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AC Electromagnet Core Material

  1. Jul 9, 2014 #1

    mugginsjr

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    I'm attempting to build an alternating current electromagnetic coil and want to maximize its potential field. My initial query was to find the very best core material for the electromagnet. I found an older post (2005) on the subject and saw some very interesting answers there. Being new here I learned that we can not ask new questions on old posts so I'm presenting these questions here. Thank you for any responses.

    What are the benefits/downfalls of using a permanent magnet as the core of an electromagnet? (like a series of round Neodymium magnets - stacked in opposing faces - n/s,s/n/n/s, etc)

    Will the electromagnet (with a magnet core stacked in the opposing nature as above) change polarity when the current through its coil changes polarity? I know that using a ferrite core will allow the polarity change but am not sure about the magnet core.

    A poster named Intuitive said that he had made several magnets from joining broken bits of old magnets in that opposing fashion, with n/s, s/n, n/s, s/n, etc. I'm not sure if he was implying that he was physically joining these opposing faces together with some sort of cement or something. Is it possible to physically join magnets together with some sort of bonding agent?

    Also (and this is just for curiosity's sake) he said that certain magnets need to be packaged in a special way when being shipped because they can actually wipe out credit card info and hard drive data info at up to 5 feet away. I did not know any material existed that could insulate magnetic fields. Was I wrong about that? And if so, what is the material and is it possible to use that insulating material to concentrate a magnetic field or flux?
     
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  3. Jul 9, 2014 #2

    Baluncore

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    Welcome to PF.

    The core of an AC solenoid needs to be constructed from the same materials as an AC transformer core. That requires a laminated steel, ferrite or iron powder core. What frequency AC will you be using? That will determine the lamination thickness or iron powder grade in the same way as power transformers or audio amplifiers. I would suggest that you consider making your core from a used transformer core. http://en.wikipedia.org/wiki/Magnetic_core

    There is no advantage in using permanent magnets for the core of an AC solenoid.
    Magnets will cause asymmetric saturation characteristics that reduce the maximum AC field.

    Any magnetic material that will conduct the magnetic field is a partial magnetic insulator.
    Mumetal is often used for that application. http://en.wikipedia.org/wiki/Mumetal
     
  4. Jul 10, 2014 #3

    mugginsjr

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    Thank you for that info. I think this info will help me significantly. I have a question about your response on insulating magnetic fields with substances such as mumetal. (Great tip btw. Thank you for that.) It seems like such substances are deterring the magnetic field rather than soaking it up as lead does for radiation. If that is true, then what is the absolute best substance for deterring a magnetic field? Is it Mumetal, or is that used as insulation simply because it is widely available?
     
  5. Jul 10, 2014 #4

    Baluncore

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    Mumetal is used because it works well enough for it's cost.
    In effect, it short circuits the magnetic circuit, so there is less stray flux out there.

    A superconducting material would work better. It would also be slightly more expensive.
    Any magnetic field would be reflected from the zero resistance surface. That is because the eddy currents induced in the surface would generate a magnetic field that exactly countered the applied magnetic field.
     
  6. Jul 10, 2014 #5

    mugginsjr

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    Thank you for letting me monopolize your time. You are certainly kind and I thank you for sharing your wealth of knowledge. It seems like the deeper the answers get the more questions I have. I apologize for that.

    I believe all superconductors require cooling to around -400 F, no? That's not something I want to get into just now, of course I will later if I have to. Let me ask, though, just for clarity, when you say the eddy currents "induced" on the surface of the superconductor, are you saying that the original magnetic field does not follow the length of the superconductor but rather it induces an additional mirrored field which then travels down the length of the superconductor? And by "exactly countered" I assume the new field would have opposite poles and be of the same exact strength as the original field? Finally, in a situation where the bar magnet is only 12" and the superconductor bar is 48", would the new induced magnetic field travel the full length of the superconductor or will the new field be about the same size of the magnet and localized only where the magnet is located?
    sc_field.png
     
  7. Jul 10, 2014 #6

    Baluncore

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    A magnetic field goes on out forever from the magnet or current that causes it. It gets weaker further away.
    http://en.wikipedia.org/wiki/Magnetic_field

