Electrical World

The characteristics depends on whether generator is cumulatively compound or differentially compound generator. In cumulatively compound, Φ 2r = Φ 2r + Φ 2r . As load current increases,  I a increases hence  I se  also increases producing more flux. But as  I a increases, the various voltage drops and armature reaction drop also increases. Hence there is drop in the terminal voltage.        If drop in V t due to increasing  I L is more dominating than increase in V t due to increase in flux then generator is called under compounded and its characteristics is dropping in nature, as shown in the Fig. 1. Fig. 1 Characteristics of compound generator        If drop in V t due to armature reaction and other drops is much less than increase in V t 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 effect...

Consider a series generator shown in the Fig. 1        In case of series generator,         I a = I se =  I L        As load current increases,  I se increases. The flux Φ is directly proportional to. So flux also increases. The induced e.m.f. E is proportional to  I se flux hence induced e.m.f. also increases. Thus the characteristics of E against i.e. internal characteristics is of increasing nature. As increases I a , armature reaction increases but its effect is negligible compared to increase in E. But for high load current, saturation occurs and flux remains constant. In such case, due to the armature reaction E starts decreasing as shown by dotted line in the Fig. 2. Fig. 1 Characteristics of d.c. series motor        Now as I L = I a  increases, thus the drop I a  (R a  +R se ) increases.  ...

Consider the d.c. shunt generator shown in the Fig. 1. The internal characteristics is E V s I L while the external characteristics is V t  against I L . Fig. 1 Internal characteristics        Let us see the nature of these two characteristics.        Ideally the induced e.m.f. is not dependent on the load current I L or armature current I a . But as load current increases, the armature current I a increases to supply load demand. As I a increases, armature flux increases. Note : The effect of flux produced by armature on the main flux produced by the field winding is called an armature reaction.        Due to the armature reaction, main flux pattern gets distorted. Hence lesser flux gets linked with the armature conductors. This reduces the induced e.m.f. Note : Thus the armature conductors. This reduces the induced e.m.f.        This is shown in the Fig. 2...

The characteristics is separately excited d.c. generator are divided into two types, 1) Magnetization   and         2) Load characteristics. 1.1 Magnetization or Open Circuit Characteristics         The arrangement to obtain this characteristics is shown in the Fig. 1. Fig. 1  Obtaining O.C.C. of separately excited generator        The rheostat as a potential driver is used to control the field current and the flux. It is varied from zero and is measured on ammeter connected.        E o  = (ΦPNZ) / (60A)        As I f is varied, then Φ change and hence induced e.m.f. E o  also varies. It is measured on voltmeter connected across armature. No Load is connected to machine, hence characteristics are also called no load characteristics which is graph of E o  against field current I f as sho...

The d.c. generators have following characteristics in general, 1) Magnetization characteristics 2) Load characteristics 1.1 Magnetization Characteristics        This characteristics is the graph of generated no load voltage E against the field current I f , when speed of, generator is maintained constant. As it is plotted without load with open output terminals it is also called No load characteristics or Open circuit characteristics.        E o V s I f  is magnetization characteristics        Where  E o = No load induced e.m.f.        But for generator,        E = (ΦPNZ) / (60A) . . .     E α   Φ                 with (PNZ) / (60A)  constant . . .     E  α   I f                 ...

In this type, the part of the field winding is connected in parallel with armature and part in series with the armature. Both series and shunt field windings are mounted on the same poles. Depending upon the connection of shunt and series field winding, compound generator is further classified as : i) Long shunt compound generator, ii) Short shunt compound generator. 1.1 Long Shunt Compound Generator        In this type, shunt field winding is connected across the series combination of armature and series field winding as shown in the Fig. 1. Fig. 1 Long shunt compound generator        Voltage and current relations are as follows.        From the Fig. 1.        I a = I se         and  I a = I sh +  I L        Voltage across shunt field winding is V t .        ...

   When the field winding is connected in series with the armature winding while supplying the load then the generator is called series generator. It is shown in the Fig. 1.        Field winding, in this case is denoted as S 1 and S 2 . The resistance of series field winding is very small and hence naturally it has less number of turns of thick cross-section wire as shown in the Fig. 1. Fig. 1 Series generators       Let R se be the resistance of the series field winding. 1.1 Voltage and current Relations        As all armature, field and load are in series they carry the same current. . . .         I a = I se =  I L Where   I se = Current through series field winding.        Now in addition to drop I a R a , induced e.m.f. has to supply voltage drop across series field winding too. This is I se R se i...