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Inductors are deceptively simple. Importance of proper SG core construction material

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  • Inductors are deceptively simple. Importance of proper SG core construction material

    Inductor Core Material: The Heart of an Inductor

    By Andy Chow, Chief Design Engineer, J. W. Miller Magnetics, Gardena, Calif.

    Inductors are deceptively simple. However, a closer look reveals underlying complexity. We'll focus on understanding the properties of inductor core material — the heart of an inductor.

    What's a Magnetic Core?

    An inductor's magnetic core is made of specially formed material with “soft” magnetic properties. Although physically hard, a magnetic core is said to be “soft” when it doesn't retain significant magnetism. A magnetic core is usually surrounded by carefully arranged windings of wire. The combination of magnetic core and windings results in a measurable property called inductance. There are various types of “soft” magnetic materials as well as different types and shapes of magnetic cores. Magnetic cores plus their windings can be thought of as miniature electromagnets.

    There are many possible inductor core geometries. A core's geometry depends on various factors, including the application; the available mounting area and volume; the allowable radiation; the limitations on windings; the operating temperature; and how the inductor will be mounted. Consequently, a core's geometrical shape can take the form of a cylinder, bobbin, toroid or several other complex shapes.

    In addition, an inductor's magnetic core doesn't have to be made in one piece. Multi-piece cores, each piece made of the same magnetic material, are sometimes used for extremely complex shapes or larger inductors.

    Cores must be constructed and finished with an understanding of how windings will be installed. Sometimes, windings are wound directly around the core. Other times, the windings may be wound on a sleeve that is slipped over the core. Note that the wire used for the inductor windings is usually insulated, because when closely wound, adjacent turns would short out. However, thin insulation is fragile. When wound directly on the core, the magnetic material must not subject the thinly insulated wire to rough surfaces or sharp edges that could cut through insulation. To accommodate direct windings, a well-designed magnetic core will have a smooth winding surface. If appropriate, the core will provide a corner radius.

    Examples of core materials for inductors include silicon steel, iron powder and ferrites. Each of these different materials has different properties at different frequencies, temperatures and power levels.

    What Does a Magnetic Core Do?

    Functionally, an inductor's magnetic core stores recoverable energy. Circuit designers specify inductors that are capable of receiving and returning energy in prescribed intervals. Mechanically, an inductor's core provides support for its windings. Magnetically, an inductor's core provides the medium to concentrate and contain magnetic flux. The combination of winding turns and volume of magnetic material sets an upper limit on the maximum allowable magnetic flux a core can sustain. Flux density is important because it's related to energy. Higher flux densities imply greater amounts of stored energy. Magnetic flux is analogous to electrical current in a purely resistive electrical circuit. Magnetic reluctance is analogous to resistance. A core with low reluctance can support a relatively high flux density. The same size core with high reluctance can support a lower flux density.

    Another important core parameter is called permeability. Permeability is inversely related to reluctance. A core with high reluctance has low permeability and vice versa. Permeability is an important parameter because it can be thought of as a flux multiplier. For reference, consider the flux multiplier of free space to be unity (cgs system). Core permeability is always relative to the permeability of free space. Thus, the relative permeability of useful magnetic materials ranges from 10 to 10,000. More practical values of relative permeability are in the range of 100 to 1000. An inductor transforms electrical energy into magnetic energy. That magnetic energy is stored in the inductor's magnetic field. Consequently, energy stored at one instant in time can be retained in the core until it's needed later. By controlling the rate at which energy is stored and removed from the magnetic field, designers can implement switched-mode power supplies. For example, switching power supplies may operate in the range of tens of kilohertz to a few megahertz. Slower switching supplies must store more energy per cycle than higher frequency switchers. The result is that core size is larger for lower switching frequencies and smaller for higher switching frequencies.

    For a given winding configuration and core size, an inductor's value of inductance will be higher for a core with higher permeability. For the same electrical conditions, an inductor with a higher value of inductance can store more energy than an inductor with a lower value of inductance. Table 2 illustrates a few of the applications where magnetic cores are required.

