Wire preference - stranded or solid

[Teched3]

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]

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]

Al,
Excellent point made on stranded wire compression at a terminal connection.

[MikeE]

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]

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]

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]

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]

Originally Posted by wireties

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]

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]

Originally Posted by eagle33199

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]

Originally Posted by Al Skierkiewicz

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]

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]

Originally Posted by Al Skierkiewicz

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]

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.