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| Transformer |
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| For a given power requirement, one has the choice of the relative values of Irms and Erms. That is, for the product to be a constant, we can choose a relatively large current I and a relatively small potential difference v or just the reverse. In an electric power distribution system, it is desirable - both for reasons of safety and the efficient design of equipment; to have relatively low voltage at both the generating end and receiving end. But for transmission of electrical energy from the generating plant to the consumer, we want the lowest practical current so as to minimize the I2R energy dissipation in transmission line. This mismatch between the requirements for transmission and consumption calls for a device which raises or lowers the potential difference in a circuit, keeping the product IrmsErms essentially constant. This device is a transformer whose operations are based on Faraday's law of induction. |
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| An electrical device is used to change the AC voltage. A transformer which increases the AC voltage is called a 'step up transformer' and a transformer which decreases the AC voltage is called a 'step down transformer'. |
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| A transformer is based on the principle of mutual induction. |
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| It consists of a soft iron core made of laminated sheets well insulated from each other. The coils P1 P2 and S1 S2 are wounded on the same core. |
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| The coil P1 P2 is a primary coil connected to AC source and S1 S2 is a secondary coil connected across a load resistance R. |
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| As the current in the primary varies, the flux linked with P1 P2 and hence S1 S2, changes. |
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| If Np is the number of turns in P1 P2, and Ep is the alternating EMF fed to P1 P2 at instant t under ideal conditions; |
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| Self induced EMF in P1 P2 at instant t = EMF fed to P1 P2 at this instant. |
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| Assuming there is no flux leakage, the rate of change of flux through each turn of S1 S2 is df/dt, |
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| Since |
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| Es > Ep |
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| i.e., K >1 Ns > Np |
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| Es < Ep |
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| i.e., K <1 Ns < Np |
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| If we assume there is no loss of power, |
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| Out put power = Input power |
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| EsIs = EpIp |
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| (i) Copper loss is the energy lost due to heating of copper coils of transformers. |
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| (ii) Iron loss due formation of induced current in the iron line resulting in lot of heat. |
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| (iii) Leakage of magnetic flux. All flux linked with primary may not be linked with secondary. |
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| (iv) Magnetostriction i.e., humming noise of a transformer. |
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