The name separately excited tells you, The field winding is excited by an external independent DC power supply.i.e, There is no electrical connection between armature winding and field winding.
The DC generator has no load and load characteristics. The load characteristics is again classified by internal and external characteristics.
Open Circuit or magnetization or No load characteristics of separately excited dc generator
Consider the load is removed from the dc generator. In this situation the load current (IL=0).
By Kirchoff voltage law, We can write the equation,
Vt = Ea - IaRa
Vt- Terminal voltage
Ea- generated voltage
Ia- Armature current
Ra- Armature resistance
Practically, there will be voltage drop across the brush in the range of one or two volt per brush. So, it is neglected to get a round off voltage.
Even though the field current is zero, There will be some emf present in armature. Because of residual magnetism in the poles.
From the emf equation of DC generator,
we can say generator voltage is directly proportional to the speed of the Electrical Machine.
Ea α N
At No load condition, generation of voltage across the dc generator varies with field current at fixed RPM N1,N2,N3.The graph shows that for the generation of voltage is increased with increase in fixed rpm for different values.
Load characteristics:
External characteristic (I vs Vt)
The plot between output parameters terminal voltage vs load current gives terminal characteristic.
When the terminal voltage Vt decrease with increase in demand of current from the more load. Because of drop across the armature ohmic drop. There is No load at terminal the Load current is Zero (Ia=0).
Vt = Ea - IaRa
Vt = Ea
Internal characteristics (Ia vs Ea)
We all know that compensating winding is provided in between the poles for neglecting Armature Reaction.
Consider dc generator is not present with compensating winding.If the load current increases IL= Ia for separately excited dc generator. The armature reaction causes flux weakening, there will be reducing flux so the terminal voltage will be decreases.
Conclusion
The Major advantage of the separately excited machine is separate DC power supply to field winding.This is more stable than other machine when we operate.
For example, The separately excited dc generator has a terminal voltage of 240v and induced emf of 250v. If Ra = 0.1 ohm, What will be full load current and output power?
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