Question

explain separately exited dc generator

Answer 1

The separately excited dc generator, the field winding is connected to a separate source of dc power. This source may be another dc generator, a controlled rectifier, or a diode rectifier, or a battery.
                                             (OR)
A separately-excited dc generator is a dc generator whose field magnet winding current is supplied by an external dc voltage source.
For comparison, a self-excited dc generator is a dc generator whose field magnet winding is supplied by current from the output of the generator itself.