Characteristics of D.C. Generators

Characteristics of D.C. Generators

• The d.c. generators have following basic characteristics,

1) Magnetization characteristics and 2) Load characteristics

1. Magnetization Characteristics (O.C.C.)

• 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 (O.C.C.).

E₀ Vs I_f is magnetization characteristics

Where E₀ = No load induced e.m.f.

But for generator,

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

Key Point : 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 Fig. 3.23.1. Eo in volts

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

Actually, E ∞ N ϕ

So if magnetization characteristics for various speeds are plotted we will get family of parallel characteristics as shown in the Fig. 3.23.2. For lower speeds, generated voltages are less so characteristics for lower speeds are below the characteristics for higher speeds.

2.  Load Characteristics

These are further divided into two categories,

1) External characteristics and 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. E against the armature current l_a.

Key Point: While plotting both the characteristics, the speed N of the generator is maintained constant.

Note: 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.

Review Question

1. State and explain the basic characteristics of d.c. generators.