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MAGNETIC CORE OF TRANSFORMER:

The purpose of magnetic core is to provide a low reluctance path for the magnetic flux linking primary and secondary winding. Most materials have low permeability, so they are poor conductors of magnetic flux. Vacuum has a permeability of 1.0 and non magnetic materials such as paper, air and copper have permeability of the same order. But some materials such as iron, nickel, cobalt and their alloys have some high permeability, ranging hundreds to thousands. So, a magnetic core with these types of materials which have high permeability is better choice than air core. By this way the path of magnetic flux is defined. But there is also a limit of how much magnetic flux can be generated in a magnetic material before the magnetic core goes into saturation. After that magnetic core act as air core.

CHARACTERISTICS OF MAGNETIC CORE:

PERMEABILITY: Permeability is the ability of a magnetic material to conduct flux. The magnitude of the permeability represents the magnetic materials quality. It is the ratio of flux density B and magnetizing force H.
Permeability =B/H.
RELUCTANCE: Reluctance is the resistance of the magnetic flux, so we need a low reluctance path for magnetic core.
SATURATION OF MAGNETIC CORE: If we apply an external magnetizing force to a completely demagnetized ferromagnetic material, resultant flux density increases very slowly at first. After certain moment it increases very rapidly. But after a certain value it stops increasing. At that point the magnetic core materials act as air core.
B/H=1.
This is the saturation level of magnetic core.


B-H CURVE & HYSTERESIS LOOP:

For actual transformer core materials, the relationship between B & H depends on the flux that periodically changes. So, this curve depends on the magnitude of flux density and the periodic frequency.
If we pass a magnetic material through a complete cycle of magnetisation and demagnetisation, we know that it will follow a loop, known as hysteresis loop or B-H loop. This loop represents energy lost in the core. The enclosed area of the loop is the measured area of energy lost in the core material during the cycle. In a.c application, the process is repeated continuously and the total hysteresis loss depends upon the frequency. This loss generates heat in the core material.

CORE MATERIAL:

Magnetic core are made of three types of materials like bulk metal, powdered materials and ferrite materials. Bulk metals are processed from the furnace into ingots. Then it also processed by hot and cold rolling. The rolling produces a sheet thickness ranging from 0.004 to 0.031 inches and punched into lamination.
Powder cores such as powder molly Permalloy and powdered iron materials are die pressed into toroids, EE cores and slugs. It’s processing starts at the ingots then goes through various steps of grinding until the powder is the right consistency for the required performance.
Ferrites are ceramic materials of iron oxide, alloyed with oxides of carbonate of manganese, zinc, nickel, magnesium or cobult. Based on the required permeability of the core, alloys are selected and mixed. Then these mixtures are molded into the desired shape with pressure and fired at temperature above 200 degree F. Ferrites can be machined to almost any shape as desire.

CORE STEEL:

As the core of transformer experiences iron losses due to hysteresis and eddy currents flowing within it which heats the core material. So, the core losses though in small amount are present whenever the transformer is energized.
Core loss is made up two components; first hysteresis loss which is proportional to the frequency and depends on the area of the hysteresis loop. Second is the eddy current loss which depends on the square of the frequency and square of the thickness of the material. So, choosing a material with minimum area of hysteresis loop minimize the hysteresis loss, while minimizing eddy current loss is achieved by building up the core from a stack of thin laminations and increasing resistivity of the material in order to make it less easy for eddy currents to flow.
The addition of small amount of silicon or alluminium to the iron greatly reduces the magnetic losses.

HOT ROLLED STEEL:
With the addition of silicon to the iron, not only reduces hysteresis loss, also increases permeability and resistances. Thus reduces eddy current losses. The disadvantage is that the steel become brittle and hard. The elimination of impurities like carbon is also necessary for reduction of losses. Electrical steel sheets have a crystalline structure so that the magnetic properties of the sheet derived from the magnetic properties of the individual crystal or grain and depend on the direction of the crystal in which they are measure. Firstly laminations of thickness around 0.35 mm used in transformers, then the hot rolling process came, in which the grains are packed together in a random way so that magnetic properties observed in a sheet have similar values independent of the direction in which they are measured. The materials are known as isotropic.

GRAIN ORINTED STEEL:
With the thickness 0.32 mm with a loss of 1.5 W/Kg at 1.5 T, 50 HZ, grain oriented cold-rolled silicon steel is introduced in the year 1939. The material is cold reduced by a set of process. Initially hot rolled strip is pickled to remove surface oxides and is then cold rolled. The material is given an annealed to re-crystallize the cold worked structure before cold rolling again to the final gauge. Then it layered by thin magnesium oxide (MgO). Finally the material is given a flattering anneal, when excess magnesium oxide is removed and a thin phosphate coating is applied which reacts with the magnesium silicate to form a strong, highly insulating coating.