    Behavior of Different Core Materials

    In this section, we'll review the behavior of silicon steel, iron powder, and ferrite materials. These soft magnetic materials have properties of permeability and resistivity. It's the disparity in these properties that make the different materials appropriate for different design applications. This is another way of saying there's no “best” material for all applications.

    Silicon steel is relatively inexpensive and easy to form. In addition, silicon steel is a metal with low resistivity. Low-core resistivity means silicon steel readily conducts electrical current. The result is that undesirable eddy currents can flow in the core material. Eddy currents contribute to heating and core loss. In addition, a silicon steel core tends to reach the point of saturation rather easily. When saturated, a core is unable to store additional magnetic energy. Rapid saturation results in reduced operating range.

    The solution to rapid saturation is to introduce an air gap in the magnetic flux path. An air gap increases the reluctance of the flux path, which has the effect of reducing the permeability and the inductance. Consequently, the amount of current the core can handle is extended.

    Soft iron powder has higher resistivity than silicon steel. By special processing, iron particles are insulated from each other. The particles are mixed with a binder (such as phenolic or epoxy). The cores are then pressed into their final shape. Next, a baking process is used to cure the cores. After curing, many tiny air gaps combine to provide a distributed air gap effect. In other words, the air gap has been distributed throughout the core. Iron powder cores have found wide use when core loss is a consideration.

    When compared to other magnetic materials, such as ferrites, the distributed air gap allows powder cores to store higher levels of magnetic flux. The distributed air gap also allows higher dc current levels before saturation occurs.

    Ferrite is a crystalline magnetic material made of iron oxide and other elements. The mixture is processed at a high temperature and formed into a crystalline molecular structure. Unlike others, ferrites are ceramic materials with magnetic properties. Ferrites have high magnetic permeability and high electrical resistivity. Consequently, undesirable eddy currents are greatly reduced by ferrite cores. With their high resistivity, ferrites are ideal for use as inductors. For example, ferrite beads are frequently used to reduce parasitic oscillations and for general filtering at the component lead level. This type of broadband component requires a broadband low-Q in order to provide high impedance over a wide frequency range. Table 3 summarizes some of the important properties of these magnetic cores.
    Magnetic Domains or Why Materials are Different

    Different materials have different magnetic characteristics. Intuitively, there must be some underlying mechanism that's different for different materials. The answer is found in what we call “magnetic domains.” Magnetic domains are much more than the model of simple bar magnets that are either aligned or out of alignment. A magnetic domain is a volumetric space within a material. This volume has certain elemental properties. However, there are many magnetic domains of different sizes and shapes, within a single magnetic core. Furthermore, impurities and material imperfections contribute to the differences.

    Work is required to alter the energy state of each (different) domain. Because the domains are of varying shapes and sizes, different amounts of work are required for different domains. Certainly, at the macro level, we can ignore the micro properties. However, it's these very properties that give rise to the particular characteristics of each material. Thus, we can understand why it's virtually impossible to provide two cores with identical properties.

    Consider magnetic flux acting on and within magnetic domains. The magnetic domains expand and shrink, much like bubbles. Magnetic domains coalesce and meander around like rivers. Sometimes, the domains flow within set channels and sometimes they spread out, as in a flood. The sizes of the various domains, the proximity of the other domains and the various topological considerations ensure lack of uniformity. It's small wonder that different magnetic materials have different characteristics.

