The three most important characteristics or curves of a d.c generator are
1. No-load saturation characteristic (E0/If)
It is also know as Magnetic characteristic or Open circuit Characteristic (O.C.C). It shows the reation between the no-load generated e.m.f in armature, E0 and the field or exciting current If at a given fixed speed. It is just te magnetisation curve for the material of the electromagnets.Its shape is practically the same for all generators whether separately-excited or self-excited.
A typical no load saturation curve is shown in Figure.It has generator output voltage plotted against field current.The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.Separately-excited Generator
The No-load saturation curve of a separately excited generator will be as shown in the above figure.It is obvous that when If is increased from its initial small value, the flux and hence generated e.m.f Eg increase irectly as curent so long as the poles are unsaturated.This is represented by straight portion in figure.But as the flux denity increases,the poles become saturated, so a greater increase If is required to produce a given increase in voltage than on the lower part of the curve.That is why the upper portion of the curve bends.
Self-excited Generator (Series or Shunt )
The O.C.C curve for self-excited generators whether shunt or series wound is shown in above figure.Due to the residal magnetism in the poles, some e.m.f (=OA) is gnerated even when If =0.Hence, the curve starts a little way up.The slight curvature at the lower end is due to magnetic inertia.It is seen that the first part of the curve is practically straight.This is due to fact that at low flux densities reluctance of iron path being negligible,total reluctance is given by the air gap reluctance which is constant.Hence,the flux and consequently,the generated e.m.f is directly proportional to the exciting current.However, at high flux densities, where μ is small,iron path reluctance becomes appreciable and straight relation between E and If no longer holds good.In other words,after point B, saturation of pole starts.However, the initial slope of the curve is determined by air-gap width.O.C.C for higher speed would lie above this curve and for lower speed,would lie below it. Compound-wound Generator
If the series field amp-turns are such as to produce the same voltage at rate load as at no load.then th generator is flat-compounded. It should be noted, however, that even in the case of a flat-cmpounded generator, the voltage is not constant form no load to rated load. At half load, the voltage is actually greater than the rated voltage as seen from figure.If the series field amp-turns are such that the rated voltage is greater than the no-load voltage, then generator is over-compounded.If rated voltage is less than the no -load voltage, then the generator is under-compound.
2. Internal or Total characteristic (E/Ia)
It gives the relation between the e.m.f E atually induces in the armature (after allowing for the demagnetising effect of armature reaction) and the armature current Ia.
Separately-excited Generator
Let us consider a separately-excited generator giving its rated no-load voltage of E0 for a certain constant field current.If there were no armature reaction and armature voltage drop,then this voltage would have remained constant as shown in figure by the horizontal line 1. But when the generator is loaded, the voltage falls due to these two causes, thereby gving slightly dropping characteristics.If we subtract from E0 the values of voltage drops due to armature reaction for different loads, then we get the value of E-the e.m.f actually induced in the armature under load conditions.Curve 2 is plotted in this way and is known as the internal characteristic.
Series Generator
In this genarator, because field windings are in series with the armature, they carry full armature current Ia. As Ia is increased, flux and hence generated e.m.f. is also increased as shown by the curve. Curve Oa is the O.C.C. The extra exciting current necessary to neutralize the weakening effect of armature reaction at full load is given by the horizontal distance ab. Hence, point b is on the internal characteristic.3. External characteristic (V/I)It is also referred to as performance characteristic or sometimes voltage-regulating curve.
It gives relation between the terminal voltage V and the load current I.This curve lies below the internal characteristic because it takes in to account the voltage drop over the armature circuit resistance.The values of V are obtained by subtracting IaRa from corresponding values of E.This characteristic is of great importnce in judging the suitability of a generator for a particular purpose.It may be obtained in two ways (i) by making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or (ii) graphically from the O.C.C provided the armature and field resistances are known and also if the demagnetising effect or the armature reaction is known.
Figure above shows the external characteristic curves for generators with various types of excitation. If a generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage.
In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. At the short circuit condition, the only flux available to produce current is the residual magnetism of the armature.
To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at zero, reaches a peak, and then falls back to zero.
The combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine is known as an over-compound generator.
