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Electromagnetic shorting

  1. Oct 12, 2013 #1
    Hi Guys,
    I'm hoping that someone here can help me.

    I have an electromagnet that when pulsed creates a short across the 30v supply. If I increase the resistance in series with the electromagnet to stop the short, will this reduce the strength of the electromagnet? Is there a way to store the energy in a capacitor then isolate the electromagnet from the ground to discharge the capacitor through the electromagnet? The electromagnet has a resistance of 5 ohms. Has 600 turns of 18 AWG wire and the iron core is 200mm long.

    Regards,
    Dale.
     
  2. jcsd
  3. Oct 12, 2013 #2

    UltrafastPED

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    You probably have an insulation breakdown. Did you use transformer wire?

    Also your current is 30 V/ 5 Ohms = 6 Amps ... but 18 AWG is only rated for 2.3 Amps.
     
  4. Oct 12, 2013 #3

    Simon Bridge

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    DC supply?
    You mean that the coil trips the PSU circuit breaker?

    The coil is a 5Ohm resistor - what is the current it will try to draw at 30V?
    What is the current the PSU is limited to?
    You can, temporarily, get high currents by discharging a capacitor through the coil - yep - or use a tougher power supply. You can increase the strength of the magnetic field by using more turns on the coil too.
    Do you know how the magnetic field depends on the current and the number of turns?
    For a fixed DC voltage, the current also depends on the number of turns.


    Or do you mean that it was 5Ohms, but it is now closer to zero (that would be a "sort circuit")... in which case, see post #2 above.
     
  5. Oct 12, 2013 #4
    Thanks for your help guys,

    So I need to increase the number of turns to increase the resistance to have less amps across the wire? Will this reduce the magnetic field generated by the electromagnet?

    Yes I have used brand new enamelled copper wire bought in a roll and not reused transformer wire.

    I measured the 5 Ohm resistance of the coil with a multimeter. Is this wrong? The idea is to generate a strong magnetic field without shorting the dc psu.

    Using the bottom formula on: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html I should be generating a large magnetic field but doesn't work out that way. Am I using the wrong formula?
     
  6. Oct 13, 2013 #5

    meBigGuy

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    Your question is a little confusing. If you apply 30V across a 5 ohm coil the current will quickly ramp to 6 amps. Are you saying that appears as a short to your 30V supply?

    Or, are you saying that it ramps to 6A and then becomes shorted?

    Does your 30V supply still have 30V across it with the coil connected?
     
  7. Oct 13, 2013 #6
    A high current pulse applied to 600 turns of 18 gage wire will produce a hefty magnetic pulse if the power supply can handle the high current pulse. Many more turns of 18 gage magnet wire would most likely increase the strength of the field and provide a better match between it and the power source. The capacitor bank idea would be like the capacitive discharge ignitions used on older cars. Good idea.
     
  8. Oct 13, 2013 #7

    Sorry to be confusing. When the electromagnet has 30V applied, it does short to my 30V supply.

    Thanks,
    Dale.
     
  9. Oct 13, 2013 #8

    meBigGuy

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    Sorry, I want to get to the bottom of the "short". It requires over 238 meters of 18G wire to produce 5 ohms. What does your meter read when the meter leads are shorted? 238/600 = 0.4 average meters of wire per turn, which implies a pretty thick core.

    What are the specifications for your power supply? Can it really supply >6A at 30V (180 watts). If not, I assume that is what you mean by "becomes shorted". Does it blow a fuse? Current limit? Shut down?

    Doubling the turns will increase the resistance, reducing the current. But, the increased turns will compensate. Limiting the current with a resistor is an option, but it would need to be a big power resistor.

    What are you using for a core? At what point will it saturate? Do you know about magnetic saturation? http://en.wikipedia.org/wiki/Saturation_(magnetic)

    As for a capacitor bank, charging and discharging such a bank is non-trivial also. It will also short your supply, so needs to be current limited as it is charged. Needs a pretty hefty switch also. How long of a pulse do you need? Does it need to be automatic?
     