    Inductor Core Material: The Heart of an Inductor

  • #2

    I am coming at this area as an outsider so I rarely know when I am being obvious or heretical. So without being a wiseguy, how does this relate to the induction seen in a Tesla Coil, air coil arrangement. I mean you can get ten foot sparks, if you're Tesla, from only an air coil. I have given this, I won't say a lot, but a small amount of thought. Where does all that "electricity" come from? Electricity is supposed to come from a changing magnetic field and the inductive spike from a very rapidly collapsing magnetic field. So why can I get a higher voltage more powerful inductive spike off an air coil? I mean really, there is no core. Is this all self induction between the circles on a coil? If so why is it muted when an iron core is present but comes to the fore elsewise? I think it very much has to do with self induction between the individual strands of a coil, this is what I guess. If not, it brings up a terrible question of how are electricity and magnetism related, i.e. if electricity (the inductive/back EMF spike) is generated (in an air coil) without significant magnetic flux ... well you think about it. You can, from what little I have seen, using a pulse generator or 555 timer get a higher voltage and power inductive spike from an air coil than with an iron core. And again the Tesla coil type induction, how can this be? Anyone???
    Last edited by ZPDM; 01-26-2013, 10:04 AM.


    • #3

      I am not trying too be wise, but I did not post this in relation to air coils or the Tesla coil, so possibly you were mistaken in your assumptions; sorry for the confusion.

      In relationship to the SG project I felt the Inductor Core Material: The Heart of an Inductor information might be of some use to the SG group.

      I keep going back to the words of Bedini "Build it as I have built it", well if you were to follow this sage advice and delved into what Bedini built you would find that he spec'ed a paticular type of material to be used in the construction of the iron core of the SG coil, R45 welding rod was his preferred material. But what makes this particular product so special?

      Could not Bedini just as easily have spec'ed bailing wire? When one goes into this aspect of what Bedini has spec'ed and stated many times, why is it that there is even a question on the subject? Some on this forum have stated that they are using rusty wire, others tie wire, who knows others chicken wire. But then state that they are not achieving what Bedini has achieved. I imagine that if Bedini were to be frank on this subject he knows exactly why R45 welding wire is preferred in the construction of the SG coil, but he is silent on the matter, leaving the curious to delve deeper into the inner sanctum of the why's.

      It is the material composition that goes into the R45 welding rod that is important. Several years ago, Westinghouse had a trademark material called Hipersil which was used in constructing the iron cores of transformers. Hipersils was a a cold rolled, grain oriented, silicon steel with about 3.5% silicon added.

      But why silicon? Why add silicon?

      Silicon was added to control residual magnetism (core staying magnetized ). See the following for a deeper explanation: [Amps] Hipersil, the myth and the truth. (Updated) which states that:

      "A cores power handling ability (in watts or volt amperes) comes from its ability to cram all the magnetic lines of force (flux) into a small core area (flux density). Every core has a maximum flux density (Bmax) which is determined by the cross sectional area of the core (A) and the material the core is made from, nothing more."

      I am not going too second guess Bedini or Westinghouse on this matter, if there is a material that lends itself to better performance factors in the SG coil, then it would behoove other SG builders to follow suit.

      R45 welding rod composition is as follows AWS A5.2 Class R45. CHEMICAL COMPOSITIONCarbon .08Manganese .50Silicon .10Phosphorus .035Sulfur .040Copper .30Chromium .20Nickel .30Molybdenum .20Aluminum .02

      It is my opinion that Bedini selected this particular material for its material composition, especially its silicon content of .50%.

      In addressing your other concern that an air coil versus a wire wound induction coil you stated that "Electricity is supposed to come from a changing magnetic field and the inductive spike from a very rapidly collapsing magnetic field." This is exactly what is happening in a properly tuned and CONSTRUCTED SG motor.
      The SG is an induction device, the inductive spike from the SG is somewhere in the mid 600 volt range, this is the back EMF not lost to hysteresis or saturation of the iron core maximizing potential of the magnetic flux in the core of the coil during an induction cycle.

      I hope I have cleared this matter up between the SG induction coil, and the Tesla air coil. And the importance of building a proper iron core for the SG coil, remembering John Bedini's words "Build it as I have built it."


      • #4
        there are several reasons for the rod, the rods with the air spaces between them help isolate the mag field of each rod. it does not behave like a solid iron core or even like metglass. its easy to get, has the right magnetic properties, can be made to fit just about any sized coil, do not retain residual magnetism. my best coils have welding rod in them. well I shouldnt say that I have built coils woth a core too big. the 3/4 coil core seems to be just the right size!!

