TRANSFORMERPresentation By:

Group Member:

M Ali (BEE-FA13-029)

M Tahir Shaheen (BEE-FA13-038)

Anum Hassan (BEE-FA13-045)

Ahmer Sameer (BEE-FA13-074)

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.

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.

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.

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 coreThe changing magnetic flux is cut by the secondary coil, hence induces an e.m.f in the secondary coil

WORKING

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

Transformer ConstructionFor 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.

Classification of Transformer

• As per phase1. single phase2. Three phase

• As per core1. Core type2. Shell type

• As per cooling system

1. Self-cooled2. Air cooled3. Oil cooled

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

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.

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

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:

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.

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.

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 VoltageV2 – Secondary VoltageE1 – Primary induced VoltageE2 – secondary induced VoltageN1:N2 – Transformer ratio

Transformer EfficiencyTo 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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Transformer Losses Generally, there are two types of losses;

i. Iron losses :- occur in core parametersii. Copper losses :- occur in winding resistance

i. Iron Losses

ii Copper Losses

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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).

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.

THANKYOUBYE