When the field winding is supplied from the armature of the generator itself then it is said to be self excited generator. Now without generated e.m.f., field can not be excited in such generator and without excitation there can not be generated e.m.f. So one may obviously wonder, how this type of generator works. The answer to this is residual magnetism possessed by the field poles, under normal condition.
Practically through the generator is not working, without any current through field winding, the field poles possess some magnetic flux. This is called residual flux and the property is called residual magnetism. Thus when the generator is started, due to such residual flux, it develops a small e.m.f. which now drives a small current through the field winding. This tends to increase the flux produced. This in turn increases the induced e.m.f. This further increases the field current and the flux. The process is cumulative and continues till the generator develops rated voltage across its armature. This is voltage building process in self excited generators.
Based on how field winding is connected to the armature to drive its excitation, this type is further divided into following three types.
i) Shunt generator
ii) Series generator
iii) Compound generator
Shunt Generator
When the field winding is connected in parallel with the armature and the combination across the load then the generator is called shunt generator.
The field winding has large number of turns of thin wire so it has high resistance. Let Rsh be the resistance of the field winding.
Fig. 1 Shunt generator
1.1 Voltage and Current Relations
From the Fig. 1, we can write
Ia = IL + Ish
Now voltage across load is Vt which is same across field winding as both are in parallel with each other.
... Ish = Vt /Rsh
While induced e.m.f. E, still requires to supply voltage drop Ia Ra and brush contact drop.
... E = Vt + Ia Ra + Vbrush
Where E = (ΦPNZ)/(60A)
In practical, brush contact drop can be neglected.
Series Generators
When the field winding is connected in series with the armature winding while supplying the load then the generator is called series generator. It is shown in the Fig. 1.
Field winding, in this case is denoted as S1 and S2. The resistance of series field winding is very small and hence naturally it has less number of turns of thick cross-section wire as shown in the Fig. 1.
Fig. 1 Series generators
Let Rse be the resistance of the series field winding.
1.1 Voltage and current Relations
As all armature, field and load are in series they carry the same current.
... Ia = Ise = IL
Where Ise = Current through series field winding.
Now in addition to drop Ia Ra, induced e.m.f. has to supply voltage drop across series field winding too. This is Ise Rse i.e. Ia Rse as Ia = Ise. So voltage equations can be written as,
E = Vt + Ia Ra + Ia Rse + Vbrush
... E = Vt + Ia (Ra + Rse) + Vbrush
where E = (ΦPNZ)/(60A)
Compound Generator
In this type, the part of the field winding is connected in parallel with armature and part in series with the armature. Both series and shunt field windings are mounted on the same poles. Depending upon the connection of shunt and series field winding, compound generator is further classified as : i) Long shunt compound generator, ii) Short shunt compound generator.
1.1 Long Shunt Compound Generator
In this type, shunt field winding is connected across the series combination of armature and series field winding as shown in the Fig. 1.
Fig. 1 Long shunt compound generator
Voltage and current relations are as follows.
From the Fig. 1.
Ia = Ise
and Ia = Ish + IL
Voltage across shunt field winding is Vt.
Ish = Vt /Rsh
where Rsh = Resistance of shunt field winding
And voltage equation is,
E = Vt + Ia Ra + Ia Rse + Vbrush
Where Rse = Resistance of series field winding
1.2 Short Shunt Compound Generator
In this type, shunt field winding is connected, only across the armature, excluding series field winding as shown in the Fig. 2.
Fig. 2 Short shunt compound generator
Voltage and current relations are as follows.
For the Fig. 2, Ia = Ise + Ish
and Ise = IL
... Ia = IL + Ish
The drop across shunt field winding is drop across the armature only and not the total Vt, in this case. So drop across shunt field winding is E -Ia Ra .
Ish = (E - Ia Ra ) / ( Rsh)
Now the voltage equation is E = Vt + Ia Ra + Ise Rse + Vbrush
... Ise = IL
... E = Vt + Ia Ra + IL Rse + Vbrush
Neglecting Vbrush , we can write,
E = Vt + Ia Ra + IL Rse
E - Ia Ra = Vt + IL Rse
Ish = (Vt + IL Rse) / ( Rsh)
Any of the two above expression of Ish can be used, depending on the quantities known while solving the problems.
1.3 Cumulative and Differential Compound Generator
It is mentioned earlier that the two windings, shunt and series field are wound on the same pole. Depending on the direction of winding on the pole, two fluxes produced by shunt and series field may help or may oppose each other. This facts decides whether generator is cumulative or differential compound. If the two fluxes help each other as shown in Fig. 3 the generator is called cumulative compound generator.
Fig. 3 Cumulative compound generator
ΦT = Φsh + Φse
Where Φsh = Flux producd by shunt
Φse = Flux produced by series, field winding
If the two windings are wound in such a direction that the fluxes produced by them oppose each other then the generator is called differential compound generator. This is shown in the Fig. 4.
ΦT = Φsh - Φse
Where Φsh = Flux produced by shunt field winding.
Φse = Flux produced by series field winding.
Fig. 4 Differential compound generator
