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Maximum Voltage To Apply To An Air core Electromagnet

  1. Aug 20, 2015 #1
    Hello everyone.
    I am glad I came across this science oriented Forum site.

    I actually get there from Google and what I am deeply curious to know about is the amount of Maximum Voltage my 3.5kg Air Core Electromagnet can bear.

    The Core of the Solenoid is 120mm in height and 13inches in Diameter.
    It core is 47mm wide and 130mm long.
    I used 0.31mm thick enameled copper wire to make it and after winding it reads 1.14KOhms.

    Please may I know the maximum Voltage to apply to it in other not to get it burnt?
    How do I calculate that? IMG-20150528-00586.jpg IMG-20150526-00563.jpg

    Attached Files:

  2. jcsd
  3. Aug 20, 2015 #2
    Sorry people, I'm wrong by saying the Core of the Solenoid is 120mm in height and 13inches in Diameter.

    What actually mean to say is the Air core Windings is 120mm in height and 13inches in Diameter.
  4. Aug 21, 2015 #3


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    Welcome to PF.

    The voltage handling will be determined by the insulation on the wire and by the temperature.

    Heat generated will be w = I2R and must escape somehow. Temperature will be determined by heat generated and thermal conductivity.

    If you disconnect the coil while a current is flowing, the coil inductance will produce a voltage spike that may puncture the electrical insulation. For a DC coil you should have a “flyback diode” to prevent that spike.

    What is resistance of the coil at what temperature?
    What is the insulation and temperature rating of the magnet wire?
  5. Aug 21, 2015 #4


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    Na way any part of the solenoid as shown has 13" diameter if it is 120 mm high. Perhaps you mean 13" circumference?
  6. Aug 21, 2015 #5


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    You need to know the rating for the insulation of the wire you are using. For example, Polyimide insulated magnet wire is capable of operation at up to 250 °C.

    To calculate the power required to cause that increase you need to know the thermal conductivity from the coil to the ambient. I don't know how to calculate that, or whether there are rule of thumb numbers based on exposes surface area. Also, you need to understand the internal heat transfer through the layers of insulation and wire to determine the internal temperatures.

    I'd just measure the temp rise for a certain power level, and then extrapolate.

    Here is a paper on the temperature of heating coils that illustrates all the variables, but its a very different application:

    http://www.researchgate.net/publication/265928890_Thermal_Design_of_Transformers_and_Inductors_in_Power_Electronics [Broken]
    Has some rules of thumb for things with cores.
    Last edited by a moderator: May 7, 2017
  7. Aug 22, 2015 #6
    Thank you all for your reply so far.
    There is no way for me to get the temperature rating of the coil as I bought it in Spool from a local electronic spare part store her in Nigeria.
    But I after I finished winding the coil and supplied 500vdc (using capacitor voltage doubler) to it via the wall AC mains, it bears the voltage. But when I allowed it to stay powered up longer for about 2mins, it temperature start to rise. But when I use a battery powered inverter, it takes longer at same voltage before it starts to get Hot.

    Now apart from knowing the maximum amount of High Voltage that this Air Core E.Magnet can bear, I really want to know how best to wind the Coil to generate an enormously strong Flux.

    Should I reduce the height of the magnet and increase the diameter of it Core?
    Is that one of the best ways to make a Powerful Electromagnet?

    My reason for making this magnet is to make an High Voltage Electromagnetic Pulse Motor that uses very low Power (say 30w).

    I can't use any of the thicker gauges because from my personal research so far, they suck a great amount of current but have very very low resistance which is not important as the current because Voltage which can be easily multiplied unlike current can be used to suppress high resistance in a Solenoid made with thinner gauge.

    Now my people, if I rewind the coil, and reduce the height of the coil and increase the diameter of it core 2 fold or more by using wider plastic pipe, will that make it repulsive power stronger?

    Or should I leave the height as it is but increase the diameter of it core by using a wider plastic pipe?

    What actually is the primary dominant factor that determines how strong an Electromagnet will be?

    Is it the length of the Magnet or the width of it core putting into mind thousands of turns as well do increase the flux but raises the resistance and voltage needed to suppress it to make it work and still decrease the needed amp to run it when Pulsed?


    Yes 13" Circumference I mean to say.
    Last edited: Aug 22, 2015
  8. Aug 22, 2015 #7
    Could you please help me to understand what you said from line 4 to line 5 with a schematics diagram?
    Last edited: Aug 22, 2015
  9. Aug 22, 2015 #8


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    Do you understand how inductors work? Do you understand that voltage spike that they cause if you disconnect them while they are in an active circuit carrying current? If not, you should study inductors.
  10. Aug 22, 2015 #9
    Yes I know what you mean by how inductors work as regards acting as capacitor and thereby multiplying voltage during back EMF process.
    But that is not what my request is about now. I need your help by interpreting how to do what you said that "If you disconnect the coil while a current is flowing, the coil inductance will produce a voltage spike that may puncture the electrical insulation. For a DC coil you should have a “flyback diode” to prevent that spike. "

    Would you mind helping me with that now?
  11. Aug 22, 2015 #10


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    Google is your friend. Try "flyback diode"
  12. Aug 22, 2015 #11


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    An inductor never acts like a capacitor. So don't say things like that. That whole sentence is total gibberish to me. I have no idea what you are visualizing, but it is wrong.

    Moving on to explain how make a spark:
    Think about an inductor carrying a current, with an EMF field. Now, disconnect the power source. The inductor is suspended in space connected to nothing. What happens to the energy in the field? The inductor "wants" to keep the current flow constant, but the resistance just became infinite. To keep the current constant it just needs to develop an infinite voltage. So, it tries, and something has to give. (the collapsing field has to dissipate its energy somehow)

    Since V =Ldi/dt and the current just suddenly stepped to zero, what do you get for V.

    V = Ldi/dt describes an inductor (inductors use their energy to resist current changes)
    I = C dv/dt explains a capacitor (capacitors use their energy to resist voltage changes)
    Last edited: Aug 22, 2015
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