  10. Feb 10, 2014 #9
    Electromagnets and voltage

    Hi Guys,
    I have a short DC Pulse of 30V 1A and I am trying to feed this through a electromagnet to get the biggest magnetic field possible. I have been told that electromagnets require voltage and not current. Is this correct? Using different circuits, I have boosted the voltage to 200V DC. My question is: Will boosting the voltage give me a stronger magnetic field? As now I have 200V through the 5 ohm electromagnet, therefore my current across the electromagnet is 20 Amps. Am I right in assuming this?

    Any feedback would be appreciated
    -Dale
     
  11. Feb 10, 2014 #10

    Baluncore

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    No. The voltage is needed across the coil to get the current flowing through it, but current is then limited by the resistance.

    No. Ohms law says I = V/R. Therefore I = 200/5 = 40 amp maximum current.


    How short is a short DC pulse? One word has not been mentioned; Inductance.

    The magnetic field as measured in “ampere*turns” is proportional to the number of turns but there are two problems. 1. Resistance is proportional to the number of turns. That limits current. 2. Inductance is proportional to the square of the number of turns. Inductance limits the rise time of the current.

    The voltage across the coil due to inductance will be; V = inductance * di/dt
    The voltage across the coil due to resistance will be; V = I * R
    The total voltage is the sum of those two.

    I suspect the negative inductive voltage spike that must occur when you stop the current has broken through the insulation of your coil. You now have a partial short circuit in your coil.

    You need to have some device such as a reverse biassed power diode across the coil to catch that spike and so prevent damage to the coil.

    For a, four times faster rising pulse, you need to use half as many turns with twice the thickness wire. That needs half the voltage and twice the current.
     
  12. Feb 11, 2014 #11

    Thanks for the help. Sorry I can't seem to grasp this. I meant to say 40 Amps across the coil. The coil is 600 turns (3 layers of 200) of 1mm enamelled copper wire. My pulse is rather short, just a discharge from a 4700uF Cap. I'm trying to make the biggest magnetic field I can with the 30V 1Amp stored in the cap. I generally do have a reversing diode across the coil.

    I will try your idea of twice as thick wire with half the turns. Do you know anyone who could tutor me on this?

    Regards,
    Dale.
     
  13. Feb 11, 2014 #12

    Baluncore

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    What is the inside diameter of the coil you have now?

    You must charge the capacitor from your supply through a resistor of about 30 ohms to limit the supply current into the capacitor. That will prevent the supply fuse blowing.

    You must have a reverse biassed power diode, rated at about 30 amps, across your coil. That will protect the coil from a spike, and the capacitor and supply from the negative 30V expected after the pulse.

    You should disconnect the charging supply from the capacitor before connecting the capacitor to the coil.

    Your 4700uF capacitor will have some resistance in it's structure. That will also tend to limit the maximum current.

    To understand what is happening with your circuit you need to develop an understanding of voltage, current and resistance = Ohm's Law. Then charge, power and energy. Then capacitance and inductance.

    Tutor? You are probably better asking questions here on PF in the short term. That way you will get the help you need from those who can give it.
     
    Last edited: Feb 11, 2014
  14. Feb 11, 2014 #13

    meBigGuy

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    It is certainly difficult to piece together exactly what you are doing from all the words. A drawing would certainly help. You didn't mention what you are using for a switch and how long it is closed. Does it stay closed for full discharge?

    When you connect the 4700uF capacitor to the inductor you have an RLC circuit which will behave differently based on the C,L and R terms. Eventually all the energy will be dissipated. But how it behaves while dissipating is value dependent (you should read about series RLC circuits).

    Saying you have 30V 1A stored in the Capacitor is inaccurate. You have 30V stored on the capacitor (a capacitor charged to 30V). It can supply huge currents (but the inductor will "resist" and the current will ramp up).

    When you apply the capacitor to the Inductor the 30V appears across the coil instantly. The coil current starts at 0 and ramps up, building a field. At the same time the voltage across the capacitor starts to drop since you are removing charge. What happens when the capacitor voltage gets to zero will depend on several factors.

