The characteristics depends
on whether generator is cumulatively compound or differentially compound
generator. In cumulatively compound, Φ2r = Φ2r + Φ2r. As load current increases,
Ia increases hence
Ise also increases producing more flux. But as
Ia increases, the various voltage drops and armature reaction drop also increases. Hence there is drop in the terminal voltage.
If drop in Vt due to increasing
IL is more dominating than increase in Vt due
to increase in flux then generator is called under compounded and its
characteristics is dropping in nature, as shown in the Fig. 1.
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| Fig. 1 Characteristics of compound generator |
If drop in Vt due to armature reaction and other drops is much less than increase in Vt
due to increase in flux then generator is called over compound and its
characteristics is rising in nature, as shown in the Fig. 1. If the
effects of the two are such that on full load current Vt is same as no load induced e.m.f. i.e. the effects are neutralizing each other on full then then generator is called flat compounded or level compound. Its characteristics is shown in the Fig. 1.
In differentially compound, ΦT = Φsh ~ Φse.. The net flux is difference between the two. As IL increases, Φsh is almost constant but Φse increases rapidly. Hence the resultant flux ΦT reduces.
There is drop due to armature resistance, series field resistance,
armature reaction due to which terminal voltage drops further. Thus we
get the characteristics of such differentially compound generator as
shown in the Fig. 1.