.

I've politely amended your posts a few times over the years and I apologize if you were/are upset or offended in any way. Next time you amend something I've posted, please (at a minimum) quote the relevant text. Done.

Kind Regards,
Keith

Al Skierkiewicz
03-30-2011, 04:51 PM
Keith,
OK, now I am confused. Are we in agreement? What did I amend?
I used the calculator for skin depth on this page...
http://www.daycounter.com/Calculators/ and then did a quick conversion to mm.
I often point ham radio students to pages like this when discussing RF. It is easy to demo reasons for using copperweld for antennas and to discuss why shields on coaxial cable work.

wireties
03-31-2011, 04:34 AM
Keith,
OK, now I am confused. Are we in agreement? What did I amend?
I used the calculator for skin depth on this page...
http://www.daycounter.com/Calculators/ and then did a quick conversion to mm.
I often point ham radio students to pages like this when discussing RF. It is easy to demo reasons for using copperweld for antennas and to discuss why shields on coaxial cable work.

We were never at odds, thats the weird thing. You disagreed with something I did not say. I think this topic is better off dead.

Kind Regards