1. 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 changesAC electrical power at one voltage level into AC electrical power at anothervoltage level through the action of magnetic field but with a proportionalincrease or decrease in the current ratings., without a change in frequency. It can be either to step-up or step down.

2. The main principle of operation of a transformer is mutual inductance between twocircuits which is linked by a common magnetic flux. A basic transformer consists of twocoils that are electrically separate and inductive, but are magnetically linked through apath of reluctance. The working principle of the transformer can be understood fromthe figure belowWORKING OF TRANSFORMERWORKING 3. As shown above the transformer has primary and secondary windings. The corelaminations are joined in the form of strips in between the strips you can see thatthere are some narrow gaps right through the cross-section of the core. Thesestaggered joints are said to be imbricated. Both the coils have high mutualinductance. A mutual electro-motive force is induced in the transformer from thealternating flux that is set up in the laminated core, due to the coil that isconnected to a source of alternating voltage. Most of the alternating flux developedby this coil is linked with the other coil and thus produces the mutual inducedelectro-motive force. The so produced electro-motive force can be explained withthe help of Faradays laws of Electromagnetic Induction ase=M*dI/dtIf the second coil circuit is closed, a current flows in it and thus electrical energy istransferred magnetically from the first to the second coil.The alternating current supply is given to the first coil and hence it can be calledas the primary winding. The energy is drawn out from the second coil and thus canbe called as the secondary winding.In short, a transformer carries the operations shown below: Transfer of electric power from one circuit to another. Transfer of electric power without any change in frequency. Transfer with the principle of electromagnetic induction. The two electrical circuits are linked by mutual induction. 4. For the simple construction of a transformer, you must need two coils having mutualinductance and a laminated steel core. The two coils are insulated from each otherand from the steel core. The device will also need some suitable container for theassembled core and windings, a medium with which the core and its windings fromits container can be insulated.In order to insulate and to bring out the terminals of the winding from the tank, aptbushings 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 transformersheet steel laminations assembled to provide a continuous magnetic path withminimum of air-gap included. The steel should have high permeability and lowhysteresis loss. For this to happen, the steel should be made of high silicon contentand must also be heat treated. By effectively laminating the core, the eddy-currentlosses can be reduced. The lamination can be done with the help of a light coat of coreplate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, thethickness of the lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz. 5. Classification of transformer As per phase1. single phase2. Three phase As per core1. Core type2. Shell type As per cooling system1. Self-cooled2. Air cooled3. Oil cooled 6. Transformer classified asper core CORE TYPE TRANSFORMER:-In core-type transformer, the windings are given to aconsiderable part of the core. The coils used for this transformer are form-wound and are ofcylindrical type. Such a type of transformer can be applicable for small sized and large sizedtransformers. In the small sized type, the core will be rectangular in shape and the coils used arecylindrical. The figure below shows the large sized type. You can see that the round or cylindricalcoils are wound in such a way as to fit over a cruciform core section. In the case of circularcylindrical coils, they have a fair advantage of having good mechanical strength. The cylindricalcoils will have different layers and each layer will be insulated from the other with the help ofmaterials like paper, cloth, macerate board and so on. The general arrangement of the core-typetransformer with respect to the core is shown below. Both low-voltage (LV) and high voltage (HV)windings are shown. 7. The low voltage windings are placed nearer to the core as it isthe easiest to insulate. The effective core area of thetransformer can be reduced with the use of laminations andinsulation 8. 2. Shell-Type TransformerIn shell-type transformers the core surrounds a considerable portion ofthe 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 consistsof discs stacked with insulation spaces between the coils. These insulation spaces form thehorizontal cooling and insulating ducts. Such a transformer may have the shape of a simplerectangle or may also have a distributed form. Both designs are shown in the figure below: 9. A strong rigid mechanical bracing must be given to the cores and coils of the transformers. This willhelp in minimizing the movement of the device and also prevents the device from getting anyinsulation damage. A transformer with good bracing will not produce any humming noise during itsworking and will also reduce vibration.A special housing platform must be provided for transformers. Usually, the device is placed in tightly-fittedsheet-metal tanks filled with special insulating oil. This oil is needed to circulate through thedevice and cool the coils. It is also responsible for providing the additional insulation for the devicewhen it is left in the air. 10. 1. Oil Filled Self-Cooled TypeOil filled self cooled type uses small and medium-sized distribution transformers. The assembledwindings and core of such transformers are mounted in a welded, oil-tight steel tanks providedwith a steel cover. The tank is filled with purified, high quality insulating oil as soon as the core isput back at its proper place. The oil helps in transferring the heat from the core and the windingsto the case from where it is radiated out to the surroundings. For smaller sized transformers thetanks are usually smooth surfaced, but for large size transformers a greater heat radiation area isneeded, and that too without disturbing the cubical capacity of the tank. This is achieved byfrequently corrugating the cases. Still larger sizes are provided with radiation or pipes.2. Oil Filled Water Cooled TypeThis type is used for much more economic construction of large transformers, as the above toldself cooled method is very expensive. The same method is used here as well- the windings and thecore are immersed in the oil. The only difference is that a cooling coil is mounted near the surfaceof 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 transmissionlines. The biggest advantage of such a design is that such transformers do not require housingother than their own. This reduces the costs by a huge amount. Another advantage is that themaintenance and inspection of this type is only needed once or twice in a year.3. Air Blast TypeThis type is used for transformers that use voltages below 25,000 volts. The transformer is housedin a thin sheet metal box open at both ends through which air is blown from the bottom to the top. 11. Three phase transformerNormally , when three-phase is required, a single enclosure with threeprimary and three secondary windings wound on a common core is all thatis required. However three single-phase transformers with the same ratingcan be connected to form a three-phase bank. Since each single-phasetransformer has a primary and a secondary winding, then 3 single-phasetransformers will have the required 3 primary and 3 secondary windings andcan be connected in the field either Delta-Delta or Delta-Wye toachieve the required three-phased transformer bank 12. Ideal transformer An ideal transformer is a transformer which has no loses, i.e. itswinding has no ohmic resistance, no magnetic leakage, and thereforeno I2R and core loses. However, it is impossible to realize such a transformer in practice. Yet, the approximate characteristic of ideal transformer will be used incharacterized the practical transformer.N1 : N2I1 I2V1 E1 E2V2V1 Primary VoltageV2 Secondary VoltageE1 Primary induced VoltageE2 secondary induced VoltageN1:N2 Transformer ratio 13. Transformer Efficiency To check the performance of the device, bycomparing the output with respect to the input. The higher the efficiency, the better the system.OutputPower, 100%100% InputPowerPoutP PV Icos100%cos2 22 2 c cuout lossesV I P PEf f iciency100%VAVA P PnVAcoscos100%coscos( )( ) 2 c cuload nc cufull loadnVA P n PWhere, if load, hence n = , load, n= ,90% of full load, n =0.9Where Pcu = PscPc = Poc 14. Transformer Losses Generally, there are two types of losses;i. Iron losses :- occur in core parametersii. Copper losses :- occur in winding resistancei. Iron Lossesii Copper Lossesiron c c c opencircuit P P I R P 2 ( ) copper cu short circuit02P P I R I R P2( ) ( )01 221221 221, ( ) ( )or if referred P I R I Rcu Chetan Upadhyay 15. BYE