2. TRANSFORMER
• A transformer is a static device.
• The word ‘transformer’ comes form the word ‘transform’.
• Transformer is not an energy conversion device, but it is device that changes
electrical power at one voltage level into electrical power at another voltage
level through the action of magnetic field but with a proportional increase or
decrease in the current ratings., without a change in frequency.
• It can be either to step-up or step down.
3.
4. TYPES OF TRANSFORMER
STEP UP TRANSFORMER:
A transformer in which voltage across
secondary is greater than primary voltage
is called a step-up transformer (shown in
figure)
In this type of transformer, Number of
turns in secondary coil is greater than that
in Primary coil, so this creates greater
voltage across secondary coil to get more
output voltage than given through primary
coil.
5. TYPES OF TRANSFORMER
STEP DOWN TRANSFORMER:
•A transformer in which voltage across
secondary is lesser than primary voltage is
called a step-down transformer (shown in
figure)
•In this type of transformer, Number of
turns in secondary coil is lesser than that in
Primary coil, so this creates lesser voltage
across secondary coil, so we get low
output voltage than given through primary
coil.
6. The transformer works in the principle of mutual induction
“The principle of mutual induction states that when the two
coils are inductively coupled and if the current in coil change
uniformly then the e.m.f. induced in the other coils. This e.m.f
can drive a current when a closed path is provide to it.”
When the alternating current flows in the primary coils, a changing magnetic
flux is generatedaround the primary coil.
The changing magnetic flux is transferred to the secondary coil through the iron
core
The changing magnetic flux is cut by the secondary coil, hence induces an e.m.f
in the secondary coil
WORKING
7. Now if load is connected to a secondary winding, this e.m.f drives a current
through it
The magnitude of the output voltage can be controlled by the ratio of the
no. of primary coil and secondary coil
The frequency of mutually induced e.m.f as same that of
the alternating source which supplying to the primary
winding b
8. For the simple construction of a transformer, you must need two coils having
mutual inductance and a laminated steel core. The two coils are insulated
from each other and from the steel core. The device will also need some
suitable container for the assembled core and windings, a medium with
which the core and its windings from its container can be insulated.
In order to insulate and to bring out the terminals of the winding from the
tank, apt bushings that are made from either porcelain or capacitor type must
be used.
In all transformers that are used commercially, the core is made out of
transformer sheet steel laminations assembled to provide a continuous
magnetic path with minimum of air-gap included. The steel should have high
permeability and low hysteresis loss. For this to happen, the steel should be
made of high silicon content and must also be heat treated. By effectively
laminating the core, the eddy-current losses can be reduced. The lamination
can be done with the help of a light coat of core plate varnish or lay an oxide
layer on the surface. For a frequency of 50 Hertz, the thickness of the
lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.
9. Classification of Transformer
• As per phase
1. single phase
2. Three phase
• As per core
1. Core type
2. Shell type
• As per cooling system
1. Self-cooled
2. Air cooled
3. Oil cooled
10.
11. Three phase transformer
Normally , when three-phase is required, a single enclosure with three
primary and three secondary windings wound on a common core is all that
is required. However three single-phase transformers with the same rating
can be connected to form a three-phase bank. Since each single-phase
transformer has a primary and a secondary winding, then 3 single-phase
transformers will have the required 3 primary and 3 secondary windings and
can be connected in the field either Delta-Delta or Delta-Wye to
achieve the required three-phased transformer bank
12. Transformer classified
as per core
CORE TYPE TRANSFORMER:-
In core-type transformer, the windings are given to a
considerable part of the core. The coils used for this transformer are form-wound and are of
cylindrical type. Such a type of transformer can be applicable for small sized and large sized
transformers. In the small sized type, the core will be rectangular in shape and the coils used are
cylindrical. The figure below shows the large sized type. You can see that the round or cylindrical
coils are wound in such a way as to fit over a cruciform core section. In the case of circular
cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical
coils will have different layers and each layer will be insulated from the other with the help of
materials like paper, cloth, macerate board and so on. The general arrangement of the core-type
transformer with respect to the core is shown below. Both low-voltage (LV) and high voltage (HV)
windings are shown.
13. The low voltage windings are placed nearer to the core as it is
the easiest to insulate. The effective core area of the
transformer can be reduced with the use of laminations and
insulation
14. 2. Shell-Type Transformer
In shell-type transformers the core surrounds a considerable portion of
the windings. The comparison is shown in the figure below.
