I hope you have come across or have known about DC Generator in our previous article. If you have not seen or read it yet, read it here:
DC Generator: Working Principle, Constructions, EMF Equation, and Types
Link:- https://electricalguide360.com/dc-generator-working-principle/

This article will elaborately explain to you the types of DC generators, which is also classified in the above article.

The magnetic field in a d.c. generator is normally produced by electromagnets
rather than permanent magnets. Generators are generally classified according to
their methods of field excitation. On this basis, d.c. generators are divided into
the following two classes:

(i) Separately excited d.c. generators
(ii) Self-excited d.c. generator

The behavior of a d.c. generator on load depends upon the method of field
excitation adopted.

1. Separately Excited D.C. Generators
A d.c. generator whose field magnet winding is supplied from an independent external d.c. source (e.g., a battery, etc.) is called a separately excited generator. The figure above shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of the armature and the field current (Eg = Pf ZN/60 A). The greater the speed and field current, the greater the generated e.m.f. It may be noted that separately excited d.c. generators are rarely used in practice. The d.c. generators are normally of the self-excited type.

2. Self-Excited D.C. Generators
A d.c. generator whose field magnet winding is supplied current from the output
of the generator itself is called a self-excited generator. There are three types of
self-excited generators depending upon the manner in which the field winding is
connected to the armature, namely;

Full Article on https://electricalguide360.com/types-of-d-c-generators/