    You can read about RLC circuits here http://en.wikipedia.org/wiki/RLC_circuit.
     
  15. Feb 12, 2014 #14
    Hi Guys,
    Attached is the simple diagram.

    When the DPST switch closes, the capacitor charges up. Then the DPST is open to isolate the electromagnet from the supply. When the push button switch closes, the cap discharges through the 20 ohm resistor into the electromagnet. The 20ohm resistor is to protect from switch bounce and the inrush of current to the electromagnet. Hope this diagram helps in explaining things.

    The size of the iron core coil is 40mm x 10mm x 200mm wrapped with 600 turns of 1mm enamelled copper wire.

    Yes the push button switch stays closed for the full discharge of the cap to the electromagnet.

    -Dale.
     

    Attached Files:

  16. Feb 12, 2014 #15

    Baluncore

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    1. The 20 ohm resistor is quite unnecessary. Initial current will be limited by the coil's inductance, then final current by the coil's resistance.

    2. The diode across the coil protects the winding's insulation and the switch from an inductive spike and arc. But the diode will keep the current flowing in the coil, which will lengthen the tail of the magnetic pulse time.

    3. The iron core will need to be laminated or of an iron powder or ferrite material. (Transformers are wound on laminated cores that have laminations thin enough to become magnetised in about 5 milliseconds). If it is a solid iron core as suggested then it will take one second or more to build up the magnetic field. If you want the fastest rise in magnetic field you cannot use a solid mild steel core, you must avoid magnetic materials and use air. How fast do you want the field to rise?
     
  17. Feb 12, 2014 #16
    Hi,
    The core is a laminated core, exactly the same as a transformer core. Sorry I have another electromagnet that is a solid core. Got them mixed up. But the one I am using has a laminated core. 5 milliseconds I'm sure is enough. I have been looking into RLC circuits.

    The aim is to create the biggest and sustained magnetic field I can create with the voltage stored in the capacitor. With the info and the diagram I provided, do you think that I need to make another laminated core electromagnet with 400 turns of 2mm wire?

    Also I can take out the de-bounce resistor. From my understanding, this should give me more current across the coil. The formula's is what tricks me up. Slowly it is sinking in. Besides a PHD, Is there anything else I can do to achieve my goal?

    Thanks,
    Dale.
     
  18. Feb 12, 2014 #17

    meBigGuy

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    1. You definitely want a protection diode in case of switch bounce and pre-mature release.
    2. Not sure you need a DPDT to charge the capacitor. A SPST might suffice.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html says your inductance with an iron core and 600 turns is over 18mH. (I said 200 relative perm, 0.36cm area, 20cm long)

    If the circuit is underdamped then the polarity on the capacitor will reverse, which you don't want.

    Damping factor less than 0.707 (critical damping) will have ringing.

    [STRIKE]damping = (R/2) sqrt (L/C)[/STRIKE] <------ EDIT: THIS FORMULA IS WRONG

    Assuming R is 5 ohms and C=4700uF, there will be adequate damping if sqrt (L/C) > 0.2828
    L/C > 0.53
    L > 2.4mH

    You need to consider losses in the core and ESR of the capacitor as part of the total effective R.

    Check my numbers because I'm notoriously bad at cranking out numbers.
     
    Last edited by a moderator: Feb 13, 2014
  19. Feb 13, 2014 #18

    Baluncore

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    @meBigGuy. There is a diode across the coil in the diagram.

    The capacitor is therefore prevented from becoming significantly reverse charged, there can be no oscillation.

    Once the capacitor is discharged, the inductor current circulates through the coil and diode only. The diode keeps the voltage across the inductor low, which will increases the current and field decay time, since V=L*di/dt.
     
  20. Feb 13, 2014 #19

    meBigGuy

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    You are right, of course.
     
  21. Feb 13, 2014 #20
    @meBigGuy
    Forgive my ignorance but how did you work out the coil radius?

    -Dale.
     
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