Modern DC generators with field coils are self-excited generators which get started with the initial current in the field coils. When generator is switched off, a small magnetism is developed in rotor iron which induced electromotive force in the armature due to which current is produced in the field windings. Initially, weak magnetic field creates less current in the coil, but to sustain self-excitation, the additional magnetic flux increases the electromotive force in the rotor, due to which voltage keep on increasing until the machine takes the full load.
What Are Self Excited DC Generators
The small amount of magnetism is present in the rotor iron. This residual magnetic field of the main poles, induced an electromotive force in the stator coils, which produces initial current in the field windings.
Due to flow of small current in the coil, an increase in magnetic field occurs. As a result, voltage output increases, in turn, increases the field current. This process continues as long as the electromotive force in the armature is more than the voltage drop in the field winding. But, after at certain level, field poles get saturated, and at that point electric equilibrium is reached, and no further increase in armature emf and increase in current. The resistance of the field winding has certain fixed value, at which self-excitation can be achieved. This resistance value may vary according to electric parameters of the generator.
Types of DC Generators
DC Generators are classified based on the position of the field coils and armature in the circuit, and how the generator’s magnetic field is excited (i.e. produced). Some portable generators are DC generators.
Series Wound Generators
In series wound generators, field winding and the armature winding are connected in series so that current that passes through external circuit and through field windings, passes from armature as shown in the figure below. The field coil of series-wound generator has low resistance, consist of a few turns of thick wire. If the load resistance decreases, then current flow increases.
As a result magnetic field and output voltage increases in the circuit. In such type generator, output voltage varies directly with respect to load current which is not required in most of the application. Due to this reason, such types of generators are rarely used.
Shunt Wound DC Generators
In this type of generator, the field winding is wired parallel to the armature winding so that voltage is same across the circuit. Here, field winding has many numbers of turns for the desired high resistance so that fewer armatures current can pass through field winding and the reaming passes through load.
So, 
In shunt wound generator, as they are connected parallel, current in the parallel branches are independent of each other. Hence, the output voltage almost constant and if it varies then it varies inversely with respect to load current. This is because of the voltage drop as armature resistance increases.
Combination Wound Generator
Compound wound generator is advanced version of series wound generator and shunt wound generator. the working principle of the generator is the combination of two types so that it prevail over the disadvantages of both. It has both types of winding; series field and shunt field winding. On the basis of their connection, compound wound generators are of two types- short shunt compound generator and long shunt compound generator.
Long Shunt Compound Generator
Here the shunt field winding is connected parallel to armature only as shown in figure. Series winding is then connected in series.
Short Shunt Compound Generator
Here the shunt field winding is connected parallel to armature only as shown in figure. Series winding is then connected in series.
Advantages of Compound DC Generator
In compound generator, the armature voltage decreases automatically, when load current increases, due to which magnetic field produced by shunt winding decreases. But the same increase in load current flows through the series winding causes an increase in the magnetic field. So the decrease in magnetic field in the shunt field is compensated by increase in magnetic field in series field. In his manner, the output voltage remains constant as shown in figure.
Commutative and Differential Compound DC Generator
As compound wound generator has both the fields-shunt field and series field, their combination makes lot of difference. When series field aided the shunt field, their impact is more and it is said to be commutatively compound wound. On the other hand, if the series field opposes the shunt field, then their impact is less and it is said to be differential compound generator.
