Electric Transformer and its working complete notes in pdf format for more article visit http://asmeuol.blogspot.com/

1. Electrical Transformer

3. Electrical Transformer, How it works?
Transformers are capable of receiving AC power at one voltage and delivering it at another voltage. In this article, we will
go through the working and construction of a 3 phase transformer by starting from its simplest form. We will also
understand what power transformer is and how it is constructed.
A detailed webpage version of the video is given below.
Why Transformers are used?
Transformers are ubiquitous devices. They are used to either step-up the A.C voltage or to step-down it. But, why should
we do this voltage transformation?. It is a science fact that a stepped-up voltage is associated with a reduced current. A
reduced current leads to low eddy current energy loss. In this way, transformers help achieve better transmission efficiency
while transferring the power over longer distances.
Fig.1 Transformers help in step-up or step-down the voltage; this in turn increases the transmission efficiency
After the electrical power has transmitted to the desired spot, the voltage can be reduced to the desired
level, using a step-down transformer.
The Basic Working Principle

4. The basic working principle of a transformer is simple, electromagnetic induction. According to this principle, a varying
magnetic flux associated with a loop will induce an electromotive force across it. Such a fluctuating magnetic field can
easily be produced by a coil and an alternating E.M.F (EP) system. A current carrying conductor produces a magnetic field
around it. The magnetic field produced by a coil will be as shown in the first part of Fig.2. With the fluctuating nature of
the alternating current, the magnetic field associated with the coil will also fluctuate.
This magnetic flux can be effectively linked to a secondary winding with the help of a core made up of a ferromagnetic
material. The linked magnetic flux is shown in the second part of Fig.2. This fluctuating magnetic field will induce an
E.M.F in the secondary coils due to electromagnetic induction. The induced E.M.F is denoted by ES.
Fig.2 AC current in a coil produces a fluctuating magnetic field; this magnetic field can effectively linked to a secondary coil with the help of a core
Since the turns are arranged in a series, the net E.M.F induced across the winding will be sum of the
individual E.M.Fs (eS) induced in each turn. Ns represents, number of turns at the secondary winding.
Since the same magnetic flux is passing through the primary and secondary coils, the EMF per turn for both
the primary and secondary coils will be the same.
The E.M.F per turn for the primary coil is related to the applied input voltage as shown.
By rearraging the above equations, it can be established that, the induced E.M.F at the secondary coil is
expressed as follows.
This simply means that with fewer turns in the secondary than in primary, one can lower the voltage. Such
transformers are known as step-down transformers. For the reverse case, one can increase the voltage (step-
up transformer).
But since energy is conserved, the primary and secondary currents have to obey the following relationship.

5. 3 Phase Transformer
Three phase transformers use 3 such single-phase transformers, as shown in the figure below.
Fig.3 A 3 phase transformer can be considered as three independent single phase transformers
It is clear from Fig.3 that, independent 3 phase transformer will require a huge amount of core material and
results in a bulky design. As a result practical 3 phase transformers use a slightly different coil configuration.
To make it more economical the design illustrated in Fig.4 is used. Here, the primary and secondary coils sit
concentrically. Three such concentric pairs are used in 3 phase transformer.
Fig.4 HV and LV windings are placed concentrically in 3 phase transformers
The concentric windings are made to sit on three transformer core limbs as shown in the Fig.9. We will learn
more about the core constriction in the coming sessions.
Power Transformer - Construction Features
The transformers which are used in high voltage applications are referred as 'Power Transformers'. They handle voltage in
the range of 33 to 400 kV. The winding of a power transformer is quite different from that of a low voltage transformer
(Distribution Transformer). We will explore the construction and connection details of the power transformer winding in
this session.

