Characteristics Of Separately Excited D.C. Generators

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  = (?PNZ)/(60A)

As I_f is varied, then ? 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 shown in the Fig. 2. As I_f increases, flux ? increases and E_o increases. After point A, saturation occurs when ? becomes constant and hence E_o  saturates.

Fig. 2  Open circuit characteristics

1.2 Load Saturation Curve
This is the graph of terminal voltage V_t against field current I_f. When generator is loaded, armature current I_a flows and armature reaction exists. Due to this, terminal voltage V_t is less than the no load rated voltage. On no load, current I_a is zero and armature reaction is absent. Hence less number of ampere turns are required to produce rated voltage E_o .

These ampere-turns are equal to OB as shown in the Fig. 3. On load, to produce same voltage more field ampere-turns are required due to demagnetizating effect of armature reaction. These are equal to BC as shown in the Fig. 3. Similarly there is drop I_a R_a across armature resistance. Hence terminal voltage V = E – I_a R_a . This graph OR is also shown in the Fig. 3. The triangle PQR is called drop reaction triangle. Thus OP is no load saturation curve, OQ is the graph of generated voltage on load and OR is the graph of terminal voltage on load.

Fig. 3  Load saturation curves

1.3 Internal and External Characteristics External

Let be the no load rated voltage which drops to E due to armature reaction on load and further drops to V_t due to armature resistance drop I_a R_a on load.

The graph of V_t against load current I_L is called external characteristics while the graph of E against load current I_L is called internal characteristics. These are shown in the Fig. 4. for separately excited d.c.generator. The graphs are to be plotted for constant field current. In case of separately excited d.c. generator induced e.m.f. is totally dependent on flux ? i.e. field current I_f . Hence to have control over the field current, in case of separately excited d.c. generators field regulator is necessary.

Fig. 4  Load characteristics