Magnetic cores are selected in accordance with the desirability for a specific application. So, in the equipment of wide application (players, televisions, tape recorders, etc.), the minimum cost of a magnetic circuit and a winding wire, as well as efficiency in production, can serve as an optimality criterion. In meteorological probes, airplanes and other aircraft, the criterion of optimality may consist of obtaining winding components of minimum mass. In special equipment, the optimality criterion may be a specific indicator. For example, a product operating in a robot when exposed to penetrating radiation may be presented with the basic requirement of high radiation resistance.
Based on Magnetic Core Selection
Toroidal (ring core) transformers without a gap help to obtain high winding inductance at low magnetic core material consumption and small product dimensions which is an advantage. Additionally, toroidal transformers without a gap create very small scattering fields during the operation of the devices, which is why it is advisable to use them in noise-sensitive equipment. The downside of toroidal transformers is the difficulty of manufacturing and, as a result, low manufacturing capacity, which raises the manufacturing costs of a winding product. The outer winding of the toroidal transformer can be protected from damage, for example, by a layer of coating insulation.
Toroidal magnetic cores with a gap, as well as toroidal magnetic cores without a gap, make it possible to conduct small dimensional winding parts, having consumed a minimum of materials. The scattering fields of toroidal magnetic cores with a gap, however, are slightly larger than the gapless toroidal magnetic cores.
Cup (pot core) or pot-shaped magnetic cores, as they are also called, are good for low-current, signal circuits. Unlike toroidal magnetic cores, because of the shielding of the core winding, they have high scattering fields if the sidewalls do not have a large non-magnetic gap, but the scattering fields of the toroidal magnetic cores are usually smaller without a gap. In addition, the center of the cup protects the windings from mechanical damage. Cup magnetic cores are advanced technologically; coil-wound components made on them are cheaper in production than toroidal ones. The disadvantage of cup cores is a larger volume and a larger amount of magnetic core material relative to toroidal magnetic cores.
Winding components with C Cores are technologically advanced, however, compared to components with toroidal or cup cores they typically have large scattering fields. Within C cores, a non-magnetic void can be arranged with no difficulty. A W-shaped magnetic circuit closes part of the winding which protects it from mechanical damage.
To reduce the scattering fields of the C magnetic cores, one turn of copper or brass tape often covers all three cores, in which the beginning and the end are soldered. The central cores of the W-shaped magnetic cores may have a circular (ETD core) or rectangular portion on which the windings are mounted
The windings are placed on dielectric sleeves which repeat the magnetic cores across sections. A shorter length of winding wires is needed when placing the same number of turns on a sleeve of a shaped magnetic core with a round core than when placing windings on a sleeve of an Sh-shaped magnetic core with a rectangular magnetic core of the same cross-sectional area. When the first layers of winding are placed on the sharp edges of the sleeve of a C magnetic core with a rectangular magnetic core, the state of the wire’s insulation coated with a diameter of about 0.5 mm or more should be controlled, as it can be impaired by excess. In addition, usually, the windings of magnetic cores with a toroidal magnetic core can be laid more densely on the sleeve than on a rectangular sleeve.
C core is more advanced technologically than toroidal ones. Components with C cores have wide scattering fields compared with toroidal winding components which are a downside. Windings that cover (and shield) the junction of the magnetic circuit’s C cores are halved and are usually positioned on both rods. Winding components made on rod cores are usually highly technical in the form of rods with round (rot core) or rectangular (plate core) parts, but are distinguished by very broad scattering fields. Manufacturing firms produce a much wider array of magnetic cores.
Based on Losses at different Frequencies
Losses in magnetic circuits of components operating at low frequencies.
The French scientist Dominic Francois Arago (Arago Dominique Francois) first discovered eddy currents in 1824, and the study of those currents was carried out by another French physicist: Jean Bernard Léon Foucault. Eddy currents are called Foucault currents, in honor of the latter. An alternating electromagnetic field induces foucault currents in every material that is electrically conductive. Such currents flow in the direction in which they have the greatest resistance to the origin of their incidence over closed circular trails.
An alternating electromagnetic field induces foucault currents in every material that is electrically conductive. These currents flow in the direction in which they have the greatest resistance to the cause of their occurrence along closed circular trails. The lower the material’s resistance and the higher the magnetic flux change rate, the higher the foucault currents and the higher the foucault currents, the higher their thermal influence. Due to the thermal influence of the Foucault currents the metal is melted in the induction heating furnaces. Foucault currents are trying to reduce as much as possible in magnetic cores and in winding wires.
The magnetic cores of devices that work at dozens of hertz at low frequencies are often made of permalloys or transformer steels. If the magnetic circuit is continuous, then the Foucault currents in it would be high, there would be a lot of heat released in the magnetic circuit, which could lead to overheating failure of the coil components.
Low power low frequency transformer The magnetic circuit is not continuous, but from a series of thin plates or tapes, which are electrically isolated from each other, greatly weakening the detrimental effect of Foucault currents.
These are typically wrapped around a ring structure to create a magnetic toroidal circuit. A “ring” can be dissected into two parts, and then such a magnetic circuit is called broken, to the ease of placing on a coil with windings.
A layer of oxide or varnish can form insulation. The thickness of the plates and tape for components operating at 50 Hz household network frequency is typically 0.3 to 0.4 mm, components operating at 400 Hz-0.05 to 0.1 mm frequency, and components operating at 1 kHz-0.02 frequency. And 0.05 mm. The higher the frequency, the thinner the metal thickness should be, however it is extremely difficult to produce with a thickness of less than 0.02 mm, therefore metal magnetic cores are not used for the manufacture of winding components operating at higher frequencies.
Losses in magnetic circuits of components operating at high frequencies.
One can note a decrease in effective permeability and magnetic induction in the process of core magnetization reversal in winding products operating at a high frequency, which occurs due to magnetic viscosity. Magnetic viscosity, or, in other words, magnetic aftereffect, is called the delay in magnetic induction transition with a change in field strength. The magnetic parameters of ferrite deteriorate as a result of magnetic viscosity manifestations as high-frequency or pulsed currents pass through the device winding. A return to the initial state depends on the magnetic circuit material, and for some materials, hundreds of picoseconds are the relaxation time.
As applied to ferrites, magnetic relaxation is a mechanism that results in a substance’s thermodynamic equilibrium due to the establishment of a balance between the electron spins and the crystal lattice. The rate of change in induction can be much higher with a mechanical impact on ferrite, and the magnetic viscosity may be less than without compression.
Summing up, it should be emphasized that the magnetic induction and permeability of ferrite magnetic cores in pulsed power supplies as a result of magnetic viscosity is reduced, which must be taken into account when entering the margin for these parameters during the calculations of transformers and chokes.