Transformer the essential line voltage, VL and its

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Transformer rating specified in (kVA)
because copper loss of a transformer               depends on current and iron loss
on voltage. So, total transformer depends on volt ampere (VA) and not on phase
angle between voltage and current. It is independent of load power factor.
That’s why rating of transformer in (kVA) and not in KW(kilowatt).


                        5. Why transformer
rating specified in KVA?

four fundamental setups of a three-stage transformer, we can list the
transformers optional voltages and streams as for the essential line voltage,
VL and its essential line current IL as appeared in the accompanying table.
Where: n breaks even with the transformers “turns proportion” (T.R.)
of the quantity of optional windings NS, partitioned by the quantity of
essential windings NP. (NS/NP) and VL is the line-to-fix voltage with VP being
the stage to-unbiased voltage.

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Three phase transformer
line and voltage current.

 Where once more, VL is the line-to-line
voltage, and VP is the stage to-unbiased voltage on either the essential or the
optional side.


Three phase voltage and current.







(vi)  Scott
connection-  Two
transformers are utilized as a part of this sort of association. One of the
transformers has focus taps on both essential and auxiliary windings.
Scott-association can likewise be utilized for three stage to two stage change.
The association is made as appeared in the figure above.




Open delta connection- Open
delta association can be used when one of the transformers in delta-delta is
incapacitated and the administration is to be continued until the point when
the defective transformers is repaired or supplanted. It can in like manner be
used for little three stage loads where establishment of full three transformer
bank is un-fundamental. The aggregate load passing on limit of open delta
association is 57.7% than that would be for delta-delta association.


Formulae shows
delta star turns ratio.

Delta-star connection-
The essential winding is related in delta and the optional winding is
related in star with unbiased grounded. This sort of connection is
fundamentally used in step up transformer toward the begin of transmission
line. The extent of auxiliary to essential line voltage is ?3 times the change
proportion. There is 30 degree move between the essential and optional line


Formulae shows star-delta turns ratio.


(iii) Star-delta connection-
The essential winding is star-star associated with grounded impartial and the
secondary winding is delta associated. Step down transformer at the substation
end of transmission line is mainly utilized for this connection. The proportion
of secondary to primary line is 1/?3 times the transformation proportion. The
shift between the primary and secondary voltage line voltages is 30 degree.

connection –Generally, large and low voltage transformer use this connection.
Number of required turns is decently greater than star-star association. The
transformation proportion of the transformer is equivalent to the proportion of
line voltages on the primary and secondary side. This connection can be used
notwithstanding for unbalanced loading. Regardless of whether one transformer
is not balanced, system can keep on operating in open delta association is one
of the benefit of the connection


Usually small, high voltage transformers
use Star-star connection. In star
connection number of required turn is reduced. Thus, reduction of the amount of
insulation also takes place. On the off chance that the associated load is
balanced, this association can be utilized.


delta and





or delta can be connected in the winding of primary or secondary windings. It
will provide a total of 4 possible connections for 3 phase transformer.

phase transformer connection


In a shell type three phase
transformer, three stages are more autonomous than they are in core type. Each
stage has its individual magnetic circuit. The development of shell type three
stage transformer is shown in the figure above. The development is like that of
three single stage shell compose transformers kept on the highest point of each

The development of a core kind three
stage transformer is as appeared in the figure. The core comprises of three
legs or limbs. Of course, the coreis comprised of thin overlaid sheets to
decrease eddy current misfortunes. Every limb has essential and secondary windings
in cylindrical shape arranged concentrically.


Three phase ac system
present in almost all major generation and distribution systems in the world. It
constructed from 3 single phase transformers. Three phase transformer framework
introduce in all significant age and conveyance frameworks on the planet. It
developed from 3 single phase transformers. A three-stage transformer is set up
by three arrangements of essential and auxiliary windings, each set wound
around one leg of an iron center get together. Fundamentally it would seem that
three single-stage transformers sharing a joined center as in figure. Three
phase transformer center contains three arrangements of windings. Those
arrangements of essential and optional windings will be associated in either ?
or Y setups to shape a whole unit. Three phase ac system present in almost all
major generation and distribution systems in the world. It constructed from 3
single phase transformers. A three-phase transformer is set up by three sets of primary and secondary
windings, each set wound around one leg of an iron core assembly. Basically it
looks like three single-phase transformers sharing a joined core as in figure.
Three phase transformer core contains three sets of windings. Those sets of
primary and secondary windings will be connected in either ? or Y
configurations to shape an entire unit. This winding can be associated together
in different combinations.


Three phase transformer (operation
and types)


Now we will discuss the
savings of copper in auto transformer compared to conventional two windings
electrical power transformer. We know that weight of copper of any winding
depends upon its length and cross – sectional area. Again length of conductor
in winding is proportional to its number of turns and cross – sectional area
varies with rated current. So weight of copper in winding is directly
proportional to product of number of turns and rated current of the winding.
Therefore, weight of copper in the section AC proportional to (N1 ?

How copper is saving in autotransformer?








Absent of galvanic isolation.

