Self Excited Generator

A DC generator whose field magnet windings are supplied current from the generator output itself is termed as self-excited generator. Self-Excited generator can be categorized into three types relying upon the design in which the field windings are coupled to the armature winding. They are

1. Series Generator
2. Shunt Generator
3. Compound Generator

Series Generator:

The fig above shows the connection of a series wound generator. In a series generator, the field winding is connected in series with armature winding in order that whole armature current flows through the field winding likewise because the load. Since the field winding carries the entire load current, it's some turns of thick wire having low resistance. Series generators are seldom used aside from special functions e.g., Voltage boosters.

Terminal Voltage V = E_g-I(R_a+R_se)

Armature Current I_a = I_se = I_L = I

Power developed in armature = E_gI_a

Power delivered to load = E_gI_a-I_a²(R_a+R_se)

                                       = I_a [E_g-I_aR_a-I_aR_se] 

                                       = VI_a or VI_L

Shunt Generator:

The fig shows that the field winding connected is in parallel with the armature winding in order that supply voltage of the generator is applied across it. Generally in shunt generator the shunt coil has large number of turns with fine wire having high resistance. As a result, solely a part of armature current flows through shunt winding and also the rest flows through the load.

Terminal Voltage V=E_g-I_aR_a

Armature Current I_a=I_L+I_sh

Shunt field current I_sh = V/R_sh

Power Developed in armature = E_gI_a

Power delivered to load = VI_L

Compound Generator:

In a compound generator, there are 2 sets of field winding on every pole—one is in series and also the different in parallel with the armature. This generator can be classified into two:

(a) Short Shunt during which solely shunt winding is connected in parallel with the armature winding which is shown in the figure.

(b) Another one is long Shunt during which shunt winding is connected in parallel with each series field winding and armature winding as shown in the figure.

Short Shunt Compound Generator

Terminal Voltage V = E_g – (I_aR_a) – (I_seR_se)

Series field current I_se = I_L

Shunt field current I_sh = (V+I_seR_se) / R_sh

Power developed in armature = E_gI_a

Power delivered to load = VI_L

Long shunt Compound Generator

Terminal voltage, V = E_g - I_a(R_a + R_se)

Series field current, I_se = I_a = I_L + I_sh

Shunt field current, I_sh = V/R_sh

Power developed in armature = E_gI_a

Power delivered to load = VI_L

CSV Nov 22, 2014

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