The mmf necessary to establish flux in the magnetic circuit of a dc generator can be obtained by means of
- i. Permanent magnets
- ii. Field coils excited from some external source and
- iii. Field coils excited by the generator itself.
Generators are generally classified according to these methods of field excitation. On this basis, dc generators are divided into the following two classes:
- Permanent magnet dc generators
- Separately excited dc generators
- Self-excited dc generators
In permanent magnet dc machines, a permanent magnet is used to establish flux in the magnetic circuit.
These generators are not found in industrial applications because of the low power generated from it. Such generators are employed only in small sizes like dynamos in motorcycles.
The behavior of a dc generator on load depends upon the method of field excitation adopted.
Separately Excited D.C. Generators
|Seperately Excited DC Generator|
Armature current, Ia = IL
Terminal voltage, V = Eg – IaRa
Self-Excited D.C. Generators
A d.c. generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;
- i. Series generator
- ii. Shunt generator
- iii. Compound generator
DC Series generator
The figure shows the connections of a series wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., as boosters.
DC Shunt generator
The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. Also, read the characteristics of a shunt generator.
The figure below shows the connections of a shunt-wound generator.
DC Compound generator
- Short Shunt in which only shunt field winding is in parallel with the armature winding.
- Long Shunt in which shunt field winding is in parallel with both series field and armature winding.
Long shunt Compound Generator
Short Shunt Compound Generator
In cumulative wound generators the series field assists the shunt field, whereas, in differential wound generators, series field opposes the shunt field.