Characteristics of D.C. Generators

The d.c. generators have following characteristics in general,

1) Magnetization characteristics

2) Load characteristics

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                    as   Φ  α   I_f

       Thus induced e.m.f. increases directly as I_f  increases. But after certain I_f  core gets saturated and flux also remain constant though I_f  increases. Hence after saturation, voltage also remains constatnt.

*GOOD TO KNOW:
 Thus characteristics is linear till saturation and after that bends such that voltage remains constant though I_f  increases.

        The characteristics is shown in the BELOW figure.

magnetisation characteristics

Now the induced e.m.f. also varies with speed.

       Actually,             E   α   N Φ

       So if magnetization characteristics for various speeds are plotted we will get family of parallel characteristics as shown in the BELOW FIG.1. For lower speeds, generated voltages are less so characteristics for lower speeds are below the characteristics for higher speeds.

 

fig.1

2 .Load Characteristics

These are further divided into two categories,

1) External characteristics

2) Internal characteristics

       The external characteristics is the graph of the terminal voltage V_t against load current I_L.

       The internal characteristics is the graph of the generated induced e.m.f. against the armature current I_a.

*GOOD TO KNOW:  While plotting both the characteristics, the speed N of the generator is maintained constant.

*GOOD TO KNOW: In most of the cases, the shunt field current is very small as compared with load current I_L. Hence in practice, the internal characteristics shows the graph of induced e.m.f. E against load current I_L, instead of I_a, neglecting I_sh.