In Separately Excited Generator, the field windings are connected in:

• Separately excited dc generator requires an independent dc external source for energizing the field winding.
• The field windings in a separately excited generator connected in series with dc external source.
• The equivalent circuit representation under steady-state condition is shown in Figure.

The equations under steady-state are:

V_f = I_fR_t

R_t = R_f + R_ext

V_a = E + I_LR_a

I_L = I_a

Where V_f = External dc source

I_f = Field winding current

R_t = Net resistance across external source side

R_f = Field winding resistance

V_a = Voltage across load

E = Generated emf

I_L = Load current

I_a = Armature current

R_a = Armature resistance

• When the field current is held constant and the armature is rotating at a constant speed, the induced emf in an ideal separately excited dc generator is independent of the armature current.
• As the load current IL increases, the terminal voltage decreases as indicated by a solid line. If the armature reaction is neglected, decrement in V_a should be linear and equal to the voltage drop across R_a and carbon brushes.
• However, if the generator is operated at the knee point in the magnetization curve the armature reaction causes a further drop in terminal voltage.