Self Excited DC Generators

Key learnings:
  • Self Excited DC Generators: These generators initiate power generation using the residual magnetism in their rotor, which helps start the electromotive force needed for operation.
  • Operation Mechanism: As the magnetic field increases due to the initial current, the voltage output of the generator rises, continuing this cycle until saturation occurs at the field poles.
  • Types of DC Generators: DC generators are mainly classified as series wound, shunt wound, and compound wound, each differing in coil arrangement and voltage regulation characteristics.
  • Compound Wound Generators: These generators use a combination of series and shunt windings to overcome the limitations of each type, ensuring a more consistent output voltage.
  • Voltage Regulation: In compound DC generators, voltage stability is maintained by adjusting the magnetic fields in response to changes in load, demonstrating effective voltage regulation.

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

A small amount of magnetism remains in the rotor iron. This residual magnetic field in the main poles induces an electromotive force in the stator coils, generating the initial current in the field windings.

A small current flowing through the coil enhances the magnetic field. Consequently, the voltage output and field current increase. This cycle persists until the electromotive force in the armature exceeds the voltage drop across 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 by the arrangement of field coils and armature, and the method used to excite the magnetic field. This classification includes common portable generators.

Series Wound Generators

In series wound generators, the field and armature winding are connected in series, allowing current to flow through both the external circuit and the windings. The field coil has low resistance and consists of a few turns of thick wire, which increases current flow as load resistance decreases.

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 shunt wound generators, the field winding is connected in parallel to the armature, maintaining consistent voltage across the circuit. The field winding features many turns to achieve high resistance, limiting the current passing through it and directing the remainder to the load.
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.

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