For designing of transformer core first we take a concept about air gap and fringing flux in magnetic core.


A magnetic material with high permeability is one which has low reluctance path. If an air gap is included with a magnetic circuit like iron, almost all of the reluctance in the circuit will be at the gap, because the reluctivity of air is much greater, than that of a magnetic material. So, controlling the size of the air gap controls the reluctance.
Types of air gap:
There are two types of gaps used in the design of magnetic components. Bulk and distributed. Bulk gaps are maintained with materials such as paper, Mylar or even glass. The gapping materials are designed to be inserted in series with the magnetic path to increase the reluctance. Placement of gap in each leg and gapping materials in gap needs a proper design. Design of gap placement also required for minimizing the noise for fringing.
So, placement of small air gap in magnetic core will lower and stabilize the effective permeability.


Whenever an air gap is inserted into the magnetic path, there is an induced fringing flux at the gap. Its effect is to shorten the air gap. Fringing flux decreases the total reluctance of the magnetic path and therefore increases the inductance.
As the air gap increases, the fringing flux will increase. If a coil is wound tightly around the core and encompasses the gap, the flux generated around the magnet wire will force the fringing flux back into the core.
Flux will always take the path of highest permeability. This can best be seen in transformers with interleave laminations. The flux will traverse along the lamination until it meets its mating, I or E. At this point, the flux will jump to the adjacent lamination and bypass the mating point,
At low levels of excitation, the flux is taking the high permeability path, by jumping to the adjacent lamination. As the excitation is increased, the adjoining lamination will start to saturate, and the exciting current will increase and become nonlinear. When the adjacent lamination approaches saturation, the permeability drops. It is then that the flux will go in a straight line and cross the minute air gap. Air gaps in a magnetic core will add considerable reluctance to the magnetic circuit. Remembering that the inductance of a coil and the magnetic reluctance are inversely proportional, air gaps reduce the inductance of the coil and increase the magnitude of magnetizing currents.
In practical transformers, we want to reduce magnetizing currents to almost negligible levels; it is therefore important to eliminate all air gaps if possible.
One approach would be to make the core from a solid block of material. This is impractical from the standpoint of fabricating the transformer, since the coils would have to be wound through the core window. Also, since metallic core materials conduct electric current as well as magnetic flux, the induced voltages would produce large circulating currents in a solid core. The circulating currents would oppose the changing flux and effectively ‘‘short out’’ the transformer.
A practical solution is to fabricate the core from thin laminated steel sheets that are stacked together and to coat the surfaces of the laminations with a thin film that electrically insulates the sheets from each other.


If an alternating voltage is applied to the primary winding, it will induce an alternating flux in the core. The alternating flux will, in turn, induce a voltage on the secondary winding.
This alternating flux also induces a small alternating voltage in the core material. These voltages produce currents called eddy currents, which are proportional to the voltage. The magnitude of these eddy currents is also limited by the resistivity of the material. The alternating flux is proportional to the applied voltage. Doubling the applied voltage will double the eddy currents. This will raise the core loss by a factor of four.
Eddy currents not only flow in the lamination itself, but could flow within the core as a unit, if the lamination is not properly stamped, and if the lamination is not adequately insulated.
There are two eddy currents, as shown ia & ib. The intra laminar eddy current, ia, is governed by flux, per lamination and resistance of the lamination. It is, therefore, dependent on lamination width, thickness, and volume resistivity. The interlaminar eddy current, ib, is governed by total flux and resistance of the core stack. It is primarily dependent upon stack width and height, the number of laminations, and the surface insulation resistance, per lamination.
The magnetic materials used for tape cores and laminations are coated with an insulating material. The insulating coating is applied to reduce eddy currents.


Core laminations are built up to form a limb or leg having as near as possible, a circular cross-section in order to obtain optimum use of space within the cylindrical windings. The stepped cross-section approximates to a circular shape depending only on how many different widths of strip a manufacturer is prepared to cut and build. For smaller cores of distribution transformers this could be as few as seven. For a larger generator transformer, for example, this might be 11 or more.
Laminations are available in scores of different shapes and sizes. The punch press technology for fabricating lamination has been well-developed. Most lamination sizes have been around forever. The most commonly used laminations are the El, EE, FF, UI, LL, and the DU. The laminations differ from each other by the location of the cut in the magnetic path length. This cut introduces an air gap, which results in the loss of permeability. To minimize the resulting air gap, the laminations are generally stacked in such a way; the air gaps in each layer are staggered.

Stacking Laminations and Polarity:

If the laminations are stacked correctly, all of the burred ends will be aligned. If the laminations are stacked randomly, such as the burr ends facing each other, then, the stacking factor would be affected. The stacking factor has a direct impact on the cross-section of the core. The end result would be less iron. This could lead to premature saturation, as an increase in the magnetizing current, or a loss of inductance.
There are several methods used in stacking transformer laminations. The most common technique used in stacking laminations is the alternate method. The alternate method is where one set of laminations, such as an E and an I, are assembled. Then, the laminations are reversed, as shown in Figure. This technique, used in stacking, provides the lowest air gap and the highest permeability. Another method for stacking laminations is to interleave two-by-two. The second method of stacking would be in groups of two or more. The loss in performance in stacking, other than one by one, is the increase in magnetizing current and a loss of permeability. The actual laminations are too thin to show individually. The empty spaces between the core and the circular coil are filled with wooden dowels or other spacer materials to improve the mechanical strength of the transformer.


