Chris wrote:> > This may be a stupid question, but here goes. > We all know that cable is graded for its current carrying capabilities > according to its cross-sectional area. BUT, could one conceivably pass > excessive amounts of current through a cable not rated to carry it by > pulsing the current in short bursts at a very low duty cycle? >** Cable current ratings are given in amps rms - so the heating effect of the current is what matters, not its peak or average value. For the same reason, circuit breakers are thermally operated, responding to the rms value of whatever current is flowing in the wire. The most efficient use of a cable occurs with DC current. .... Phil
Cables and Currents
Started by ●March 3, 2017
Reply by ●March 4, 20172017-03-04
Reply by ●March 4, 20172017-03-04
On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote:> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> > wrote: > > >This may be a stupid question, but here goes. > >We all know that cable is graded for its current carrying capabilities > >according to its cross-sectional area. BUT, could one conceivably pass > >excessive amounts of current through a cable not rated to carry it by > >pulsing the current in short bursts at a very low duty cycle? > > Sure. The cable heats up from the current (current squared, > approximately) and has some heat storage capacity. So you can really > whack it for a short time, milliseconds to tens of seconds maybe, > before the copper gets too hot. > > Wire can handle a lot of current if you cool it, too. Most power > wiring stuff assumes that wires are inside jackets, inside walls > maybe, where there's not much cooling. So power wire is conservatively > rated for current. > > Pulse bursts don't increase the long-term RMS current capacity of a > wire. They actully reduce it. > > (Which could restart the argument about "average RMS current.") > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.comFor thing like magnet coils (we do mostly air coils) you can totally run 'em high, we have one instrument, that limits the duty cycle... up to a 20 second period. For a hunk of copper, there should be some current, that raises the piece 1 deg K/ sec. (Well at least for small changes in T) A related question, (and currently of more interest to me. NPI) is how much current can a wire carry in vacuum. I've got some graphs on my computer at work, but I'm not sure I believe them.... The wire is phosphor-bronze, this looked good, but they didn't model radiation..? https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf I've never heard of this, but do people paint their wires black? Better radiators. George H.
Reply by ●March 4, 20172017-03-04
On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >> wrote: >> >> >This may be a stupid question, but here goes. >> >We all know that cable is graded for its current carrying capabilities >> >according to its cross-sectional area. BUT, could one conceivably pass >> >excessive amounts of current through a cable not rated to carry it by >> >pulsing the current in short bursts at a very low duty cycle? >> >> Sure. The cable heats up from the current (current squared, >> approximately) and has some heat storage capacity. So you can really >> whack it for a short time, milliseconds to tens of seconds maybe, >> before the copper gets too hot. >> >> Wire can handle a lot of current if you cool it, too. Most power >> wiring stuff assumes that wires are inside jackets, inside walls >> maybe, where there's not much cooling. So power wire is conservatively >> rated for current. >> >> Pulse bursts don't increase the long-term RMS current capacity of a >> wire. They actully reduce it. >> >> (Which could restart the argument about "average RMS current.") >> >> >> -- >> >> John Larkin Highland Technology, Inc >> picosecond timing precision measurement >> >> jlarkin att highlandtechnology dott com >> http://www.highlandtechnology.com > >For thing like magnet coils (we do mostly air coils) >you can totally run 'em high, we have one instrument, >that limits the duty cycle... up to a 20 second period.Big (non-superconducting) electromagnets are usually water cooled. They have a lot of copper volume to surface area ratio, so get hot.