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ELECTRICAL MACHINE 140070110029 140070110046 140070110022 140070110006 140070110064 RUSHABH MEHTA UDIT PATEL JAIMIN PUJ BHARGAV KAKADIYA MAHENDRA SHIYAL BRANCH ELECTRONICS ENGINEERING Guided By:J.C.Baria Single Phase Transformer 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 AC electrical power at one voltage level into AC 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. WORKING The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance. The working principle of the transformer can be understood from the figure below WORKING OF TRANSFORMER As shown above the transformer has primary and secondary windings. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil hat is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electromotive force. The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction as e=M*dI/dt If the second coil circuit is closed, a current flows in it and thus electrical energy is transferred magnetically from the first to the second coil. The alternating current supply is given to the first coil and hence it can be called as the primary winding. The energy is drawn out from the second coil and thus can be called as the secondary winding. Types of Transformer CORE Type Transformer: It has a single magnetic circuit. In single phase type, the core has two limbs. The windings encircle the core. The cylindrical type coil is used. As winding are distributed natural cooling is effective. It is preferred for Low Voltage transformer. 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 Shell Type Transformer: It has double magnetic circuit. In single phase type, the core has three limbs. The core encircle most part of winding. Multilayer disc type or sandwich coils are used. As winding are surround by core the natural cooling is not possible. The construction is for high voltage transformer. OPEN CIRCUIT TEST The purpose of this test is to find the iron loss(core loss). A wattmeter, voltmeter and an ammeter are connected in low voltage side. Secondary side of transformer we will keep open. We give the normal voltage applied to the primary, normal flux will be set up in the core, hence normal iron losses will occur which are recorded by the wattmeter. As the primary no-load current is measured by ammeter and its value is small, Cu-loss is negligibly small in primary and nil in secondary (it being open). Hence, the wattmeter reading represents practically the CORE LOSS under no-load condition. SHORT CIRCUIT TEST A wattmeter, voltmeter and an ammeter are connected in high voltage side. The low voltage side of transformer is short circuited. The applied voltage is a small percentage of the normal voltage. Hence the core losses are very small with the result of the wattmeter reading represent the full-load CU-LOSS or loss for the whole transformer . 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. Efficiency, Output Power 100% Input Power Pout 100% Pout Plosses ( fullload) V2 I 2 cos 100% V2 I 2 cos Pc Pcu (load n ) Where Pcu = Psc Pi = Poc kVAcos 1000 (kVAcos 1000) Pc Pcu nkVAcos 1000 (nkVAcos 1000) Pc n 2 Pcu Where, if ½ load, hence n = ½ , ¼ load, n= ¼ , 90% of full load, n =0.9 Transformer Losses Copper Loss: The total power loss in winding resistance of transformer is copper loss. Copper Loss=Power loss in primary resistance + Power loss in secondary resistance Copper Losses Pcopper Pcu ( I 1) 2 R1 ( I 2) 2 R2 Pshort circuit or if referred , Pcu ( I 1) 2 R01 ( I 2) 2 R02 Copper loss should be kept as low as possible to increase efficiency. Iron loss Pi = Hysteresis loss + eddy current loss Iron Losses Piron Pc ( I c) 2 Rc Popen circuit Both hysteresis loss and eddy current loss are dependent on frequency. Hence iron loss Pi of the total loss is dependent on frequency but the copper loss Pcu is constant irrespective of frequency.