why dc supply used in generator filed for generating voltage.

use to produce magnetic field of generator.

A generator works by moving a coil through a field. This is usually a field by either a permanent magnet of a coil in which you run a current. The ones where you run a current give you a means of output control by varying that current.

The output is either AC of DC depending on the commutation/slip rings or rectification.

Often the make AC and rectify it with solid state rectifiers. In the past they made DC with commutators.

Suggest you read up on DC generators via Google, and AC generators(also called alternators)

Very common question. I'll try and find a drawing that shows how the N/S poles flip as they rotate in a simple generator. In this case it might be best to hold the coils fixed and rotate the magnet inside, then graph what happens to the voltage as the magnet spins through 360.

Oftentimes called the "exciter" - see Google.