TRANSFORMER CORE BUILDING: In this stage of manufacture of transformers the core is constructed. Magnetic circuit is one of the important active parts of a transformer. it consists of laminated core & carries flux linked to winding energy is transferred from one circuit to another through the magnetic field current by core.

The transformer are classified into 2 general type based on their construction
1. Core type
2. Shell type

Core Type

In this type of construction winding surrounds the considerable portion of core. a part of which is surrounded by winding is called limb or leg remaining part of core which is not surrounded binding, but essentially for completing the path of flux is called yoke . this type of construction is more common and has following advantages
1. Construction is simple.
2. cooling is better.

Shell Type

In this type of construction the core surrounds he considerable portion of the winding. Shell type construction in which a cross section of winding is surrounded by the limb and yoke is also used. it has advantage that one use sandwich construction of LV and HV winding to get very low impendence, if desired which is easily possible in the core type construction. in all type of transformers, the core is constructed of lamination’s assembled to provide a continuous path with minimum air gap included.
Raw materials and grade of core core is manufactured from lamination’s of cold rolled grain oriented (crgo)silica steel which gives low specific power loss at operated flux densities joints are designed such that the electromagnetic flux always in the direction of grain oriented.
Further Magnetic Flux reverses direction cut across the core structure it includes a voltage in the core is known as eddy voltages. This action in turn causes eddy currents to circulate in the core.  Also lines of flux that link copper conductor winding of transformer pass through the core itself and contribute to including the electrical currents in it.
These eddy currents heat up the core then waste the power. Eddy Currents tend to flow in a direction to oppose the flux change. That is this field of the current is opposite to the applied field there is shielding the interior of the core thus wasting power.
Eddy currents tend to flow in a direction oppose the flux change. The field of the current tend to flow in a direction to oppose the flux change. The field of the current is opposite to the applied field. There by shielding the interior core from applied field. The loss due to eddy current losses can be determined using the formula.
Pe=1.65*Ve*b*2*d*2*t*2/r Ve is the volume of the cube in cubic meters. F= Frequency T= thickness of core in mts R = Resistivity of the core material in ohm meters It is apparent that both the hysteresis and eddy current losses increases and increasing frequency at the applied voltage.

Material Used in the core building

1. Core Clamps
2. Tie rods
3. Core studs
4.yoke studs with nuts
5. Tap switch cleats
6. Pressure screws
7. Ms washers
8. Cross pads insulation

9. MS rings


Core inspection

After receiving the core materials ensure packing of the core dimensions. thickness, watts loss.
1. And packing of core material should be per standard
2. Procedure for checking of core lamination.
3. Keep the core lamination’s at different sizes or step wise properly.
4. Ensure core lamination’s dimensions by using long scale or measuring tape.
5. If any moisture content& Oil present in the core clean it properly.
 6. If any bends are present at end correct it.
7. Ensure watt loss using watt loss meter.
a. Take watt loss measuring meter
b. Switch on the supply.
 c. Set thickness and flux density,
d. Please the probe on the surface of specimen and there should be no air gap probe and specimen
e. Correct values of thickness and flux density is oversexed and note it down history book.
f. The tester measures the iron loss in the limited are covered by the probe. recommended ti take several values at different points and calculate the the mean which nearly equals to the value specified by the manufacturer.
8. Core loss matter specification
 a. Model-DW20.range-0.2 to 19.00watt/kg
b. Induction-1.0 to 1.8 tesla per m^2
c. Specimen size-30*70mm minimum
d. Thickness -0.1 to 0.65mm to the test
e.Test Object- CRGO,CRNO,HR etc..,
f.Set thickness=B*thickness/set induction on knob
g. Actual watt*set thickness/thickness.

Core Building process

1. First bed is et, Check the level of top and bottom clamps by proper wooden packing and fix the tie rods.
2. Instrument used in leveling.
3. Spirit level.
4. Measuring tape.
5. Hammer
a. For transformers with limb od >300mm flith plates are use instead of tie rods.
b. Before placing the lamination’s on the core bed the diagonal length of the clamps are checked.
c. lamination’s are placed in the core bed the obtain locking in the end. If v notch is present at the corner is called mitered type of building otherwise if rectangular notch is present then it is called spy type core building.
d. After completion of each step check the od, diagonal length and distance between two limbs.
e. The distance above listed should match the design specification.
f. Alternate lamination’s are place reversely to obtain locking at the corners.
g. After all steps completed fix the top clamps.
h. Along with clamp core studs and yoke studs are attached.
i. Insulation board is placed in between core and core clamp.

Conduct meggar test between

a. Conduct clamp and core limb
b. Yoke stud and core limb
c. Tie rods and core limb Ensure the tightening of all tie rods, yoke studs and core studs. Chalk the powder with araldite is provided at cross pads to avoid slipping of bottom core and projection on 1v side should be more then HV as per drawing.

Transformer Core coil Assembly

The core assembly is vertically placed with the foot plate touching the ground. the top yoke of the core is removed. The limbs of the core are tightly wrapped with cotton tape.
Cylinder made out of insulating press board/pressphan paper is wrapped on all the three limbs. Low Voltage Coil is placed on the insulated core limbs
Insulating block of specified thickness and number are placed both at the top and bottom of the L.V.Coil. Cylinder made out of corrugated paper or plain cylinder with oil ducts are provided over L.V.Coil. H.V.Coils are placed over the cylinder.
Gap between each section of  H.V.Coils including top & bottom clearances is maintained with the help of oil ducts, as per the design/drawings.
The Top Yoke is refilled. Top core frame including core bolts and tie rods are fixed in position.
Primary and secondary windings are connected as per the requirements. Phase barrier between H.V.phases are placed as per requirement. Connections to the tapping switch or oltc (if required) are made.
Finally, the component is placed in the oven.


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