Types of DC Generators - Pedagogy Zone

DC generators can be classified according to their methods of field excitation. There are two types of DC generators on the basis of excitation.

Separately excited DC generators
Self excited DC generators

Separately excited DC generators

If the field winding is excited by a separate DC supply, then the generator is called separately DC generator.

Figure shows the diagram of a separately excited generator. The field winding has large number of turns of thin wire.

From this diagram,

Armature current I_a = Load current I_L
R_a = Resistance of the armature winding
Terminal voltage V = E_g – I_a R_a – V_brush
V_brush = voltage drop at the contacts of the brush.
Generally V_brush is neglected because of very low value.
Generated emf E_g = V + I_a R_a – V_brush
Electric power developed = E_g I_a
Power delivered to load = V I_a

Self-excited DC Generators

If in a D.C generator field winding is supplied from the armature of the generator itself, then it is called a self-excited D.C generator. Residual flux is present in the poles. When the armature is rotated, a small emf is produced in the armature winding because of residual flux.

This emf produces a small field current in the field winding. Then flux per pole increases. The increased flux increases the induced emf, which further increases the field current. Because of this cumulative process, generator produces its rated voltage. The self-excited generators can be classified depending upon how the field winding is connected to the armature. There are three types,

Series generator
Shunt generator
Compound generator.

Series Generator

The field winding is connected in series with the armature. This type of D.C. generator is called D.C. series generator. Here, the armature current flows through the field winding as well as the load. The connection diagram for dc series generator is shown in figure.

The field winding las less number of turns of thick wire. It has low resistance. It is denoted by R_se. Here, armature, field and load are all in series. So they carry, the same current.

∴ I_a = I_se = I_L

Generated emf E_g = V + I_a R_a + I_a R_se + V_brush

where V = terminal voltage in volts
I_a R_a = voltage drop in the armature resistance.
I_a R_se = voltage drop in the series field winding resistance
V_brush = brush drop
∴ Terminal voltage V = E – I_a R_a – I_a R_se – V_brush
Power developed in the armature = E_g I_a
Power delivered to load = V I_a (or) V I_L

Shunt Generator

In a d.c shunt generator, field winding is connected across the armature. The load is also connected across the armature. The shunt field winding has more number of turns of thin wire. It has high resistance. Therefore, a small amount of current flows through the field winding and large amount of current flows through the armature. Figure shows connections diagram of a D.C shunt generator.

Terminal voltage V = E_g – I_a R_a
Shunt field current I_sh = V/R_sh
Armature current I_a = I_L + I_sh
Power developed by armature = E_g I_a
Power delivered to load = V I_L

Compound Generator

The compound generator consists of both shunt field and series field windings. One winding is in series and other winding is in parallel with the armature. Depending upon the shunt field and series field connections, compound generator can be classified as

Long shunt compound generator
Short shunt compound generator

Long shunt compound generator

Figure shows connection diagram of a long shunt compound generator. Here, shunt field winding is connected across both series field and armature windings.

I_se = I_a = I_L + I_sh
Shunt field current I_sh = V/R_sh
Generated emf E = V + I_a (R_a + R_se) + V_brush
Terminal voltage V = E_g – I_a (R_a + R_se) – V_brush
Power developed in armature = E_g I_a
Power delivered to load = V I_L

Short Shunt Compound Generator

Figure shows short shunt compound generator. Here, shunt field winding is connected in parallel with the armature and this combination is connected in series with series field winding.

Series field current = I_se = I_L
Load current = I_L
I_a = I_sh + I_se
Generated emf E_g = V + I_a R_a + I_se R_se + V_brush

Voltage across shunt field winding = I_sh R_sh
I_sh R_sh = E_g – I_a R_a – V_brush
= V + I_a R_a + I_se R_se + V_brush – I_a R_a – V_brush
= V + I_se R_se
Shunt field current I_sh = V+ I_se R_se/R_sh
Terminal voltage V = E_g – I_a R_a – I_se R_se – V_brush
Power developed in armature = E_g I_a
Power delivered to load = V I_L

What are the different types of D.C Generator?

1. Separately excited D.C Generator 2. Self excited D.C Generator a) Series generator b) Shunt generator c) Compound generator i) Long shunt compound ii) Short shunt compound