    An electric current flows in the surface of a superconductor due to an incident magnetic fields.
    That surface current causes another magnetic field that cancels the incident field.
    A perfect mirror makes a perfect shield. There is no current or field inside the superconductor.
    But beyond the edges of a mirror there is no shield so the field can wrap around.
    http://en.wikipedia.org/wiki/Magnetic_levitation
     
  8. Jul 11, 2014 #7

    mugginsjr

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    Thank you. Your help has probably saved a good deal of time on my project. Superconductors would be perfect for my experiment, but since the project involves increasing the efficiency of generating alternating current I fear the added cost of cooling the substrate would be contrary to the goal. Only time, trial and error will establish that I suppose. I wish that we had a more stable conductor which could perform similarly at normal Earth temperatures.

    My understanding of superconductors is that they become affectively viable at that point where their thermal vibration stops (Zero Kelvin), requiring a drastic drop in temperature. While valiant efforts are currently being made to find substrates requiring less of a drastic drop in temperature, I wonder if the temporary solution to efficiency could be discovering if only the useable surface of the substrate needs to undergo a series of brief temperature drops rather than dropping the temperature of it's entire thickness.

    The Wikipedia page on Magnetic Field you posted above explains the field's origin partially as "the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin." This is exciting to me because the notion of "elementary particles" at least indicates that most of our scientific community is on board with the quantum physical nature of magnetism and electricity. (I believe that physical particles in a magnetic field are what causes interaction with the free electrons in the outer shells of substrate atoms like copper and silver, which makes the generation of electricity more of a chemical reaction.) It goes on to suggest "photons" as one possible rudimentary culprit for the interactions within an electromagnetic field. I haven't heard that before and while I think that is an absolutely genius correlation I'm wondering if it's not just a bit premature. There are dozens of questions floating around in my head over that. It is certainly wonderful fodder for thought.

    Thank you kindly again for your advice.
     
  9. Jul 11, 2014 #8

    Baluncore

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    You are welcome.

    You may have now realised that asking questions on PF can narrow your view and internalises your experience.
    Working out an answer by reading Wikipedia will widen your view as you explore the adjacent universe.
    When you get stuck, ask PF for an explanation or a reference.

    The answer you get will be proportional to the precision and quality of your question.
    If you can ask the perfect question, then the answer will be obvious to you.
    We can really only help you to ask the right question. That is what we are here for.
     
  10. Jul 11, 2014 #9

    mugginsjr

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    Couldn't have been said better! I'm guessing you are pretty good at philosophy also... Looking into a very narrow box of known data discounts vision for the unknown, I think. To that end I think it narrows the perspective so greatly that even the simplest ideas become overlooked.

    I may have discombobulated an engineer friend recently by asking him if there is any reason why the stator in a generator/alternator must remain stationary. "Wouldn't it be twice as efficient to have both elements necessary for electricity (conductive coils and magnetic fields) moving in opposite directions instead of having one of them remain stationary? Looking at it in its simplest form of a conductive coil moving through a magnetic field, it seems that you could produce almost twice the energy (discounting for the necessary inverting gear to rotate them in opposite directions) or spend half the fuel to create the same amount of energy by having both elements moving in opposite directions." Even the word "stator" is restricting because it implies stationary. Sometimes we humans are silly, quantifying known properties to the nth degree but not seeing that the simplest physical change could alter the entire design. And sometimes we resist those new ideas because the formulas and laws we have already established for the original design tell us that only the original design will work. But of course the formulas and laws are based on the original design which means that they could not apply to an altered design. To me that is the silliest notion of all. We see what we believe and completely block out everything else. That is why we are still trapped in the infancy stage of the technological curve in my opinion.
     
  11. Jul 11, 2014 #10
    you threw him for a loop because your idea doesn't make any sense whatsoever. You've gained absolutely nothing. Moving the stator in the opposite direction is exactly the same as moving the motor with a true stator twice as fast. If you want that, just adjust the gearing for moving part. This is a basic understanding of having a reference frame. Its silly to speak from a position of complete ignorance and complain about experts using confining terms.
     
