The curve showing relation between terminal voltage V and field current If when the generator is loaded is known as load saturation curve.
Ob = OCC curve at no load. Eo = Open circuit voltage.
Oc = Open circuit field current at no load.
E = Load voltage at load.
The curve Ld is practically parallel with the curve Ob. The terminal voltage V will be less then this generated voltage E by an amount of IaRa.
At load condition the terminal voltage will decrease due to demagnetize effect
∆ bdc = called drop reaction for full load.
∆ mnt = called drop reaction triangle for half load.
FACTORS EFFECTIVE VOLTAGE BUILDING OF A DC GENERATOR:
Its represent the normal operation the prime mover rotation is clockwise and both the residual flux ΦR and the field flux ΦF are directed to the left.
It represent the reversed connection of the field circuit which cases ΦF to oppose ΦR consequently the generator voltage builds down from its original residual value.
Its represent revered residual magnetism. Armature voltage is reversed w hich farther reversed the field current.
So both ΦF ΦRare reversed. So the voltage buildup is in the reversed direction.
It represents the reversed armature rotation causes the reversal of the voltage produced by the residual magnetism.
The reversed field current flow causes ΦF to oppose ΦR so that the voltage builds down from its original residual value.