In dc generator, there are three power stages
Mechanical power input Electrical power input Electrical power output
Mechanical efficiency is ratio of electrical power output and mechanical power input.
Electrical efficiency is ratio of electrical power output and electrical power input.
And overall efficiency is ratio of electrical power output and mechanical power input.
In order to find the copper loss
Do subtraction of electrical power output and electrical power input. By finding I2R losses of shunt field, armature field and series field.
As @Apollos asked in this question
if you try to find the copper loss by first method, then you will have to carefully calculate the back emf Eb.
If only Ra-armature resistance is given, then Eb=V+IaRa
If Ra and Rse-series resistance is given then Eb=V+IaRa+IaRse
If Ra and Rse and brush voltage drop is given,then Eb=V+IaRa+IaRse+2*drop
And then you can calulate the value of copper loss as Output power - Eb*Ia
Now for second method, you can find value of Ia and Ish - armature current and shunt current and then find the copper loss of series field, shunt field and armature field and then do addition.