Instead of with permanent magnets to form the magnetic field, we will use a combine of electromagnets, referred to as field poles, which is shown in the Fig. once the field current in such a generator is provided by a separate supply (such as a battery or other generator, referred to as an exciter), then the generator is termed to be singly (separately) excited. Thus, the DC supply connected to terminals a and b sources an exciting current I to flow. If the armature coil is driven by a motor or diesel engine, a voltage EO seems between brush terminals x and y.
No-Load Operation and Saturation Curve:
When a singly / separately excited DC generator runs at no load (i.e. armature circuit open), a modification within the exciting current causes a corresponding modification within the induced voltage. We have a tendency to currently examine the link between field flux and exciting current.
Field flux Vs exciting current :
Let us bit by bit raise the exciting current Ix, in order that the mmf of the field will increase, that will increase the flux (f per pole). If we have a tendency to plot f as a function of Ix, we have a tendency to get the saturation curve shown below. This curve is obtained whether or not or not the generator is popping.
When the exciting current is comparatively little, the flux is little and therefore the iron within the machine is unsaturated. Little or no mmf is required to ascertain the flux through the iron, with the result that the mmf developed by the field coils is nearly entirely out there to drive the flux through the air gap. As a result of the permeableness of air is constant, the flux will increase in directly proportion to the exciting current, which is shown in the curve by linear portion 0a of saturation curve..
However, as we have a tendency to still raise the exciting current, the iron within the field and therefore the coil begins to saturate. An outsized increase within the mmf is currently needed to supply a little increase in flux, shown in the curve by portion b to c of the curve. The machine is currently said to be saturated. Saturation of the iron begins to be vital after we reach the supposed “knee" ab of saturation curve.
The output voltage varies to the field current for singly excited DC generator is shown below.