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 <[email protected]> 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) > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA ( > http://groups.yahoo.com/group/NEDRA) > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140223/7f3480ec/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