> >For a hunk of copper, there should be some current, that >raises the piece 1 deg K/ sec. (Well at least for small changes in T) > >A related question, (and currently of more interest to me. NPI) >is how much current can a wire carry in vacuum. >I've got some graphs on my computer at work, but I'm not sure I believe >them.... The wire is phosphor-bronze, >this looked good, but they didn't model radiation..? >https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf > >I've never heard of this, but do people paint their >wires black? Better radiators.Most shiny metals have low emissivities at thermal wavelengths. Copper is an almost perfect mirror at thermal IR. So in a hard vacuum, practically the only cooling will be conduction to the end terminations. Making the wire black (at thermal wavelengths!) would really help. Smashing it into a ribbon would increase the surface area, too. Painted or insulated wire is better than bare metal, unless you can run literally red hot. Most organics have high emissivity. Interesting experiment: try bare copper wire vs magnet wire, same sizes, same current, in vacuum. Inferr the temperature from the resistance. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●March 6, 20172017-03-06
On 03/04/2017 07:22 PM, John Larkin wrote:> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > >> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >>> wrote: >>> >>>> This may be a stupid question, but here goes. >>>> We all know that cable is graded for its current carrying capabilities >>>> according to its cross-sectional area. BUT, could one conceivably pass >>>> excessive amounts of current through a cable not rated to carry it by >>>> pulsing the current in short bursts at a very low duty cycle? >>> >>> Sure. The cable heats up from the current (current squared, >>> approximately) and has some heat storage capacity. So you can really >>> whack it for a short time, milliseconds to tens of seconds maybe, >>> before the copper gets too hot. >>> >>> Wire can handle a lot of current if you cool it, too. Most power >>> wiring stuff assumes that wires are inside jackets, inside walls >>> maybe, where there's not much cooling. So power wire is conservatively >>> rated for current. >>> >>> Pulse bursts don't increase the long-term RMS current capacity of a >>> wire. They actully reduce it. >>> >>> (Which could restart the argument about "average RMS current.") >>> >>> >>> -- >>> >>> John Larkin Highland Technology, Inc >>> picosecond timing precision measurement >>> >>> jlarkin att highlandtechnology dott com >>> http://www.highlandtechnology.com >> >> For thing like magnet coils (we do mostly air coils) >> you can totally run 'em high, we have one instrument, >> that limits the duty cycle... up to a 20 second period. > > Big (non-superconducting) electromagnets are usually water cooled. > They have a lot of copper volume to surface area ratio, so get hot. > >> >> For a hunk of copper, there should be some current, that >> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >> >> A related question, (and currently of more interest to me. NPI) >> is how much current can a wire carry in vacuum. >> I've got some graphs on my computer at work, but I'm not sure I believe >> them.... The wire is phosphor-bronze, >> this looked good, but they didn't model radiation..? >> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >> >> I've never heard of this, but do people paint their >> wires black? Better radiators. > > Most shiny metals have low emissivities at thermal wavelengths. Copper > is an almost perfect mirror at thermal IR. So in a hard vacuum, > practically the only cooling will be conduction to the end > terminations. > > Making the wire black (at thermal wavelengths!) would really help. > Smashing it into a ribbon would increase the surface area, too. > > Painted or insulated wire is better than bare metal, unless you can > run literally red hot. Most organics have high emissivity. > > Interesting experiment: try bare copper wire vs magnet wire, same > sizes, same current, in vacuum. Inferr the temperature from the > resistance. > >Thick black copper oxide has an emissivity of about 0.78 in the thermal IR, according to <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 6, 20172017-03-06
On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 03/04/2017 07:22 PM, John Larkin wrote: >> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold >> <gherold@teachspin.com> wrote: >> >>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >>>> wrote: >>>> >>>>> This may be a stupid question, but here goes. >>>>> We all know that cable is graded for its current carrying capabilities >>>>> according to its cross-sectional area. BUT, could one conceivably pass >>>>> excessive amounts of current through a cable not rated to carry it by >>>>> pulsing the current in short bursts at a very low duty cycle? >>>> >>>> Sure. The cable heats up from the current (current squared, >>>> approximately) and has some heat storage capacity. So you can really >>>> whack it for a short time, milliseconds to tens of seconds maybe, >>>> before the copper gets too hot. >>>> >>>> Wire can handle a lot of current if you cool it, too. Most power >>>> wiring stuff assumes that wires are inside jackets, inside walls >>>> maybe, where there's not much cooling. So power wire is conservatively >>>> rated for current. >>>> >>>> Pulse bursts don't increase the long-term RMS current capacity of a >>>> wire. They actully reduce it. >>>> >>>> (Which could restart the argument about "average RMS current.") >>>> >>>> >>>> -- >>>> >>>> John Larkin Highland Technology, Inc >>>> picosecond timing precision measurement >>>> >>>> jlarkin att highlandtechnology dott com >>>> http://www.highlandtechnology.com >>> >>> For thing like magnet coils (we do mostly air coils) >>> you can totally run 'em high, we have one instrument, >>> that limits the duty cycle... up to a 20 second period. >> >> Big (non-superconducting) electromagnets are usually water cooled. >> They have a lot of copper volume to surface area ratio, so get hot. >> >>> >>> For a hunk of copper, there should be some current, that >>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >>> >>> A related question, (and currently of more interest to me. NPI) >>> is how much current can a wire carry in vacuum. >>> I've got some graphs on my computer at work, but I'm not sure I believe >>> them.... The wire is phosphor-bronze, >>> this looked good, but they didn't model radiation..? >>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >>> >>> I've never heard of this, but do people paint their >>> wires black? Better radiators. >> >> Most shiny metals have low emissivities at thermal wavelengths. Copper >> is an almost perfect mirror at thermal IR. So in a hard vacuum, >> practically the only cooling will be conduction to the end >> terminations. >> >> Making the wire black (at thermal wavelengths!) would really help. >> Smashing it into a ribbon would increase the surface area, too. >> >> Painted or insulated wire is better than bare metal, unless you can >> run literally red hot. Most organics have high emissivity. >> >> Interesting experiment: try bare copper wire vs magnet wire, same >> sizes, same current, in vacuum. Inferr the temperature from the >> resistance. >> >> > >Thick black copper oxide has an emissivity of about 0.78 in the thermal >IR, according to ><http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> > >Cheers > >Phil HobbsI guess that copper will get hot and tarnish some. Not to change the subject, but regular office white-out has a very high emissivity. So you can dab it on shiny things, like the metal tops of some FPGAs and such, to read the temps better. Kapton tape is pretty good. https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 7, 20172017-03-07
On 03/06/2017 05:17 PM, John Larkin wrote:> On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 03/04/2017 07:22 PM, John Larkin wrote: >>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold >>> <gherold@teachspin.com> wrote: >>> >>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >>>>> wrote: >>>>> >>>>>> This may be a stupid question, but here goes. >>>>>> We all know that cable is graded for its current carrying capabilities >>>>>> according to its cross-sectional area. BUT, could one conceivably pass >>>>>> excessive amounts of current through a cable not rated to carry it by >>>>>> pulsing the current in short bursts at a very low duty cycle? >>>>> >>>>> Sure. The cable heats up from the current (current squared, >>>>> approximately) and has some heat storage capacity. So you can really >>>>> whack it for a short time, milliseconds to tens of seconds maybe, >>>>> before the copper gets too hot. >>>>> >>>>> Wire can handle a lot of current if you cool it, too. Most power >>>>> wiring stuff assumes that wires are inside jackets, inside walls >>>>> maybe, where there's not much cooling. So power wire is conservatively >>>>> rated for