HIGH- PERMEABILITY STEEL:
In 1965, the Japanese, Nippon Steel Corporation announced the high permeability grain oriented silicon steel. They introduced around 0.025% of alluminium to the melt and result the use of alluminium nitrate. The final product has a better orientation than cold rolled grain oriented steel with most grains aligned within 3 degree of the ideal. Its permeability is also near about 3 times higher than that of best steel, whereas loss and magnetostriction were lower. By improved coating the hysteresis loss is also reduced.

DOMAIN-REFINED STEEL:
The same Japanese company in early 1980’s introduced improved steel with 5-8% lower loss than high permeability steel. Eddy-current loss that arises in part, due to magnetic domain wall movements, during the cycles of magnetisation is minimized. In a grain oriented material anomalous eddy current loss is proportional to the domain wall spacing and inversely proportional to sheet thickness. Introduction of strain into sheet steels had the effect of subdividing magnetic domains and thus reducing core loss. Here high power laser beam is trained to the surface of the sheet, the outer most layer of the sheet vaporizes and scatters instantaneously. As a result, an impact pressure of several thousand atmospheres is generated to form a local elastic-plastic area in the sheet. As the laser irradiation vaporizes and scatters the outermost layer of the sheet, an additional coating is necessary in order to make good the surface insulation layer.
The residual strains will be removed if the material is subsequently annealed at a temperature above 500 degree C, thus reversing the process. It is important therefore that any process carried out after laser etching should not take the temperature above 500 degree C.

AMORPHOUS STEELS:
It has a totally different source than the silicon core steels. Amorphous metals have a non-crystalline atomic structure. There are no axes of symmetry and the constituent atoms are randomly distributed within the bulk of the material. They rely for their structure as a very rapid cooling rate of the molten alloy and the pressure of a glass forming element such as boron. Typically they might contain 80% iron with the remaining 20% boron and silicon. Various production methods exist but the most popular involve spraying a stream of molten metal alloy to a high speed rotating copper drum. The difficulties of cutting and building this into a conventional core can tend to outweigh any benefits gained. Another practical problems associated with amorphous steel is its poor stacking factor.

MICROCRYSTALLINE STEEL:
This type of steel is also an improved version of silicon steel. It is the production of high silicon and alluminium iron alloys by rapid solidification. These materials have the advantage of far higher field permeability than that of amorphous materials.

Lastly, the cold rolled grain- oriented steels which replaced the earlier hot- rolled steels is maximum used core material. However introduction of high permeability grain oriented steels was more gradual but its higher cost tends it to restricted use. A similar situation for the laser etched steel too. Availability and cost is the main reason for using it only in special cases.

MAGNETOSTRICTION:

The noise of the power transformer is a major problem. If the audible noise is loud enough, it can be very annoying. Essentially all transformers noise is due to a phenomenon called magnetostriction. When a strip of steel is magnetized, it contracts very slightly. At the flux densities used in large power transformers, the amount of magnetostriction is only about 60µ per meter of length. For a 50 HZ transformer, this small change in dimension occurs 100 times per second. There are also harmonics of 100 HZ present in the noise. If any part of the transformer is in resonance with any of the harmonics, the noise can be amplified hundreds of times. So, analysis of resonant frequencies is required for core design.

Keep in pocket:→

✓Primary & secondary coil of a transformer wound on a steel frame named as core.
✓core is made by high silicon content laminated steel sheet with minimum air gap.
✓It provides magnetic path with high permeability & carries alternating flux.
✓After saturation magnetic core act as air core.
✓B-H curve or Hysteresis loop is very essential for choosing core material.
✓Due to magnetization of steel strip, noise of a transformer is called magnetostriction.

Short questions related to this topic:

Q. Why magnetic core is required?

A. Provide a path for flux.

Q. Permeability of magnetic core should be______?

A. High.

Q. Reluctance of magnetic path should be_____ ?

A. As low as possible.

Q. After saturation level, magnetic core act as?

A. Air core.

Q. Hysteresis loop size should be_____for choosing core material ?

A. Low.

Q. Losses in core generates what?

A. Heat.

Q. Hot rolled or cold rolled steel which one is great use?

A. Cold rolled silicon steel.

Q. How magnetostriction reduced?

A. By proper core design.

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