The coils are form-wound but are multi layer disc type usually wound in the form of pancakes.
Paper is used to insulate the different layers of the multi-layer discs. The whole winding consists
of discs stacked with insulation spaces between the coils. These insulation spaces form the
horizontal cooling and insulating ducts. Such a transformer may have the shape of a simple
rectangle or may also have a distributed form. Both designs are shown in the figure below:
15. A strong rigid mechanical bracing must be given to the cores and coils of the transformers. This will
help in minimizing the movement of the device and also prevents the device from getting any
insulation damage. A transformer with good bracing will not produce any humming noise during its
working and will also reduce vibration.
A special housing platform must be provided for transformers. Usually, the device is placed in tightly-
fitted sheet-metal tanks filled with special insulating oil. This oil is needed to circulate through the
device and cool the coils. It is also responsible for providing the additional insulation for the device
when it is left in the air.
16. CLASSIFICATION ON THE BASIS OF
COOLING EMPLOYED
1. Oil Filled Self-Cooled Type
Oil filled self cooled type uses small and medium-sized distribution transformers. The assembled
windings and core of such transformers are mounted in a welded, oil-tight steel tanks provided with a
steel cover. The tank is filled with purified, high quality insulating oil as soon as the core is put back at
its proper place. The oil helps in transferring the heat from the core and the windings to the case from
where it is radiated out to the surroundings. For smaller sized transformers the tanks are usually smooth
surfaced, but for large size transformers a greater heat radiation area is needed, and that too without
disturbing the cubical capacity of the tank. This is achieved by frequently corrugating the cases. Still
larger sizes are provided with radiation or pipes.
2. Oil Filled Water Cooled Type
This type is used for much more economic construction of large transformers, as the above told self
cooled method is very expensive. The same method is used here as well- the windings and the core are
immersed in the oil. The only difference is that a cooling coil is mounted near the surface of the oil,
through which cold water keeps circulating. This water carries the heat from the device. This design is
usually implemented on transformers that are used in high voltage transmission lines. The biggest
advantage of such a design is that such transformers do not require housing other than their own. This
reduces the costs by a huge amount. Another advantage is that the maintenance and inspection of this
type is only needed once or twice in a year.
3. Air Blast Type
This type is used for transformers that use voltages below 25,000 volts. The transformer is housed in a
thin sheet metal box open at both ends through which air is blown from the bottom to the top.
17. Ideal transformer
An ideal transformer is a transformer which has no loses, i.e. it’s
winding has no ohmic resistance, no magnetic leakage, and therefore
no I2 R and core loses.
However, it is impossible to realize such a transformer in practice.
Yet, the approximate characteristic of ideal transformer will be used in
characterized the practical transformer.
V1 V2
N1 : N2
E1 E2
I1 I2
V1 – Primary Voltage
V2 – Secondary Voltage
E1 – Primary induced Voltage
E2 – secondary induced Voltage
N1:N2 – Transformer ratio
18. Transformer Efficiency
To check the performance of the device, by comparing the
output with respect to the input.
The higher the efficiency, the better the system.
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Where Pcu = Psc
Pc = Poc
19. Transformer Losses
Generally, there are two types of losses;
i. Iron losses :- occur in core parameters
ii. Copper losses :- occur in winding resistance
i. Iron Losses
ii Copper Losses
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20. EDDY CURRENTS
By Changing Flux through a solid
conductor, induced currents are set up
within the body of a conductor in a
direction perpendicular to the flux
which are eddy currents.
Since our iron core is ferromagnetic
material, so it allows these currents to
pass through the whole body of
conductor causing heating of core of
conductor.
This is a power loss in transformer(
shown as in figure 1 ), to reduce this
the core should be made of lamination
sheets which stop the flow of eddy
currents (shown as in figure 2).
21. HYSTERESIS LOSS
The energy spent in magnetisation and
demagnetisation of the core of
transformer is called hysteresis loss.
This loss in energy is expressed by
using B-H(magnetic flux density B
and flux density H) curve for a
specific ferromagnetic material.
For reducing this loss, we should use
such a soft material for core whose
hysteresis loop is very small.
The hysteresis loops of both hard and
soft magnetic materials are shown
respectively, which shows that soft
magnetic materials have small
hysteresis loss of energy.
