Tuesday, August 28, 2012

Self Excited Generator

       When the field winding is supplied from the armature of the generator itself then it is said to be self excited generator. Now without generated e.m.f., field can not be excited in such generator and without excitation there can not be generated e.m.f. So one may obviously wonder, how this type of generator works. The answer to this is residual magnetism possessed by the field poles, under normal condition.
       Practically through the generator is not working, without any current through field winding, the field poles possess some magnetic flux. This is called residual flux and the property is called residual magnetism. Thus when the generator is started, due to such residual flux, it develops a small e.m.f. which now drives a small current through the field winding. This tends to increase the flux produced. This in turn increases the induced e.m.f. This further increases the field current and the flux. The process is cumulative and continues till the generator develops rated voltage across its armature. This is voltage building process in self excited generators.
       Based on how field winding is connected to the armature to drive its excitation, this type is further divided into following three types.
i) Shunt generator
ii) Series generator
iii) Compound generator

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