So...battery cables and "skin depth"

[lastcowboy32]

This was brought up in another thread, I was sort of dismissive of the notion.

But, I was looking up wire ampacity for another reason (we just got our old NH155 hay elevator that we bought for scrap price going...and now I want to make a good 240V wiring system for it)...

Anyway, the first chart I looked up online did have a skin depth chart along with ampacity.

What would a good "frequency" be for a Ford N tractor?

To me... I did

(C * R)/60 and multiplied by 10

C is # of cylinders... 4 cylinders
R is max RPMs...2000 RPMs

multiplying by 10 accounts for the fact that a spark has spectral content far beyond its repetition rate

A limiting frequency for a Ford N tractor, by this formula is...

1333 Hz... or 1.3 KHz

It was mentioned in another thread that race cars use welding cable vs battery cable for better skin depth performance.

So... C = 8
....... R = 10,000

For a 8 cylinder, 10,000 RPM race car, the limiting frequency would be about 10 times higher, at 13,333Hz or 13.3 KHz

BUT...

Mostly a moot point.

Why? Because skin depth tables assume a solid conductor. I've never seen a 2AWG solid battery cable. It's always stranded.

And... according to at least one set of specs, modern welding cable and battery cable are stranded identically (655 strands of 30AWG wire in both cases to make a 2AWG cable)

With 30AWG stranding, skin depth isn't a problem until all of the way up at...oh...270KHz

But, some old battery cables are coarsely stranded, yes indeed, the battery cables on my old 2N and a Ford 640 that I used to have...were inflexible bears that had strands that could be counted...none of this 655 strands of 30AWG that you get nowadays.

They would still have to be really coarsely stranded... with something like 16AWG solid wire used for the stranding.

So... long story short, your Ford N tractor doesn't care about the stranding of your battery cable.

Your brand new 10,000 RPM race car will do just fine with modern battery cable or modern welding cable... both are stranded the same.

If you went and put your old, coarsely stranded, Ford N tractor battery cable on your 10,000 RPM race car... you may have an issue.



https://www.powerstream.com/Wire_Size.htm

 

[oldsarge]

Now there's a whole pile of fascinatin' information that don't have diddley to do with the price of
hot dogs @ Wrigley Field !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[lastcowboy32]

I agree.

But at least you now know why it doesn't matter.

;-)

Seriously, this was one of those things where I said the same thing you did...and then I thought...well, is there something to this?

Definitely not for your Ford N tractor.

Maybe, possibly, but probably not, for your race car.

Definitely for your welder, but mostly for flexibility

Definitely, if you are designing an AM radio broadcast station, and you need to conduct 25KW of power at 1013.2KHz a few hundred feet up a tower.

 

[JMOR]

Why are you mixing AC/DC, starter cables & engine RPM when talking skin depth?

 

[HFJ]

(quoted from post at 16:27:19 08/24/23) Why are you mixing AC/DC, starter cables & engine RPM when talking skin depth?

as much as i love Angus Young, Ernie Ball is pretty tight-lipped about the size and number of wires used to make the Power Slinky strings Angus prefers. accordingly, i'd have left AC/DC out of this.

 

[lastcowboy32]

(quoted from post at 12:27:19 08/24/23) Why are you mixing AC/DC, starter cables & engine RPM when talking skin depth?

Good question.

Mostly because the ultimate energy source for starting and ignition is the generator (or alternator)/battery circuit.

All of the DC needs (starting, lights etc) and transient needs (spark, etc) will meet there.

The transient needs (ignition) will also meet there... but they won't have the frequency content that the ultimate ignition signals will have. It will be low pass filtered.

A good analogy is this... Power from the battery/generator circuit the to coil is just a steady flow of many electrons (mostly)... like four lanes of traffic going 5mph in a construction delay...you have the points as the flag man at the end, starting and stopping that flow at a certain rep rate. (that was what my formula took into account)

Once this energy is transformed to the secondary side...that's like getting past the flag man and cars can accelerate and zip off to their destination in the various lane of their choosing...the fastest cars travel in the outside lanes, unimpeded by the congestion in the center "skin depth"...

In that respect, to my thinking, skin depth only matters in ignition wires (coil wire and spark plug wires), to that effect, you will see hollow core wires (coaxial) used in both applications. This is where the high frequency stuff is.

Any talk about skin depth between the battery and starter would be some phenomenon that I haven't considered...maybe the initial initiation of the bendix. But I suspect it's not a big deal.

 

[lastcowboy32]

Short answer is... I was looking up wire that could carry 30Amps of 240VAC for an elevator... (where skin depth also does not matter)...

the fact that the ampacity table had a skin depth column triggered my ADHD brain about a recent string of replies on here where it was suggested that an N tractor wouldn't start due to skin depth and the inadequate stranding of a battery cable...

 

[Tony in SD]

I can only assume the skin depth is referring to SKIN EFFECT which is where then electrons only flow on the SKIN or outer layer of the cable. Skin effect is not an issue at low voltages or frequencies.
6 or even 12 VDC is too low to have any effect on voltage drop. And being a DC circuit the frequency is 0 Hz. So again no effect on voltage drop. Of course, if skin depth means something other than skin effect, then never mind what I said.
Tony BSEE

 

[Dave G9N]

(quoted from post at 15:37:12 08/25/23) I can only assume the skin depth is referring to SKIN EFFECT which is where then electrons only flow on the SKIN or outer layer of the cable. Skin effect is not an issue at low voltages or frequencies.
6 or even 12 VDC is too low to have any effect on voltage drop. And being a DC circuit the frequency is 0 Hz. So again no effect on voltage drop. Of course, if skin depth means something other than skin effect, then never mind what I said.
Tony BSEE

he spark is DC, but more of a static discharge than a normal DC current. Like lightning, DC, but a high frequency signal at the same time. So the short time it takes for the voltage to rise and fall is inducing magnetic fields and, uhm, and I don't have a BSEE, and have run completely out of gas.