Transformer - EM&T 2 Transformers Transformers Electrical...

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EM&T Transformers 2. Transformers Electrical power is generated by alternators generally at voltages in the range of 11kV to 22kV. However for economical transmission of power over long distance, higher voltage of the order of 400kV or even higher is required. Transformers are used to Step up the voltage from the generators to transmission voltages. For power distribution in urban areas the voltage is Stepped-down to 22kV or 6.6kV by means of transformers. The power at distribution voltage is further stepped-down to 400V for distribution to consumers. From what is said above, it is clear that the function of a transformer is either to step-up or step-down the voltage as required. AC Supply Primary Winding Load Secondary Winding Laminated Iron Core Figure—1. Single phase core type transformer. Basically the transformers consist of iron core with coils wound on it. The core is made up of thin steel laminations about 0.35mm thick for 50Hz transformers. In order to reduce the eddy current loss, the laminations are insulated from one another by thin layers of varnish. The schematic representation of a single phase transformer is shown in fig-1. The winding that is connected to the supply (to be transformed) is called the primary winding . The other winding, that is connected to the system utilizing the transformed voltage or to the load is called the secondary winding . NP/d4f9c2c46acf902bff8e86ef5f076088a545a9f7.doc/2-Nov-09 1
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I N μ N V 1 E 1 1 2 2 E φ I V 1 V I μ φ Ο φ μ 1 V 1 φ μ I EM&T Transformers 2.1The Ideal Transformer on no-load Figure—2. Ideal transformer on no-load. The properties of an ideal transformer are as follows. (a) winding resistances are negligible b (b) all the flux set up by the primary winding links with the secondary winding. (c) The core losses (hysteresis and eddy current losses) are negligible. (d) The core has constant permeability (i.e. the magnetization curve for the core is linear.) Fig-2 shows an ideal transformer with the primary winding connected to an AC supply of voltage V 1 and with the secondary on open circuit. A small amount of current I μ flows in the primary winding. The current I μ sets up an alternating magnetic flux φ in the iron core as shown in figure-2. When the secondary is on open circuit the primary of the Ideal transformer behaves like a pure inductance and hence the current I μ lags the voltage V 1 by 90 ° . The phasor diagram for the applied voltage V 1 , the magnetizing current I μ and the flux φ is shown in figure-3(a). the corresponding waveforms of l 1 , I μ and φ are shown in figure-3(b) NP/d4f9c2c46acf902bff8e86ef5f076088a545a9f7.doc/2-Nov-09 2
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EM&T Transformers Figure—3(a). Phasor diagram Figure—3(b). Waveform diagram 2.2Emf Equation of a Transformer Since the flux φ is sinusoidal, it may be represented by the equation φ = φ m Sin ϖ t (1) Where φ m is the maximum value of the flux. This alternating magnetic flux (which links with the primary ad secondary windings) induces emfs e 1 and e 2 in the primary and secondary windings.
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Transformer - EM&T 2 Transformers Transformers Electrical...

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