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Insulation cloth-covered wire

  1. Jun 10, 2015 #1
    Hi,

    I was conducting an experiment under high voltage and the plastic insulation of my wire started melting, so I'm looking for an alternative insulation. The wire used needs to be insulated to prevent short-circuit. May I know if cloth-covered wirings are only insulated with cloth, or with a layer of plastic insulation underneath the cloth?

    Thank you!
     
  2. jcsd
  3. Jun 10, 2015 #2

    billy_joule

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    If your wire is overheating due to resistive heating then it is undersized. Using higher temp wire is a poor & dangerous solution.

    If the wire is in a high temp environment, then use high temp wire, cloth covered wire is not necessarily suitable for high temperatures.
    Here are some options:
    http://www.awcwire.com/producttoc.aspx?id=high-temperature

    All electrical supply stores should have silicone covered cable (it's used in ovens), it's often good for ~200deg C.
     
  4. Jun 10, 2015 #3

    anorlunda

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  5. Jun 10, 2015 #4

    meBigGuy

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    Assuming you are not working in an oven:

    The only safe solution is to go to a wire size that does not overheat. (Stating the obvious .... high voltage has nothing to do with it ---- high current is the culprit, but you probably know that already).

    What currents, what wire size, what ambient temp are you working with?
     
  6. Jun 10, 2015 #5

    jim hardy

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  7. Jun 10, 2015 #6
    I'm not sure about the exact value of currents, but I'm sure it's below 10A. Using a thicker wire reduces the resistance, so doesn't the current increase in this case? Since power dissipation is P=I^2R, I suppose a thicker wire won't really help in this case..?
     
  8. Jun 10, 2015 #7
    I'm using a fairly thin (I don't know the SWG number) copper wire as it is the only wire available in my physics lab. It has not exactly broken down yet, but I could smell the plastic burning so it's not a good sign..
     
  9. Jun 10, 2015 #8
    Thank you for your replies, billy_joule, anorlunda, meBigGuy and jimhardy.

    I'm sorry I think I wasn't clear enough in stating the conditions of my experiment.

    It is carried out under room conditions, and the copper wire is used as a solenoid and is coiled around 3 iron nails. It magnetized successfully, but the current induced in another coil is too small, so I have to increase the magnetic flux in the coil (by increasing current in the coil) to a higher value. Halfway through the experiment I smelt the plastic insulation burning, and the iron nail was very hot.
     
  10. Jun 11, 2015 #9

    meBigGuy

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    The resistance of the wire will definitely will affect the current through the coil. But, you need a certain current, and seem to be able to adjust to that current. Either there is a current limit, or you are just adjusting the voltage.

    Go to larger wire and then set the current to the amount you need (or couple the coils better so you need less current).

    You may have iron saturation issues also.
     
  11. Jun 11, 2015 #10
    It sounds as if you are making a transformer. The primary winding needs many turns, so that the inductance is high and the magnetising current is not too large. I suggest using about 1 metre of wire for the primary, as a test. Also try using just 1 volt from the power supply, and make sure it is AC.
     
  12. Jun 11, 2015 #11

    Baluncore

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    It makes a big difference if you are using an AC or a DC power supply. Which is it ?
    If AC, what is the frequency of the excitation ?

    If AC, three nails in a bundle make a short circuited conductive triangle in the plane of the turns. An eddy current can flow between the nails which will generate heat in the nails and the windings. So if AC, paint or insulate the nails so as to prevent the shorted turn. Use more, thinner nails, to reduce eddy currents in the individual insulated nails.

    If AC, double the number of turns. The resistance will double but the inductance will rise to four times since it is proportional to turns2. Current is then limited more by inductance, which does not generate heat.

    If DC, double the number of turns. The resistance of the coil will double so the current will be half. The magnetic effect will be the same in amp*turns, but the heat of I2*R will halve since heat become (I/2)2 * (2*R).
     
  13. Jun 11, 2015 #12

    jim hardy

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    THE iron nail and not all 3 of them ?

    A question well stated is half answered, and yours is neither .

    How much current and what controls it? What gauge wire ? What frequency? How are nails arranged - in a bundle or forming a triangle ?


    A picture is worth a thousand words.
     
  14. Jun 11, 2015 #13
    Today I tried making a transformer using nails, and was unsuccessful. I ended up using two windings of 200 turns of 30 gauge wire on a nail 3mm dia x 75mm long. It did not overheat with 2 volts AC, but the secondary voltage was still only about 0.7 volt. So there is very large leakage.
     
  15. Jun 11, 2015 #14
    What is "High voltage"?

    What you seem to be doing is typically done at lower voltage levels. Is this a learning type experiment in school? Or are you doing some sort of high voltage research?

    Try not to blow up the lab.
     
  16. Jun 11, 2015 #15
    I suspect this "experiment" is being run with Mains....that is way too dangerous for SmokedVanilla's apparent level of knowledge.
     
  17. Jun 11, 2015 #16

    berkeman

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    @smokedvanilla -- I agree with others that this is pretty dangerous for you to be doing with so little experience. Smoked wires is a sure sign that things are starting to go wrong, and could get worse.

    What *exactly* are you trying to do? What is your power source -- what voltage and what current capacity? What gauge wire were you using when the insulation started to melt, and what was the current level (AC or DC)?

    Do you have supervision in this project?
     
  18. Jun 13, 2015 #17
    It's a school project and I'm being supervised by a teacher. I used 13V from the power pack, the frequency of current is 50Hz.
     
  19. Jun 13, 2015 #18
    I'm trying to induce a current in a coil of wire. I'm using a power pack, and the voltage is 13V. I'll find out about the gauge and current on Monday. I'm supervised by a teacher.
     
  20. Jun 13, 2015 #19
    Thank you for reassuring us. :redface:

    I would use a bigger wire or less current. To use less current you need to couple the magnetic circuit (the nails) better. Are both coils on the same nails? That would be best I think.

    Also if the nails are heating (through eddy currents for example) insulating them from each other with a sheet of paper may help the heating at a slight cost in magnetic flux coupling. You might also consider using a different type of nail. Soft iron is likely what you want. (There are better, specialty materials, but I don't think they make them in nail form.) They make nails out of all sorts of alloys now, so your nails may not be iron. Test them with a magnet.

    It is also possible the windings are already shorting, if you have old broken down lab equipment. Use paper between the nails and the winding (1 sheet thick) and try to leave a little room between the wires so they don't touch.

    Paper makes a nice, cheap, easily available insulator. It can break down unexpectedly or burn which makes it unsafe outside of the lab. Beware of the burning thing, obviously. Keep a clean lab space with a metal trash can to dump the thing in if it starts to go (which is unlikely, but safety first).
     
  21. Jun 13, 2015 #20

    jim hardy

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    Try arranging your nails in a triangle so there's a loop of all iron for the magnetic flux to circulate. It's hard to push flux through air.
     
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