current. >>>>> >>>>> Pulse bursts don't increase the long-term RMS current capacity of a >>>>> wire. They actully reduce it. >>>>> >>>>> (Which could restart the argument about "average RMS current.") >>>>> >>>>> >>>>> -- >>>>> >>>>> John Larkin Highland Technology, Inc >>>>> picosecond timing precision measurement >>>>> >>>>> jlarkin att highlandtechnology dott com >>>>> http://www.highlandtechnology.com >>>> >>>> For thing like magnet coils (we do mostly air coils) >>>> you can totally run 'em high, we have one instrument, >>>> that limits the duty cycle... up to a 20 second period. >>> >>> Big (non-superconducting) electromagnets are usually water cooled. >>> They have a lot of copper volume to surface area ratio, so get hot. >>> >>>> >>>> For a hunk of copper, there should be some current, that >>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >>>> >>>> A related question, (and currently of more interest to me. NPI) >>>> is how much current can a wire carry in vacuum. >>>> I've got some graphs on my computer at work, but I'm not sure I believe >>>> them.... The wire is phosphor-bronze, >>>> this looked good, but they didn't model radiation..? >>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >>>> >>>> I've never heard of this, but do people paint their >>>> wires black? Better radiators. >>> >>> Most shiny metals have low emissivities at thermal wavelengths. Copper >>> is an almost perfect mirror at thermal IR. So in a hard vacuum, >>> practically the only cooling will be conduction to the end >>> terminations. >>> >>> Making the wire black (at thermal wavelengths!) would really help. >>> Smashing it into a ribbon would increase the surface area, too. >>> >>> Painted or insulated wire is better than bare metal, unless you can >>> run literally red hot. Most organics have high emissivity. >>> >>> Interesting experiment: try bare copper wire vs magnet wire, same >>> sizes, same current, in vacuum. Inferr the temperature from the >>> resistance. >>> >>> >> >> Thick black copper oxide has an emissivity of about 0.78 in the thermal >> IR, according to >> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> >> >> Cheers >> >> Phil Hobbs > > I guess that copper will get hot and tarnish some. > > Not to change the subject, but regular office white-out has a very > high emissivity. So you can dab it on shiny things, like the metal > tops of some FPGAs and such, to read the temps better. > > Kapton tape is pretty good. > > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG > > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg > > >Just about any dielectric at least a few mils thick has a thermal emissivity of about 0.95. The rest is Fresnel reflection at the surface. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 7, 20172017-03-07
On Tuesday, March 7, 2017 at 9:33:38 AM UTC-5, Phil Hobbs wrote:> On 03/06/2017 05:17 PM, John Larkin wrote: > > On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs > > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > >> On 03/04/2017 07:22 PM, John Larkin wrote: > >>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold > >>> <gherold@teachspin.com> wrote: > >>> > >>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: > >>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> > >>>>> wrote: > >>>>> > >>>>>> This may be a stupid question, but here goes. > >>>>>> We all know that cable is graded for its current carrying capabilities > >>>>>> according to its cross-sectional area. BUT, could one conceivably pass > >>>>>> excessive amounts of current through a cable not rated to carry it by > >>>>>> pulsing the current in short bursts at a very low duty cycle? > >>>>> > >>>>> Sure. The cable heats up from the current (current squared, > >>>>> approximately) and has some heat storage capacity. So you can really > >>>>> whack it for a short time, milliseconds to tens of seconds maybe, > >>>>> before the copper gets too hot. > >>>>> > >>>>> Wire can handle a lot of current if you cool it, too. Most power > >>>>> wiring stuff assumes that wires are inside jackets, inside walls > >>>>> maybe, where there's not much cooling. So power wire is conservatively > >>>>> rated for current. > >>>>> > >>>>> Pulse bursts don't increase the long-term RMS current capacity of a > >>>>> wire. They actully reduce it. > >>>>> > >>>>> (Which could restart the argument about "average RMS current.") > >>>>> > >>>>> > >>>>> -- > >>>>> > >>>>> John Larkin Highland Technology, Inc > >>>>> picosecond timing precision measurement > >>>>> > >>>>> jlarkin att highlandtechnology dott com > >>>>> http://www.highlandtechnology.com > >>>> > >>>> For thing like magnet coils (we do mostly air coils) > >>>> you can totally run 'em high, we have one instrument, > >>>> that limits the duty cycle... up to a 20 second period. > >>> > >>> Big (non-superconducting) electromagnets are usually water cooled. > >>> They have a lot of copper volume to surface area ratio, so get hot. > >>> > >>>> > >>>> For a hunk of copper, there should be some current, that > >>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) > >>>> > >>>> A related question, (and currently of more interest to me. NPI) > >>>> is how much current can a wire carry in vacuum. > >>>> I've got some graphs on my computer at work, but I'm not sure I believe > >>>> them.... The wire is phosphor-bronze, > >>>> this looked good, but they didn't model radiation..? > >>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf > >>>> > >>>> I've never heard of this, but do people paint their > >>>> wires black? Better radiators. > >>> > >>> Most shiny metals have low emissivities at thermal wavelengths. Copper > >>> is an almost perfect mirror at thermal IR. So in a hard vacuum, > >>> practically the only cooling will be conduction to the end > >>> terminations. > >>> > >>> Making the wire black (at thermal wavelengths!) would really help. > >>> Smashing it into a ribbon would increase the surface area, too. > >>> > >>> Painted or insulated wire is better than bare metal, unless you can > >>> run literally red hot. Most organics have high emissivity. > >>> > >>> Interesting experiment: try bare copper wire vs magnet wire, same > >>> sizes, same current, in vacuum. Inferr the temperature from the > >>> resistance. > >>> > >>> > >> > >> Thick black copper oxide has an emissivity of about 0.78 in the thermal > >> IR, according to > >> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> > >> > >> Cheers > >> > >> Phil Hobbs > > > > I guess that copper will get hot and tarnish some. > > > > Not to change the subject, but regular office white-out has a very > > high emissivity. So you can dab it on shiny things, like the metal > > tops of some FPGAs and such, to read the temps better. > > > > Kapton tape is pretty good. > > > > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG > > > > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg > > > > > > > Just about any dielectric at least a few mils thick has a thermal > emissivity of about 0.95. The rest is Fresnel reflection at the surface.Fresnel reflection... I had to look it up. (dielectric mismatch) Is this for IR wavelengths? Certainly off white paint is higher than that in the visible. Maybe I can get the wire with a triple or quadruple build of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter. George H.> > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
Reply by ●March 7, 20172017-03-07
On 03/07/2017 09:51 AM, George Herold wrote:> On Tuesday, March 7, 2017 at 9:33:38 AM UTC-5, Phil Hobbs wrote: >> On 03/06/2017 05:17 PM, John Larkin wrote: >>> On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 03/04/2017 07:22 PM, John Larkin wrote: >>>>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold >>>>> <gherold@teachspin.com> wrote: >>>>> >>>>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >>>>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >>>>>>> wrote: >>>>>>> >>>>>>>> This may be a stupid question, but here goes. >>>>>>>> We all know that cable is graded for its current carrying capabilities >>>>>>>> according to its cross-sectional area. BUT, could one conceivably pass >>>>>>>> excessive amounts of current through a cable not rated to carry it by >>>>>>>> pulsing the current in short bursts at a very low duty cycle? >>>>>>> >>>>>>> Sure. The cable heats up from the current (current squared, >>>>>>> approximately) and has some heat storage capacity. So you can really >>>>>>> whack it for a short time, milliseconds to tens of seconds maybe, >>>>>>> before the copper gets too hot. >>>>>>> >>>>>>> Wire can handle a lot of current if you cool it, too. Most power >>>>>>> wiring stuff assumes that wires are inside jackets, inside walls >>>>>>> maybe, where there's not much cooling. So power wire is conservatively >>>>>>> rated for current. >>>>>>> >>>>>>> Pulse