Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can i generate a 370 A current?

  1. Jul 11, 2009 #1
    I need to generate a 20kOe magnetizing field in a 600 turn coil of inductance 0.58181032 microhenries, length 0.166116 m. Given the material I am magnetizing, I need to generate a current greater than 363.095 Amps for a period of about 0.5 s. I know of the use of pulse forming networks in creating large current pulses, but these are very expensive devices. Is there some simpler way to be able to get a single pulse off to meet my needs?
     
  2. jcsd
  3. Jul 11, 2009 #2

    negitron

    User Avatar
    Science Advisor

    According to this page:

    If you know someone who's into high-performance cars and they're willing to let you hook up, it might work.

    If you can make a .032 Ohm resistor rated for 4400 watts and connect it across a car battery, that'll work too.
     
    Last edited: Jul 11, 2009
  4. Jul 11, 2009 #3
    Find a weld shop with a larger machine. They easily go up to 500A or more. But, think about the fact that people find those currents useful for welding.
     
  5. Jul 11, 2009 #4
    yes clearly there will be a lot of heat generated in my wire, which is thermally rated at 155 C, however my pulse time is going to be less than a second, so i hope that the heat generated during this time will not raise my temperature above this value. Also, do welding machines use AC or DC current, because DC current is what i am after(although i do suppose turning on a 1HZ ac current for half a second will produce a 1 directional current)
     
  6. Jul 11, 2009 #5

    negitron

    User Avatar
    Science Advisor

    Welders can use either AC or DC depending on the specific weld process being used; some machines can do both. Most basic stick welders will only use 60 Hz AC.
     
  7. Jul 11, 2009 #6

    vk6kro

    User Avatar
    Science Advisor

    I need to generate a 20kOe magnetizing field in a 600 turn coil of inductance 0.58181032 microhenries, length 0.166116 m. Given the material I am magnetizing, I need to generate a current greater than 363.095 Amps for a period of about 0.5 s. I know of the use of pulse forming networks in creating large current pulses, but these are very expensive devices. Is there some simpler way to be able to get a single pulse off to meet my needs?

    I think you might have the inductance wrong. That is quite a large coil and would likely have a much greater inductance than 0.58uH. Maybe it was MilliHenrys?
    A coil of 6 inch length 3 inch diameter and 600 turns would have an inductance of about 11 millihenries or 11000 uH.

    Also, the wire in the coil would need to be very thick to carry such currents, even for a short burst.
    Could you measure the resistance of the coil ?
    If it had a resistance of even ONE ohm, it would need a DC power source of 363 Volts at 363 amps or 131 KW (or about 1200 amps from a 110 volt supply.)
     
    Last edited: Jul 11, 2009
  8. Jul 11, 2009 #7
    From my understanding:

    -dΦ/dt = Einduced = ε0LI
    L=Area(turns2/length)uo

    my field is radial in shape, so I have used the average coil area as a simplification, A=0.03032086m3:

    L = 0.03032086(6002/0.166116)(1.25663706 * 10-6)
    = 0.082524805H = 83 mH

    ah...i took the value from my calculation of the magnetizing field, where i had to multiply by ε0 and not u0.

    As for the resistance, I constructed a program which calculated my total length of wire to be 2.170 km. According to typical AWG tables, the resistance per km of 29 gauge wire is 268.4024 ohms, so my total resistance is in the region of 580 ohm.
     
  9. Jul 11, 2009 #8

    negitron

    User Avatar
    Science Advisor

    To get 370 amps through 580 ohms of wire, you need to apply 214,600 volts. Good luck with that.
     
  10. Jul 12, 2009 #9

    vk6kro

    User Avatar
    Science Advisor

    If you have 2170 metres of wire and 600 turns, each turn is 3.616 metres or 11.8 feet circumference or 3.76 feet in diameter. Is this right?

    According to a wire table, I found, the maximum current through a 29 AWG wire is 1.2 amps, so it won't cope with 263 amps. It would vapourize immediately.

    Seems like you need to rethink this a bit :)

    I have remagnetized alnico magnets with 10 amps through a heavy wire coil with about 50 turns on it and the magnet inside it.
     
  11. Jul 12, 2009 #10
    unfortunately for my design i need to use a rare earth core to achieve my desired B field. How then are typical magnetizing fields achieved for rare earth magnets? it seems to me that some absurdly high currents would have to be utilized in low resistance wire.

    just out of curiosity, if the reason i can't use thin wire is because of overheating, lets say i put my coil and magnet in an open steel drum, and pour liquid nitrogen into it, so that when i flick on my current a large volume of liquid nitrogen would rapidly evaporate, do you think it could stop my wire from vaporizing over that short period of time? coincidentally, using very low temperatures would also improve the remnance of my magnet.
     
  12. Jul 12, 2009 #11
    I didn't know that. Can you elaborate?
     
  13. Jul 12, 2009 #12

    negitron

    User Avatar
    Science Advisor

    Rare Earth magnets are magnetized commercially using coils with only a few turns, but using very thick wire--really solid copper rods thicker than your thumb bent into a helical coil--and at currents in the several thousands of amperes. These currents are usually generated from a large capacitor bank discharging though the coil.
     
