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

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ALP CBT 2 Electrician Previous Paper: Held on 22 Jan 2019 Shift 1

Answer 1

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.