bursts don't increase the long-term RMS current capacity of a >>>>>>> wire. They actully reduce it. >>>>>>> >>>>>>> (Which could restart the argument about "average RMS current.") >>>>>>> >>>>>>> >>>>>>> -- >>>>>>> >>>>>>> John Larkin Highland Technology, Inc >>>>>>> picosecond timing precision measurement >>>>>>> >>>>>>> jlarkin att highlandtechnology dott com >>>>>>> http://www.highlandtechnology.com >>>>>> >>>>>> For thing like magnet coils (we do mostly air coils) >>>>>> you can totally run 'em high, we have one instrument, >>>>>> that limits the duty cycle... up to a 20 second period. >>>>> >>>>> Big (non-superconducting) electromagnets are usually water cooled. >>>>> They have a lot of copper volume to surface area ratio, so get hot. >>>>> >>>>>> >>>>>> For a hunk of copper, there should be some current, that >>>>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >>>>>> >>>>>> A related question, (and currently of more interest to me. NPI) >>>>>> is how much current can a wire carry in vacuum. >>>>>> I've got some graphs on my computer at work, but I'm not sure I believe >>>>>> them.... The wire is phosphor-bronze, >>>>>> this looked good, but they didn't model radiation..? >>>>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >>>>>> >>>>>> I've never heard of this, but do people paint their >>>>>> wires black? Better radiators. >>>>> >>>>> Most shiny metals have low emissivities at thermal wavelengths. Copper >>>>> is an almost perfect mirror at thermal IR. So in a hard vacuum, >>>>> practically the only cooling will be conduction to the end >>>>> terminations. >>>>> >>>>> Making the wire black (at thermal wavelengths!) would really help. >>>>> Smashing it into a ribbon would increase the surface area, too. >>>>> >>>>> Painted or insulated wire is better than bare metal, unless you can >>>>> run literally red hot. Most organics have high emissivity. >>>>> >>>>> Interesting experiment: try bare copper wire vs magnet wire, same >>>>> sizes, same current, in vacuum. Inferr the temperature from the >>>>> resistance. >>>>> >>>>> >>>> >>>> Thick black copper oxide has an emissivity of about 0.78 in the thermal >>>> IR, according to >>>> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> I guess that copper will get hot and tarnish some. >>> >>> Not to change the subject, but regular office white-out has a very >>> high emissivity. So you can dab it on shiny things, like the metal >>> tops of some FPGAs and such, to read the temps better. >>> >>> Kapton tape is pretty good. >>> >>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG >>> >>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg >>> >>> >>> >> Just about any dielectric at least a few mils thick has a thermal >> emissivity of about 0.95. The rest is Fresnel reflection at the surface. > > Fresnel reflection... I had to look it up. (dielectric mismatch) > Is this for IR wavelengths? Certainly off white paint is higher than > that in the visible. > Maybe I can get the wire with a triple or quadruple build > of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter.Emissivity, not reflectance. Blackness is what you want for thermal measurements. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 7, 20172017-03-07
On Tue, 7 Mar 2017 06:51:48 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Tuesday, March 7, 2017 at 9:33:38 AM UTC-5, Phil Hobbs wrote: >> On 03/06/2017 05:17 PM, John Larkin wrote: >> > On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs >> > <pcdhSpamMeSenseless@electrooptical.net> wrote: >> > >> >> On 03/04/2017 07:22 PM, John Larkin wrote: >> >>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold >> >>> <gherold@teachspin.com> wrote: >> >>> >> >>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >> >>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >> >>>>> wrote: >> >>>>> >> >>>>>> This may be a stupid question, but here goes. >> >>>>>> We all know that cable is graded for its current carrying capabilities >> >>>>>> according to its cross-sectional area. BUT, could one conceivably pass >> >>>>>> excessive amounts of current through a cable not rated to carry it by >> >>>>>> pulsing the current in short bursts at a very low duty cycle? >> >>>>> >> >>>>> Sure. The cable heats up from the current (current squared, >> >>>>> approximately) and has some heat storage capacity. So you can really >> >>>>> whack it for a short