Any stress or strain of the magnetic materials will have an impact on the permeability. The resulting stress could cause higher magnetizing current, or a lower inductance. When the transformer is being assembled (in the stacking process), and a lamination is bent (does not return to its original shape), that lamination has been stressed and should be replaced.
Some of the important magnetic properties are lost due to stress and strain after stamping, shearing and slitting. These properties that have been lost or seriously reduced can be restored to the magnetic material by annealing. Basically, stress relief is accomplished by heating (annealing) the magnetic material to prescribed temperature, (depending on the material), followed by cooling to room temperature. The annealing must be done under controlled conditions of time, temperature and the ambient atmosphere that will avoid even minute, adverse changes in the chemistry of the steel.


The form of the complete core will, however, vary according to the type of transformer. Alternative arrangements are shown in; of these, by far the most common arrangement is the three-phase, three-limb core. Since, at all times the phasor sum of the three fluxes produced by a balanced three-phase system of voltages is zero, no return limb is necessary in a three-phase core and both the limbs and yokes can have equal cross-section. For single-phase transformers, return limbs must be provided.


Any conducting metal parts of a transformer, unless solidly bonded to earth, will acquire a potential in operation, it is more convenient to bond them to earth. The core and its framework represent the largest bulk of metalwork requiring to be bonded to earth. On large, important transformers, connections to core and frames can be individually brought outside the tank via 3.3 kV bushings and then connected to earth externally. This enables the earth connection to be readily accessed at the time of initial installation on site. To avoid circulating currents, the core and frames will need to be effectively insulated from the tank and from each other.


There are two basic types of core construction used in power transformer; core form and shell form.
In core form construction, there is a single path for the magnetic circuit. The coils are wrapped or stacked around the core. For single phase application, the windings are typically divided on both core legs. Each coil group consists of both high and low voltage winding. It ensures better magnetic coupling between two coils and lesser magnetic leakage. In three phase applications, the winding of a particular phase are typically on the same core leg.
For small core type transformers rectangular cores with rectangular cylindrical coils are used whereas for large transformers, circular cylindrical coils are used. Insulating cylinders of fuller board are used to separate the winding from core and from each other.
In shell form construction, the core provides multiple paths for the magnetic circuit. In this design, the core is wrapped or stacked around the coils. The windings are usually “pancake” type winding. Here high and low voltage windings stacked on top of each other, generally in more than one layer each in an alternating fashion.
Each of these types has its own advantages and disadvantages. As per cost, shell form is very popular in distribution transformer and low voltage applications, like electronic circuits, converters, etc. as the core can be economically wrapped around the coils and less copper required due to single leg winding. For moderate to large power transformers, the core form design is more common, possibly because the short circuit forces can be better managed with cylindrically shaped windings.
Difference between Core & shell type transformer in tabular form.

  • Copper winding surrounds the iron core.
  • Core surrounds the copper winding.
  • For single phase, has only one magnetic path.
  • Has two magnetic path.
  • More copper require for winding.
  • Less copper require.
  • More losses.
  • Less losses.
  • Less out-put.
  • More out-put.
  • Maintenance are easy.
  • Difficult.
  • Used in power transformer, auto transformer.
  • Electronic circuit, converter,small transformer.

    In both core form and shell form types of construction, the core is made of thin layers or laminations of electrical steel. The laminations can be wrapped around the coils or stacked. Wrapped or wound cores have few, if any; joints so they carry flux nearly uninterrupted by gaps. Stacked cores have gaps at the corners where the core steel changes direction. This results in poorer magnetic characteristics than for wound cores. In larger power transformers, stacked cores are much more common while in small distribution transformers, wound cores predominate. Laminations for both types of cores are coated with an insulating coating to prevent eddy current paths from developing which would lead to high losses. A wound core without a joint would need to be wound around the coils or the coils would need to be wound around the core. In stacked cores for core form transformers, the coils are circular cylinders which surround the core.

    Keep in pocket:→

    ✓Air gaps and fringing carries a vital role for transformer core design.
    ✓Air gap increases reluctance of the path, whereas, fringing reduces the reluctance.
    ✓Lamination of core is required for minimizing eddy current.
    ✓Constructionally, transformer core are two types, core type and shell type.
    ✓Cylindrical types winding are normally used in core type transformer whereas pancake type winding are used in shell type transformer.
    ✓In both core and shell form, laminations are stacked or wrapped to construct the core.

    Short questions related to this topic:

    Q. Air gap in lamination produces_____?


    Q. Why alternate method of stacking lamination are used?

    A. To reduce air-gap effect.

    Q. How eddy current loss reduces?

    A. By laminating the magnetic core.

    Q. How fringing flux reduces?

    A. By wounding coil tightly around the core.

    Q. How maximum space can be used for circular wound coil?

    A. Core build up with circular cross section.

    Q. When core form and when shell form are generally used?

    A. For power transformer, core form is used and for small transformer shell types are preferable.

    Q. Which types require less copper?

    A. Shell type.

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