11EE – 4420 Electric Machine Analysis Fall 2005 Instructor: Ernest Mendrela TRANSFORMERS 1.MAGNETIC CIRCUIT EXCITED BY ALTERNATING CURRENT According to the Faraday’s experiment the voltage einduced in a loop linking a changing magnetic field (see Fig.1) is proportional to the time rate of change of flux Φ: eddt=Φ(1) The polarity of the induced voltage can be determined by the Lenz’s law that says: “The induced voltage is always in such a direction as to tend to oppose the change in flux linkage that produces it” ΦeΦiiiFig.1 Explanation to equation (1) This is shown in Fig.1. For multitern coil the induced voltage is: eNddtddt==Φλ(2) where: N – is the number of coil turns and λ– is the flux linkage in Weber turns. Suppose we have a coil wound on one leg of a closed iron core as shown in Fig.2. To draw an equivalent circuit of such a device called inductor, and then to analyze its behavior under variable supply condition let us consider first an ideal inductor.
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