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Equivalent Circuit of the Transformer
The equivalent circuit shown in Fig. 65(a) is for an ideal transformer, and is therefore limited to problems that require only a rather approximate evaluation of transformer performance in general. The efficiency of the ideal transformer is unity, and, for a given primary voltage, the secondary voltage of the ideal transformer is constant regardless of the variations in the load. A realistic transformer, however, has real power losses and reactive power losses. The real power losses include not only the core loss but also the copper losses due to the resistance of the windings; the reactive power losses include the voltamperes associated with the magnetizing current that is required by the flux in the core as well as the voltamperes required to produce the leakage flux whose path is largely in air. As a result of the real power losses, the efficiency of a transformer is less than unity, and, because of the real power losses in combination with the reactive power losses, the secondary voltage will undergo changes in value with variations in the load even though the primary voltage is constant. It is therefore necessary to modify the equivalent circuit of the ideal transformer to take into account the imperfections of the realistic transformer.


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