CORE BUILDING
 |
| TRANSFORMER CORE BUILDING |
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
TRANSFORMER CORE BUILDING :
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.
ConversionConversion EmoticonEmoticon