6. Winding type
The power transformers generally employ a special kind of winding, known as a disc-type winding, where separate disc
windings are connected in series , through outer and inner cross-over’s.
Fig.5 the separated out disks are shown in the first part of the figure; The way discs are connected together is shown in the 2nd and 3rd part of the figure.
The first part of Fig.5 shows the separated out discs. In the second and third part of the figure, the inner
and outer cross-over are shown.
Winding Connection
The low-voltage windings of a power transformer are connected in a delta configuration and the high-voltage windings are
connected in a star configuration. The winding connections are shown in the Fig. 6 and Fig.7 respectively.
Fig.6 The low voltage winding is connected in a Delta configuration
The delta connection in low voltage windings results in 3 terminals to connect the electrical power. This is
marked as 'R','Y' and 'B' in the Fig.6.

7. Fig.7 The high voltage windings are connected in a Star configuration
On the contrary, the star connection in high voltage transformer results in 4 terminals to connect the
electric power. This is marked as 'r','y','b' and 'n' in the Fig.7. Thus, if you tap the electrical power between
any pair of the phase wires the voltage further rises to root 3 times. This voltage is known as 'line voltage'.
This also means that, from a 3 phase step-up transformer we can draw 4 output wires; 3 phase power wires
and one neutral. If you draw power between a neutral and phase wire, that is known as 'phase voltage'.
High voltage insulated bushings are required to bring out the electrical energy. It is clear from the Fig.8 that, the bushings
at the high voltage side are quite bigger compared to the low voltage bushings.
Fig.8 Insulated bushings are required for smooth transfer of electrical power

8. The Core Construction
The core of the transformer is made of thin, insulted, steel laminations. Such steel laminations are stacked together, as
shown in the Fig.9, to form 3 phase limbs. The purpose of thin laminations is to reduce energy loss due to eddy current
formation. Please note here that, the separated out layer blocks in the first part of Fig.9 is a stacked layer of much thinner
steel laminations. The thickness of each steel lamination varies from 0.25 - 0.5 mm.
Fig.9 The core is made of thin insulated steel laminations; Such laminations are stacked together to form 3 phase limbs
The low voltage windings usually sit near the core. If HV windings were placed near to the cored, due to the
winding's high voltage, a huge amount of insulation material would be required between the winding and
core. Thus by placing the LV winding near to the core, we can save a good amount of insulation material.
The output voltage of a transformer will undergo minor fluctuations due to the reasons like load variation and change in
power input supply. A tapping mechanism in the secondary coil helps in regulating the output voltage to the specified
limit. The tapping mechanism simply changes the number of active coils in the transformer action, thus controls the output
voltage. Since more number of turns are there in the HV windings, voltage fine tuning can be more accurately controlled
by providing the tapping on the HV side. This is another reason why HV windings are not placed near to the core. If they
were placed near to the core, movement of tapping mechanism would have been more difficult, causing the tapping design
more complex.
Important Link
http://asmeuol.blogspot.com/
Energy losses in a Transformer
Various kinds of energy loss happen while transferring power from the primary to secondary coil. Following are the major
source of energy losses.
 Eddy current loss

9.  Hysteresis loss
 I2
R loss
All these energy loss are dissipated as heat, so a proper cooling mechanism is necessary to keep the core and
winding temperature of the transformer below a specified limit.
Fig.10 Coolant oil circulation in the transformer is depicted in this figure
Usually the transformer is immersed in a cooling oil to dissipate the heat. The oil dissipates the heat via
natural convection. It is clear from the Fig. 10 that, hot oil at the bottom of the tank rises to the top by
natural convection (Buoyancy Force). This hot fluid is passed in to the fins, which are fitted outside of the
transformer, via fin top pipe. The oil liberates heat when it passes through the fins and it gets cooled down.
The low temperature oil naturally sinks to the bottom and enters the transformer through fin bottom pipe.
Thus a circular motion of the oil is created in the transformer.
Use of the Conservator Tank
Oil in the tank will expand as it absorbs the heat. A conservator tank helps to accommodate for this volume change. As
can been seen in Fig.11, there is a free space above the oil, in the conservator tank. When the oil expands, this space
shrinks and accommodate for the volume rise.
Fig.11 The conservator tank, which is fitted on the top of the transformer helps to accommodate for the volume change of the cooling oil

10. To know more on different types of transformer cores and windings please check the other articles.
ASME UOL Official Site
http://asmeuol.blogspot.com/