transformer and types and construction .pptx

electrical engineering

1. Transformer - Definition Transformer is a static device by means of which an electric power in one circuit is transformed to other circuit, without change in frequency. It can increase or decrease the voltage in a circuit but with a corresponding decrease or increase in current.

2. Basic Working Principle Mutual Induction between two circuits linked by a common magnetic flux. Faraday’s Law of Electromagnetic Induction “whenever a conductor is placed in a varying magnetic field, emf is induced in it, which is called induced emf. If the conductor circuit are closed, current is also induced which is called induced current.”.

3. Transformer Representation

4. Working Principle of Transformer In Transformer, there are two inductive coils, which are electrically separated and magnetically coupled. If one coil is connected to ac source  an alternating flux is set up in the core  it links with other coil and produced mutually induced emf (Faraday’s Law). If the second coil is closed, current flows in it – Thus Energy is transferred. First Coil  Primary and Second Coil  Secondary

5. 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.

6. 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 .

7. 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.

8. 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

9. 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

10. Transformer Construction 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.

11. Transformer Construction Core : Carries the flux produced by the winding It is either square or rectangular shape. The vertical portion of the core is called Limb The top and bottom horizontal portion of the core is called Yoke . Made of high grade silicon steel laminations Laminated arrangements reduces eddy current losses. The laminations are insulated from each other by using insulation like varnish.

12. Core of a Transformer

13. Transformer Construction Windings: Coils used are wound on the limbs and insulated from each other. Windings carry the current and produce the necessary flux Primary and Secondary coils are insulated from each other. Coils are made up of Copper.

14. Windings of a Transformer

15. Conservator : takes up the expansion and contraction of the oil without allowing it to come in contact with the ambient air. Explosion Vent : A bent air (using Breather : Extracts the moisture from silica gel crystals) and does not allow oil to come in contact with the moisture. fitted tank, bursts pressure pipe on main which when inside the transformer becomes excessive (which releases the pressure and protects the transformer)

16. Types of Single Phase Transformers Core Type Transformer Shell Type Transformer

17. 1. Core Type Transformers Single Magnetic Circuit with 2 Limbs – Windings placed on both the limbs  Winding encircles the core. Cylindrical type coils are used with each layers insulated by paper or mica. Low voltage coil is placed near the core and High voltage coil surrounds the low voltage coil. Natural cooling is more effective. Coils can be easily removed by removing the laminations of the tope yoke, for maintenance

18. 2. Shell Type Transformers Double Magnetic Circuit with 3 Limbs – Windings placed on central limb  Core encircles most part of the windings. Natural Cooling doesn’t exist Difficult for maintenance, as for removing any winding, large laminations to be removed Preferred for Very High Voltage Transformers.

19. Comparison of Core and Shell Type Transformers

20. Classification of Transformer As per phase single phase Three phase As per core Core type Shell type As per cooling system Self-cooled Air cooled Oil cooled

21. 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-Wy e to achieve the required three-phased transformer bank

22. 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.

23. 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

24. 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:

25. 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.

26. 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.

27. 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 I 2 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. V 1 V 2 N 1 : N 2 E 1 E 2 I 1 I 2 V1 – Primary Voltage V2 – Secondary Voltage E1 – Primary induced Voltage E2 – secondary induced Voltage N1:N2 – Transformer ratio

28. 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. Where, if ½ load, hence n = ½ , ¼ load, n= ¼ , 90% of full load, n =0.9 Where Pcu = Psc Pc = Poc

29. Transformer Losses Generally, there are two types of losses; Iron losses :- occur in core parameters Copper losses :- occur in winding resistance Iron Losses ii Copper Losses

30. 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).

31. 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.

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