2.     Used only in the
limited places where a slight variation of the output voltage from input
voltage is required.


If an auto transformer is used to
supply low voltage from a high voltage and there is a break in the secondary
winding, the full primary voltage comes across the secondary terminal which is
dangerous to the operator and the equipment. So the auto transformer should not
be used to for interconnecting high voltage and low voltage system.

1.     The secondary winding is not
insulated from the primary winding.


Disadvantages of auto transformer.


3.     A auto transformer
has higher efficiency than two winding transformer. This is because of
less ohm loss and core loss due to reduction of transformer material.

2.     Auto transformer has better voltage
regulation as voltage drop in resistance and reactance of the single
winding is less.

For an case, if transformation ratio = 2, the size of the auto transformer would be
approximately half of the corresponding size of two winding transformer (50%).
For transformation ratio say 20 however the size would be 95 %. The saving in
cost of the material is of course not in the same ratio. The saving of cost is
appreciable when the ratio of transformer is low, that is lower than 2. Thus
auto transformer is smaller in size and cheaper.

of auto transformer

of step down transformer. The picture shows that number of primary higher than
number of secondary coil.


Step Down Transformer- A
transformer in which Np >Ns is called a step down transformer. A step down
transformer is a transformer which converts high alternating voltage to low
alternating voltage.

of step up transformer. The picture shows that Ns>Np.

Step Up Transformer –At the point when
the number of secondary coil is higher than the primary coil, it will be called
as step up transformer. It converts low alternative voltage to high alternative

The currents of
the primary and secondary windings are steaming on the opposite directions, so
the total current flowing through the basic part of the winding is equivalent to
the difference between the current on the low-voltage winding and the current on the high-voltage winding. The
two windings should have the same winding sense in order for an autotransformer
to work properly.

operation of the transformer and common transformer is the same, then the connection
amongst input and output voltages and input and
output currents and the ratio of number of turns
between the primary and the secondary winding is the same.

of working operation.



is done by comparing the amount of copper saved in the winding. If the ratio
‘n’ characterized as the ratio of the lower voltage to the higher voltage, at
that point it can be demonstrated that the saving in copper is: n100%

to conventional double wound transformers of the same VA rating,
autotransformers are much cheaper. When choosing upon using an autotransformer
it is common to contrast with that of an equivalent double wound type.

any case, the regulation, leakage inductance and physical size of an
autotransformer for a given VA or Kva rating are less than for a double wound

these lines, the part of the windings passes on the contrast between the two
currents cab be made from a smaller conductor size, since the currents are much
smaller saving on the cost of an equivalent double wound transformer.

 An autotransformer is used principally for the
adjustments of line voltages to either change its value or to keep it
consistent. If the voltage modification is by a small amount, either up or
down, at that point the transformer ratio is small as Vp and Vs are almost
equal. Currents Ip and Is, are also nearly equivalent.

Auto transformer terminal


The standard procedure for denoting an auto transformer
winding is to name it with capital letters. Essentially the normal unbiased
association is set apart as N or (n). For the secondary tapping’s, suffix
numbers are utilized for all tapping points along the auto transformer
secondary winding. These number generally begin at number1 and continue in
ascending order for all tapping points as shown.


Autotransformer with multiple tapping

The autotransformer can
also be constructed with more than one single tapping point. Auto-transformers
can be utilized to provide different voltage points along its winding or
increase its supply voltage concerning its supply voltage VP as shown.

the point when the essential current Ip, is moving through the single winding
toward the arrow as shown up, the secondary current, Is, streams in the
opposite direction. Along these lines, in the part of the winding that
generates the secondary voltage, Vs the present streaming out of the winding is
the difference of Ip and Is.


The type with programmed
voltage alteration can be utilized as programmed voltage regulator, to keep up
an steady voltage at the clients administration during a wide range of line and
load conditions. Another application is a lighting dimmer that doesn’t create
EMI typical of most thyristor dimmers.

The yield voltage alteration can
be manual or automatic. The manual kind is relevant just for moderately low
voltage and it known as variable AC transformer. These are regularly utilized
as a part of repair look for testing gadgets under various voltages or to
reproduce irregular line voltages.

A constant variable turns of
ratio can be observed by uncovering some part of the winding coils and making
the secondary connection through a sliding brush. It is allowing for very
smooth control of output voltage. Discrete voltage represented by actual number
of turns does not effects the output voltage. The voltage can be smoothly
varied between turns as the brush has a relatively high resistance (compared
with a metal contact) and the actual output voltage is a function of the
relative area of brush in contact with adjacent windings. The relatively high
resistance of the brush also prevents it from acting as a short circuited turn
when it contacts two adjacent turns. Typically, the primary connection connects
to only a part of the winding allowing the output voltage to be varied smoothly
from zero to above the input voltage and thus allowing the device to be used
for testing electrical equipment at the limits of its specified voltage range.


A variable auto

·       Construction:


Auto transformer is a device that with only one winding wound on a laminated
core. An auto transformer is similar to a two winding transformer but differ in
the way the primary and secondary winding are interrelated. Both primary side
and secondary side is common to a part of the winding. On load condition, a
part of the load current is obtained directly from the supply and the remaining
part is obtained by transformer action. An Auto transformer works as a voltage regulator.



Categories: Construction


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