  12. Jul 11, 2014 #11

    Baluncore

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    I see no advantage in rotating the stator also. The same power can be extracted by rotating the rotor only. Stators are static because they have many external connections that would need brushes and slip rings if the stator rotated. It is also quite difficult to mount a stator in bearings that will be as reliable as rotor bearings can be. The stator is hollow with a large diameter, so it would have a significantly greater inertia than the rotor. Technology is where it is now because of the path it has followed while we have tried to maintain reliability and safety while reducing costs.

    But you are correct that “inverting” the design can be a valuable design technique.
    It will often lead to an insight into why it is done the way it is.
    Every now and again it will lead to an improvement.
     
  13. Jul 14, 2014 #12

    mugginsjr

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    Thanks for that vote of confidence. In my opinion it only requires being able to see the forest without having all the darned trees in the way. I understand your weight ratios and bearing problems, but there are many ways to skin a cat. Simplicity, simplicity, simplicity. Electricity is generated when a wire coil passes through a magnetic field. There is no law stating that the wire coils must rotate within the circumference of a stator. Think side by side motion, as in two gyroscopically (weight proportioned) designed disks with magnets and corresponding coils spinning on the same axle in opposing directions. I designed this 34 years ago.
     

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  14. Jul 14, 2014 #13

    jim hardy

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    It's a practical consideration not a theoretical one.


    The word "Stator" implies it's the stationary part
    and "Rotor" implies the rotating part

    the distinguishing terms should be "Armature" and "Field",
    Field being the element that creates a magnetic field
    Armature being the one whose conductors are cut by that magnetic field , and the armature is where the conversion from mechanical to electrical energy occurs.

    All you need is relative motion between magnetic field and the armature turns.
    Either one of them can be the rotor. Indeed in the 1880's machines were built both ways.
    It turned out more practical in AC machines to let the field be the rotor because field power is so much less than armature power which means your sliding contact can be much smaller.

    Here's the commutator for a DC machine where the armature is on the rotating part
    commutator-undercutter-machine_0.JPG
    courtesy http://www.rsicorp.net/electric-motor-repair/commutator-undercutter-machine-dc-motors-armatures

    Compare to the slip rings for a comparable alternator where the field rotates:
    50PTOSlipRings.JPG
    courtesy http://www.wincogen.com/HowGeneratorsWork

    Take apart an old car alternator, then an old car generator(if you can find one).
    I remember when we had to replace generator brushes at about thirty thousand miles, ~same as ignition points.
     
  15. Jul 31, 2014 #14

    mugginsjr

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    Jim, hi, yes indeed a practical matter. And yup, I have replaced a fewer alternator brushes too. That was a practical matter then too because in those days alternators were not as disposable as many are today. They were designed to be rebuilt and at a less expensive alternative (pardon the pun) to replacement. I still remember a small factory in my hometown as a boy which had the sole responsibility of rewinding the coils on used alternators and generators. I can remember thinking how powerfully consuming it must have been to tool for all the different varieties of makes and models of units.

    Practical is a really good word and I think it exactly embodies my earlier point. Sometimes I think we lose our practicality when chasing one theory from the tail of another, as in the strange resistance to the idea that neither the rotor or stator need be stationary. Of course that is fundamental truth - has been since Tesla pioneered the alternator. But our incessant human need to classify and categorize things into tidy little packages can blind us to the simplest of things like that. The damage from that is we get locked into a single design with no real fundamental changes. Sure we have improved the efficiency of generators, but they are still fundamentally the same design of 120 years ago. And except for basic improvements to combustion engine automobiles and road building techniques, these animals are fundamentally the same as well. When peripheral vision is lost all that is left is linear improvement.

    I was speaking with a geologist this morning on the phenomenon of extreme changes in the earth's magnetic field as evidenced by the alignment of certain molecules in core samples. I remarked that it must have been an extreme force like a large asteroid or comet striking the earth with such force as to change its axis rotation so abruptly as well as its magnetic core direction. She remarked off the cuff that yes but rotation probably didn't change. I'm just dumbfounded at the number of theories in the string she must have followed in full circle to reach that understanding. Because it would have been much simpler - and practical - to simply spin a basketball on the tip of her finger at one rpm while someone smashes a 90 mph softball into it to observe any rotational change. In physics there are generally two schools: the theorists and the experimentalists. Neither suits me..... I shall the now refer to my own science as practicionist, where peripheral vision can only be inspired by ignorance. You know Edison always believed Tesla was ignorant.....
     
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