Aside from the resistive heating, if you have a coil and pass an
oscillating current though it a vibrating field is created which will
interact with any conductor sufficiently close to it - including itself.
Each "line for flux" creates an eddy in a conductive material it
intersects, like little tiny motors inside the material each line as it
rises and falls has a resulting tiny circular current around it - an eddy.
All this depends on the AC nature. DC current does not cause this after
its first rise, after switching. A regularly switched DC has the same
inductive effects as continuous AC.
If you pass AC in two directions along parallel wires, there is a
possibility they will cancel - but also a possibility they do not. I doubt
the cancellation is perfect. Is the magnitude great? I don't know. The
third conductor is presumably a ground in DC? It will have eddies in it.
In three phase AC where each phase is out from the others is ther any
cancellation of inductive effects? I don't know. I am a lowly ME.
It is worth noting that frequency of oscillation is very important and
material specific.
On Sun, Feb 23, 2014 at 10:22 AM, Evan Tuer <evan.t...@gmail.com> wrote:
> Because there's current flowing through a resistance, producing heat.
> Normally it's fine, the few Watts per metre lost as heat are easily
> conducted away from the wire. If you bundle lots of cable (therefore lots
> of Watts) into a small space, the temperature can build up until the wire
> insulation melts.
>
> Makes no difference whether it's AC or DC.
>
>
> On Sun, Feb 23, 2014 at 8:24 PM, Martin WINLOW <m...@winlow.co.uk> wrote:
>
> > "three conductor wire" - Do you mean 3-phase?
> >
> > Why do ordinary domestic AC mains cable reels heat up if used coiled,
> > then? MW
> >
> >
> > On 22 Feb 2014, at 17:26, Bill Dube wrote:
> >
> > > AC, not DC powered. Transformer changes ~13 kv distribution voltage
> down
> > to 480 vac.
> > >
> > > Cord reels ("coils") consist of three conductor wire with current
> > flowing equally in both directions, so there is no inductance associated
> > with the "coil". Thus no inductive heating from the AC.
> > >
> > > On 2/22/2014 3:55 AM, Martin WINLOW wrote:
> > >> Hi Bill,
> > >>
> > >> I'm assuming the power must be DC... or otherwise wouldn't the coils
> > get V hot? Or is this what the transformer you mentioned deals with?
> > >>
> > >> MW
> > >>
> > >>
> > >> On 21 Feb 2014, at 17:53, Bill Dube wrote:
> > >>
> > >>> Electric coal mining equipment is cord-connected. Even enormous
> > draglines ("steam" shovels on steroids) are typically cord connected with
> > 13 kV cables. It is a mature science. Coal mining equipment has been
> > electric powered since the early 1900's. Specialized cables, reels,
> > transformers, and equipment for the mining industry. Easily transferable
> > technology to agriculture.
> > >>>
> > >>> They typically put a reel on the back of the mining machine that pays
> > out (or pulls in) the cable as needed. They have a "load center"
> > transformer that is moved around as the mining process proceeds.
> > >>>
> > >>> Electric shuttle car for underground coal transport from working
> face:
> > >>> http://www.undergroundcoal.com.au/fundamentals/11_face.aspx
> > >>>
> > >>> Nifty tutorial on cables for mining EVs:
> > >>> http://www.cdc.gov/niosh/mining/content/deratingfactors.html
> > >>>
> > >>> Dragline cable:
> > >>>
> >
> http://www.miningphoto.com/details/2140/dragline_working_in_background_removing_overburden_with_drill_rigs_drilling_blas
> > >>>
> > >>> Bill Dube'
> > >>> (Used to work as an Electrical Inspection intern for the Mine Safety
> > and Health Administration, MSHA)
> >
> > _______________________________________________
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> > http://groups.yahoo.com/group/NEDRA)
> >
> >
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