        Tom C

        experimental Kits, chargers and solar trackers


        • #5
          I have good results with glowed soft iron wire that has very good remanent properties.
          It's mainlu used in reinforcements for concrete constructions and since it's glowed it has a oxide skin which makes it non conductive. The diam. is 1mm therefore you can fit more material in the core. And it's bloody cheap!


          • #6

            I apologize if I may have taken things slightly afield of your original contribution. I agree with paying close attention to the details John Bedini mentions for his machines. I also agree that induction is important to the function of his SG machine. Further I would guess that materials of high magnetic permeability, silicon steel, Mu Metal, Met-glass would make outstanding cores for an SG machine. I would finally say I believe Mr. Bedini payed more than passing attention to Mr. Tesla. So in commenting upon your excelllent and useful textbook excerpt concerning magnetic cores in looking to understand both induction and magnetism I am just asking a very simple question about which there doesn't seem to be much textbook information avaialable. Again, is the induction between the primary and secondary in a Tesla coil the same phenomena as that seen in an SG machine? From the little I know, it does not seem from the textbooks it should be there at all, where does it come from? Is it a new phenomena or a behavior of the same textbook described phenomena that comes into play only at very high voltages/pulse rates? You are right it is a bit afield of the official monopole forum topic, though gaining a bit more understanding of induction might be relevant. I'm happy to forget about this question on your thread and doubt there is an easy answer in any event, as I went to write this I had started on winding a joule thief onto a silicon steel core, so I again agree that you are sharing very worthwhile information.


            • #7
              Hi all,

              Just a contribution,
              In my window motor have this:

              Click image for larger version

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              And in a Tesla coil you have this...

              Best regards,

              Marc Hamen


              • #8

                No need for apologies; we are all fellow travelers with the same destination but on different paths.

                'A journey of a thousand miles begins with the first step'

                The answer to your primary question is yes; the induction between the primary and the secondary windings in a Tesla coil are the same phenomena as seen in the SG coil.

                The SG and the Tesla coil are dealing with resonant frequency a visual indicator has been provided kindly by Mr. Bedini in the SG circuit via the neon lamp, which take wattage above and beyond that of what most would consider on the output side of the SG.

                Most electrical engineers would put the expected voltage output of the SG coil in the vicinity of 18 to 24 volts possibly 30 (estimates). Of course, if an electrical engineer built an SG they would design the coil along the lines of a choke coil to tune out unwanted frequency (back EMF), but the SG coil does not do this (that is unless in poor construction one inadvertently uses inferior core design/materials unknowingly building a choke coil).

                Electrical engineers would build the SG as a purely magnetic induction machine - calling it a dynamo.

                For more information see pages118 and 119 of "The Ultimate Tesla Coil Design and Construction Guide" by Mitch Tilbury:

       The ULTIMATE Tesla Coil Design and Construction Guide (9780071497374): Mitch Tilbury: Books

                This is a technical book written for electrical engineers, and will possibly answer any questions you may have at present and in the future on the Tesla Coil.
                But remember the Tesla coil worked in conjunction with a variable wave guide and was drawing varying resonant frequencies from the aether or the vacuum, while the SG in my opinion is a self contained self generating working model of the way the universe works, since there is no vertical capacitance involved, or is there?

                The answer should be obvious if one remembers an old Journey tune from the 1980's.

                Take care.


                • #9
                  the isolation of rods actually avoid 'eddy current' circulating between adjecent rods which can hamper the effieicency of the electromagnet (Coil).
                  'Wisdom comes from living out of the knowledge.'


                  • #10
                    Originally posted by Marc Hamen View Post
                    Hi all,

                    Just a contribution,
                    In my window motor have this:


                    And in a Tesla coil you have this...

                    Best regards,

                    Marc Hamen

                    Will you please elaborate?