  14. Jul 12, 2009 #13

    negitron

    User Avatar
    Science Advisor

    I'm about half-dead and headed for bed but, briefly, DC is generally preferred for welding since it results in easier arc strikes, smoother arcs, less spatter and deeper penetration. However, AC equipment is generally less expensive and offers higher currents for a given unit rating. So, for various high-temp steels and certain types of refractory metals, AC may be preferred. All things being equal, DC is generally superior but you pay for it in higher equipment cost.
     
  15. Jul 12, 2009 #14
    When you say a few turns, what kind of range are we looking at? less than 10 turns?

    Also if anyone can, please comment on my earlier question regarding liquid nitrogen cooling as well.

    I ran some numbers, if i use 5 turns of 0000 gauge copper, thus having a net resistance of 0.0016072 ohm, i would need 15323 A to make my field. Ohm's law suggests that a 24.62757 V source would produce this current in the copper, which seems a little too easy to me, what am i missing?
     
    Last edited: Jul 12, 2009
  16. Jul 12, 2009 #15

    vk6kro

    User Avatar
    Science Advisor

    You could have a look at this page:
    http://www.dealextreme.com/details.dx/sku.10305
    where they have some pretty cheap rare earth magnets.
    Prices are in US dollars, postage paid.

    i would need 15323 A to make my field. Ohm's law suggests that a 24.62757 V source would produce this current in the copper, which seems a little too easy to me, what am i missing?
    The problem is the 15323 AMPS and the fact it has to be DC. This is an enormous current.
    Maybe a large rotating generator could produce that current for a short time. Producing it from the mains supply would not be possible.

    If you made the wire out of a superconductor and cooled it below the superconducting temperature, you would have zero resistance, but you still have to get the power and get it to the coil.

    Incidentally, a number like 0.58181032 uH could easily be reduced to 0.58 uH because 8 significant figures implies an unlikely precision in the final product. So does 24.62757 Volts.
     
    Last edited by a moderator: Apr 24, 2017
  17. Jul 12, 2009 #16
    I know that these magnets are very cheap to buy, but the purpose of my project is to make one.

    If i understand this correctly, the power of this system would be V2/R = 377.449 kW. Given 5 turns of 0000 gauge copper, the inductance would be 13.31 uH.

    The time constant, T=L/R and after 5T, I is essentially completely discharged through my network. 5T = 41.266 ms

    Now P = E/t, so 377449 = E / 0.041266 => E = 15.576 kJ

    so if i were to build a capacitor bank similar to this one:
    http://fastmhz.com/?p=37
    it should do the job am i correct?


    EDIT: i actually was browsing around and spot welding machines do deal with ampages up to 15000A, 300kW do u think this wu be a suitable alternative?

    but even with 0000 guage wire the safe transmission currents are only around 360 A...The larger the wire i use the higher my stable current, but it's still always less than the current I would then require. do u think 0000 guage would be able to withstand a 15000A pulse?

    Incidentally, a number like 0.58181032 uH could easily be reduced to 0.58 uH because 8 significant figures implies an unlikely precision in the final product. So does 24.62757 Volts.

    i know i just like to be as precise as possible while still running numbers.
     
    Last edited: Jul 12, 2009
  18. Jul 12, 2009 #17

    negitron

    User Avatar
    Science Advisor

    4/0 (that's how industry writes 0000) should take a 15,000 A pulse; I work with the stuff daily and we occasionally do short-circuit tests with submillisecond current draws in the 10-30 kA range. There are larger wire diameters but these are sized, not in AWG, but in thousands of circular mils (MCM, or less commonly, KCM). The next few standard sizes up from 4/0 are 250, 350, 500 and 750 MCM. Note, however, that this stuff is THICK and you're going to have a tough time of bending it into a tight spiral, even 4/0.
     
  19. Jul 12, 2009 #18

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I am facinated by your specification of the length, how did you achieve that level of percision?

    My bet is that you didn't.

    Fine, carry around all the useless digits you want in your calculations, just do not show them them to us. Save yourself some effort and look more like you know what you are doing, only show us 2 or 3 digits.

    I am concerned about the hazards presented by the voltages and currents you want to work with, Please be careful !
     
  20. Jul 12, 2009 #19
    It was purely a matter of geometry actually, my coils are not perfectly circular, they are more so rectangular with rounded edges, and were designed to fit as close as possible around my mould. it was the inclusion of pi in my calculations which left such long decimal places, the precision of which was guaranteed by programming a C++ program to calculate length given my dimensions.

    And yes i am concerned about these values myself, which is why i will be sure to update this thread for advice as the design process progresses. Thanks for your concern!

    Also, to negitron, due to the geometry of my magnetizing field, the rods will only have to be bent to a 5cm turn height, not really that tight. Although your confirmation that it should handle the current is indeed very encouraging.
     
  21. Jul 12, 2009 #20
    There are commercial labs that will provide that kind of current without any problem, except you have to convince them you know what you're doing. There are several here in the Pittsburgh area (or at least they used to be here - manufacturing is slipping away quickly).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook