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Types of DC Generator – Separately Excited and Self-Excited DC Generators
A DC generator is an electrical machine which converts mechanical energy into DC electrical energy. DC generators are classified into two types on the basis of their methods of field excitation as follows −
Separately Excited DC Generator
Self-Excited DC Generator
Separately Excited DC Generator
A separately excited DC generator is the one whose field winding is supplied by an independent external DC source (like a battery). The magnitude of generated voltage depends upon the speed of rotation of armature and the field current, i.e., greater the speed and the field current, higher is the generated voltage. In practice, the separately excited DC generators are rarely used.
Refer the equivalent circuit of DC generator,
$$\mathrm{Armature\: current,I_{a}=I_{L}}$$
$$\mathrm{Terminal \:voltage,\:V_{T}=E_{g}-I_{a}R_{a}}$$
$$\mathrm{Developed\: electric\: power = E_{g}I_{a}}$$
$$\mathrm{Power\: delivered \:to\: load =\:E_{g}I_{a}\:-\:I_{a}^2R_{a}\:=\:V_{T}I_{a}=V_{T}I_{L}}$$
Self-Excited DC Generator
A self-excited DC generator is the one whose field winding is excited by the current from the output of the generator itself. Depending upon the connection of field winding with the armature, the self-excited DC generators are of three types −
Series Generator
Shunt Generator
Compound Generator
Series Generator
In case of a series generator, the field winding is connected in series with the armature of the generator so that whole armature current would flow through the field winding as well as the load. Since the load current flows through the field winding of the generator, so the field winding has a few turns of thick wire having low resistance. The DC series generators are used in special applications like boosters.
Refer the equivalent circuit of the series generator,
$$\mathrm{Armature\: current,I_{a}=\:I_{se}=\:I_{L}=\:I(Say)}$$
$$\mathrm{Terminal \:voltage,\:V_{T}=E_{g}-I(R_{a}+R_{se})}$$
$$\mathrm{Power \:developed \:in \:armature = E_{g}I_{a}}$$
$$\mathrm{Power\: delivered \:to\: load =\:E_{g}I_{a}\:-\:I_{a}^2(R_{a}+R_{se})=\:V_{T}I_{a}=V_{T}I_{L}}$$
Shunt Generator
In case of a shunt generator, the field winding is connected in parallel with the armature of the generator so that terminal voltage of the generator is applied across it. The shunt field winding has many turns of thin wire having high resistance so that only a fraction of armature current flows through the shunt field winding and the rest flows through the load.
Refer the equivalent circuit of DC shunt generator,
$$\mathrm{Armature\: current,I_{a}=\:I_{L}+I_{sh}}$$
$$\mathrm{Shunt\: field\: current,I_{sh}=\frac{V_{T}}{R_{sh}}}$$
$$\mathrm{Terminal\: voltage,V_{T} = E_{g}-I_{a}R_{a}}$$
$$\mathrm{Power \:developed \:in \:armature = E_{g}I_{a}}$$
$$\mathrm{Power\: delivered \:to\: load =\:V_{T}I_{L}}$$
Compound Generator
In case of a compound generator, there are two field winding on each pole – one is in series and the other is in parallel with the armature. The DC compound generators are of two types −
Short Shunt Compound Generator
In a short shunt generator, only shunt field winding is connected in parallel with the armature.
$$\mathrm{Series \:field \:current,I_{se}=\:I_{L}}$$
$$\mathrm{Shunt\: field\: current,I_{sh}=\frac{V_{T}+I_{se}R_{se}}{R_{sh}}}$$
$$\mathrm{Terminal\: voltage,V_{T} = E_{g}-I_{a}R_{a}-I_{se}R_{se}}$$
$$\mathrm{Power \:developed \:in \:armature = E_{g}I_{a}}$$
$$\mathrm{Power\: delivered \:to\: load =\:V_{T}I_{L}}$$
Long Shunt Compound Generator
In a long shunt generator, the shunt field winding is connected in parallel with both series field and armature winding.
$$\mathrm{Series \:field \:current,I_{se}=\:I_{a}\:=\:I_{L}+I_{sh}}$$
$$\mathrm{Shunt\: field\: current,I_{sh}=\frac{V_{T}}{R_{sh}}}$$
$$\mathrm{Terminal\: voltage,V_{T} = E_{g}-I_{a}(R_{a}+R_{se})}$$
$$\mathrm{Power \:developed \:in \:armature = E_{g}I_{a}}$$
$$\mathrm{Power\: delivered \:to\: load =\:V_{T}I_{L}}$$
Important − In compound generator, the majority of MMF is established by the shunt field winding. If the series field flux assists the shunt field then the generator is called cumulatively compounded and when they oppose each other, the generator is called differentially compounded.
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