Q. Draw the various characterstics of a DC shunt generator and explain critical resistance.
Sol. Following are three most important characterstics or curves of a d.c. generator :
1. No - load Saturation Characterstics (Eo/Io)
It is also known as Magnetic characterstics or Open - circuit Charaterstics (O.C.C.). It shows the relation b/w the no - load generated e.m.f. armature, Eo and the field or exciting current. If at a given fixed speed it is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately - excited or self - excited.
2. Internal or Total Characterstics (E/Ia)
It gives the relation b/w the e.m.f.E actually induced in the armature (after allowing for the demagnetizing effect of armature reaction)and the armature current it.
3. External Characterstics (V/I)
It gives relation b/w that terminal voltage V and the load current I. This curve lies below the internal characterstics because it takes into account the voltage drop over the armature circuit resistance. The values of Vare obtained by subtracting IaRa from corresponding values of E. This characterstics is of great importance in judging the suitability of a generator for a particular purpose. It may be obtained in two ways (i) bby making simultaneous or (ii) graphically from the O.C.C. provided the armature and the field resistance are known and also if the demagnetizing effect (under rated load condition) or the armature reaction (from the short - circuit test) is known.
The connection of a shunt wound generator. The armature current Ia splits up into two parts; a small fraction Ish flowing through shunt field windings while the major part IL goes to the external load.
(i) O.C.C. :The line OA represent the shunt field circuit resistance. When the generator is run at normal speed, it will build up a voltage OM. At no - load, the terminal voiltage of the generator will be constant (=OM) represented by the horizontal dotted line MC.
(ii) Internal characterstics : When the generator is loaded, the flux per pole is reduced due to armature reaction. Therefore, e.m.f. generated on load is less than the e.m.f. generated at no load, As a result, the internal characterstics (E/Ia) drop down slightly.
(iii) External characterstics : Curve 2 shows the external characterstics of a shunt generator. It gives the relation b/w terminal voltage V and load current IL.
V = Eg - IoRa = Eg - (IL + Ish)Ra
Therfore, external characterstics curne will lie below the internal charactercstics curve by an amount equal to drop in the armature circuit [i.e., (IL + Ish)Ra].
Critical Resistance for Shunt Generator
Due to residual magnetism in the poles, some e.m.f. and hence current would be generated. This current while passing through the field coils will strengthen the magnetism of the poles (provided fiels coils are properly connected as regards polarity). This will increase the poles flux which will further increase the generated e.m.f. Increased e.m.f. means more current which further increase the flux and so on. This mutual reinforcement of e.m.f.and flux proceed on till equilibrium is reached at some point like P. The point lies on the resistance line OA of the field windings. Let R be the resistance of the fiels winding. Line OA is drawn such that its slope equals the field winding resistance i.e. every point on this curve is such that volt/ampere = R.
The voltage Ol corresponding to point P represent M the maximum voltage to which the machine will build up L with R as field resistance. OB represents smaller resistance and the corresponding voltage OM is slightly greater than OL. If field resistance is increased, then slope of the resistance line increased., and hence the maximum voltage to which the generator will line does not cut the O.C.C. at all (like OT), then obviously the machine will fail to excite i.e. there will be no ' build up' of the voltage. If the resistance line just line along the slope, then with that value of field resistance, the machine will just excite. The value of the resistance represented by the tangent to the curve, is known as Critical Resistance Rc foa a given speed.