Please go through the construction of dc generator before reading this topic.
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
A dc generator whose field magnet winding is supplied from an independent external d.c. source (e.g., a battery etc.) is called a separately excited generator.
The figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). The greater the speed and field current, greater is the generated e.m.f.
It may be noted that separately excited d.c. generators are rarely used in practice. The d.c. generators are normally of self-excited type. Also, read characteristics of separately excited dc generator.
separately Excited DC Generator
Armature current, Ia = IL
Terminal voltage, V = Eg – Ia Ra
Electric power developed = EgIa
Power delivered to load = EgIa – I R = I E – I R = VIa
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
In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.
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.
Armature current, Ia = Ise = IL = I(say)
Terminal voltage, V = EG – I(Ra + Rse)
Power developed in armature = EgIa
Power delivered to load
DC Shunt generator
In a shunt generator, the field winding is connected in parallel with the armature winding so that the terminal voltage of the generator is applied across it. 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.
Shunt field current, Ish = V/Rsh
Armature current, Ia = IL + Ish
Terminal voltage, V = Eg – IaRa
Power developed in armature = EgIa
Power delivered to load = VIL
DC Compound generator
In a compound-wound generator, there are two sets of field windings on each pole – one is in series and the other in parallel with the armature.
A compound-wound generator may be:
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
Series field current, Ise = Ia = IL + Ish
Shunt field current, Ish = V/Rsh
Terminal voltage, V = Eg – Ia(Ra + Rse)
The power developed in armature = EgIa
Power delivered to load = VIL
Short Shunt Compound Generator
Series field current, Ise = IL
Shunt field current,
Terminal voltage, V = Eg – IaRa – IseRse
Power developed in armature = EgIa
Power delivered to load = VIL
In a compound generator, the major portion of excitation is usually supplied by the shunt field. The shunt field is slightly weaker and the series field is considerably weaker than those of the corresponding machine in which the entire excitation is produced by a single shunt or a single series winding.
Compound wound generators are of two types, known as cumulative wound and differential wound generators.
In cumulative wound generators the series field assists the shunt field, whereas, in differential wound generators, series field opposes the shunt field.
separately Excited DC Generator
No comments:
Post a Comment