Self Excited DC Generators

SELF EXCITED DC GENERATORS

In this type of generator, the field winding is excited by the same machine. When the armature is rotated, some emf is generated due to the presence of residual magnetism. Thus, some induced current is produced and this passes through the field coils. This induced current produces more flux. This action is repeated and thereby sufficient current passes through the field coils to generate the rated induced emf.

Types of self excited DC Generators

The self‒excited generators are further classified into three types, according to the way of their field winding connections to the armature as follows.

1. DC Shunt generator

2. DC Series generator

3. DC Compound generator

1. DC Shunt Generator

In this type of generator, the field winding is connected in parallel with the armature terminals.

Current equation is  l_a = I_L+ I_sh

But I_sh = V / R_sh

Voltage equation is E_g = V + I_aR_a + V_b + V_ar (or)

 E_g = V + I_aR_a    if (V_b and V_ar are negligible)

2. DC Series Generator

In this type of generator, the field winding is connected in series with the armature.

Current equation is l_a = I_L ‒ I_se

Voltage equation is E_g = V + I_a (R_a + R_se) + V_b + V_ar (or)

 E_g = V+1_a(R_a+B_se)        if (V_b and V_ar are negligible)

3. DC Compound Generator

(i) Long Shunt Compound Generator

In this type of generator, the shunt field winding is connected across the series combination of armature and series field winding and across the load.

Current equation is, I_a = I_se = I_L+I_sh, Here I_sh=V/R_sh

Voltage equation is E_g = V + I_a(R_a + R_se) + V_b + V_ar (or)

E_g = V + I_a (R_a+R_se)         (if V_b and V_ar are negligible)

(ii) Short Shunt Compound Generator

In this type of generator, the shunt field winding is connected across the series combination of load and series field winding and across the armature terminals.

Current equation is I_L = I_se

l_a = I_L+I_sh

Here

I_sh = V/R_sh and V_sh = V + I_LR_se

Voltage equation is

E_g = V + I_aR_a + I_LR_se + V_b + V_ar (or)

 E_g=V + l_aR_a + I_LR_se     (if V_b and V_ar are negligible)

Armature Reaction

The interaction of armature mmf with main field mmf is called armature reaction. It has two effects.

(i) Demagnetising effect - because of this net flux in the generator is reduced and so the emf generated and hence the terminal voltage are reduced.

(ii) Cross magnetising effect ‒ because of this there is sparking between brushes and commutator segment resulting in poor commutation.