The properties of generators are analyzed with the aid of features which give the relations between fundamental quantities which is responsible for the operation of a generator. These include the voltage across the generator terminals V, the field or exciting current If, the armature current Ia, and the speed of rotation N.
The following three are the most important characteristics of D.C. generators:
Circuit Characteristics (O.C.C). It demonstrates the relation between the field or Ir(Exciting current) and the armature induced no lead generated e.m.f, E0 at a fixed given speed. Whether sel-excited or separately excited, all generators possess similarly shaped curves. In simpler words, the material of the electromagnets is represented by these magnetization curves.
There are two ways to find the external characteristic:-
5.1. Separately Excited Generator
Fig. 29.Connection for a separately excited generator.
The field circuit is provided with adjustable struggle and would normally contain a field switch and an ammeter, these being omitted from the diagram for simplicity. The armature is connected through two poles main switch to the bus bars, between which the load is connected.
5.1.1. About No-load saturation characteristic (or O.C.C.):
5.1.2. Internal and external characteristics (or load characteristics):
We can see in the figure which shows the load characteristics for a separately excited generator. The most significant is the ‘external characteristic’ (or total characteristic), which shows the way in which the terminal voltage (V) diverges as the load current is enlarged from zero to its full load value, the speed of rotation and exciting current being persistent.
The voltage drop (drop of volts) at any particular load current, indicated by the vertical distance between the external characteristic and the no-load voltage is brought about by two causes:
(i) Armature reaction which has a demagnetizing effect upon the field.
(ii) Resistance drop, this being the product of the armature current and the total
armature-circuit resistance, consisting of the armature resistance, interpole resistance and brush contact resistance.
The ‘internal characteristic’ is found by computing the resistance drop for a few values of current and adding this to the voltage shown by the external characteristic. The upright space between the internal characteristic and no load voltage then signifies the effect of armature reaction.
When the resistance of load is R, then voltage across its terminals is V = IR, where I represents the current, so that if the values of V corresponding to various values of I are calculated, the values will all lie upon a straight line such as OL in Fig. 31. The load current and terminal voltage corresponding to this resistance is given by the inter-section of the line OL with the external characteristics.