time, milliseconds to tens of seconds maybe, >> >>>>> before the copper gets too hot. >> >>>>> >> >>>>> Wire can handle a lot of current if you cool it, too. Most power >> >>>>> wiring stuff assumes that wires are inside jackets, inside walls >> >>>>> maybe, where there's not much cooling. So power wire is conservatively >> >>>>> rated for current. >> >>>>> >> >>>>> Pulse bursts don't increase the long-term RMS current capacity of a >> >>>>> wire. They actully reduce it. >> >>>>> >> >>>>> (Which could restart the argument about "average RMS current.") >> >>>>> >> >>>>> >> >>>>> -- >> >>>>> >> >>>>> John Larkin Highland Technology, Inc >> >>>>> picosecond timing precision measurement >> >>>>> >> >>>>> jlarkin att highlandtechnology dott com >> >>>>> http://www.highlandtechnology.com >> >>>> >> >>>> For thing like magnet coils (we do mostly air coils) >> >>>> you can totally run 'em high, we have one instrument, >> >>>> that limits the duty cycle... up to a 20 second period. >> >>> >> >>> Big (non-superconducting) electromagnets are usually water cooled. >> >>> They have a lot of copper volume to surface area ratio, so get hot. >> >>> >> >>>> >> >>>> For a hunk of copper, there should be some current, that >> >>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >> >>>> >> >>>> A related question, (and currently of more interest to me. NPI) >> >>>> is how much current can a wire carry in vacuum. >> >>>> I've got some graphs on my computer at work, but I'm not sure I believe >> >>>> them.... The wire is phosphor-bronze, >> >>>> this looked good, but they didn't model radiation..? >> >>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >> >>>> >> >>>> I've never heard of this, but do people paint their >> >>>> wires black? Better radiators. >> >>> >> >>> Most shiny metals have low emissivities at thermal wavelengths. Copper >> >>> is an almost perfect mirror at thermal IR. So in a hard vacuum, >> >>> practically the only cooling will be conduction to the end >> >>> terminations. >> >>> >> >>> Making the wire black (at thermal wavelengths!) would really help. >> >>> Smashing it into a ribbon would increase the surface area, too. >> >>> >> >>> Painted or insulated wire is better than bare metal, unless you can >> >>> run literally red hot. Most organics have high emissivity. >> >>> >> >>> Interesting experiment: try bare copper wire vs magnet wire, same >> >>> sizes, same current, in vacuum. Inferr the temperature from the >> >>> resistance. >> >>> >> >>> >> >> >> >> Thick black copper oxide has an emissivity of about 0.78 in the thermal >> >> IR, according to >> >> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> >> >> >> >> Cheers >> >> >> >> Phil Hobbs >> > >> > I guess that copper will get hot and tarnish some. >> > >> > Not to change the subject, but regular office white-out has a very >> > high emissivity. So you can dab it on shiny things, like the metal >> > tops of some FPGAs and such, to read the temps better. >> > >> > Kapton tape is pretty good. >> > >> > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG >> > >> > https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg >> > >> > >> > >> Just about any dielectric at least a few mils thick has a thermal >> emissivity of about 0.95. The rest is Fresnel reflection at the surface. > >Fresnel reflection... I had to look it up. (dielectric mismatch) >Is this for IR wavelengths? Certainly off white paint is higher than >that in the visible. >Maybe I can get the wire with a triple or quadruple build >of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter. > >George H.The more insulation, the cooler the wire! A universe full of plastic conducts heat better than a universe full of vacuum. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●March 7, 20172017-03-07
On 03/07/2017 11:49 AM, John Larkin wrote:> On Tue, 7 Mar 2017 06:51:48 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > >> On Tuesday, March 7, 2017 at 9:33:38 AM UTC-5, Phil Hobbs wrote: >>> On 03/06/2017 05:17 PM, John Larkin wrote: >>>> On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 03/04/2017 07:22 PM, John Larkin wrote: >>>>>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold >>>>>> <gherold@teachspin.com> wrote: >>>>>> >>>>>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote: >>>>>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> This may be a stupid question, but here goes. >>>>>>>>> We all know that cable is graded for its current carrying capabilities >>>>>>>>> according to its cross-sectional area. BUT, could one conceivably pass >>>>>>>>> excessive amounts of current through a cable not rated to carry it by >>>>>>>>> pulsing the current in short bursts at a very low duty cycle? >>>>>>>> >>>>>>>> Sure. The cable heats up from the current (current squared, >>>>>>>> approximately) and has some heat storage capacity. So you can really >>>>>>>> whack it for a short time, milliseconds to tens of seconds maybe, >>>>>>>> before the copper gets too hot. >>>>>>>> >>>>>>>> Wire can handle a lot of current if you cool it, too. Most power >>>>>>>> wiring stuff assumes that wires are inside jackets, inside walls >>>>>>>> maybe, where there's not much cooling. So power wire is conservatively >>>>>>>> rated for current. >>>>>>>> >>>>>>>> Pulse bursts don't increase the long-term RMS current capacity of a >>>>>>>> wire. They actully reduce it. >>>>>>>> >>>>>>>> (Which could restart the argument about "average RMS current.") >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> >>>>>>>> John Larkin Highland Technology, Inc >>>>>>>> picosecond timing precision measurement >>>>>>>> >>>>>>>> jlarkin att highlandtechnology dott com >>>>>>>> http://www.highlandtechnology.com >>>>>>> >>>>>>> For thing like magnet coils (we do mostly air coils) >>>>>>> you can totally run 'em high, we have one instrument, >>>>>>> that limits the duty cycle... up to a 20 second period. >>>>>> >>>>>> Big (non-superconducting) electromagnets are usually water cooled. >>>>>> They have a lot of copper volume to surface area ratio, so get hot. >>>>>> >>>>>>> >>>>>>> For a hunk of copper, there should be some current, that >>>>>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T) >>>>>>> >>>>>>> A related question, (and currently of more interest to me. NPI) >>>>>>> is how much current can a wire carry in vacuum. >>>>>>> I've got some graphs on my computer at work, but I'm not sure I believe >>>>>>> them.... The wire is phosphor-bronze, >>>>>>> this looked good, but they didn't model radiation..? >>>>>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf >>>>>>> >>>>>>> I've never heard of this, but do people paint their >>>>>>> wires black? Better radiators. >>>>>> >>>>>> Most shiny metals have low emissivities at thermal wavelengths. Copper >>>>>> is an almost perfect mirror at thermal IR. So in a hard vacuum, >>>>>> practically the only cooling will be conduction to the end >>>>>> terminations. >>>>>> >>>>>> Making the wire black (at thermal wavelengths!) would really help. >>>>>> Smashing it into a ribbon would increase the surface area, too. >>>>>> >>>>>> Painted or insulated wire is better than bare metal, unless you can >>>>>> run literally red hot. Most organics have high emissivity. >>>>>> >>>>>> Interesting experiment: try bare copper wire vs magnet wire, same >>>>>> sizes, same current, in vacuum. Inferr the temperature from the >>>>>> resistance. >>>>>> >>>>>> >>>>> >>>>> Thick black copper oxide has an emissivity of about 0.78 in the thermal >>>>> IR, according to >>>>> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf> >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> I guess that copper will get hot and tarnish some. >>>> >>>> Not to change the subject, but regular office white-out has a very >>>> high emissivity. So you can dab it on shiny things, like the metal >>>> tops of some FPGAs and such, to read the temps better. >>>> >>>> Kapton tape is pretty good. >>>> >>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG >>>> >>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg >>>> >>>> >>>> >>> Just about any dielectric at least a few mils thick has a thermal >>> emissivity of about 0.95. The rest is Fresnel reflection at the surface. >> >> Fresnel reflection... I had to look it up. (dielectric mismatch) >> Is this for IR wavelengths? Certainly off white paint is higher than >> that in the visible. >> Maybe I can get the wire with a triple or quadruple build >> of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter. >> >> George H. > > The more insulation, the cooler the wire! A universe full of plastic > conducts heat better than a universe full of vacuum. > >T'other way round. Near room temperature, any thickness of vacuum looks like about 5 mm of air, iirc, which is 0.5 mm of plastic. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
