DC generators are generally classified according to the methods of their field excitation. Based upon the method of excitation, dc generators can be divided into;

1. Separately excited DC generator
2. Self-excited DC generator
1. Separately excited DC generator

A DC generator in which current is supplied to the field winding from an independent external DC source (e.g. a battery) is called separately excited DC generator.  The flux produced by the poles depends upon the field current. The greater the speed and field current, greater is the generated e.m.f. It may be noted that separately excited d.c. generators are rarely used in practice.

Important Relation;

Here, Ia = IL = I

Ia = armature current

IL = Line current

Ra = armature resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa

Terminal voltage, V  = Eg – IaRa – 2vb

Power developed = Eg Ia

Power output = VIL = VIa

1. Self – excited generators

Separately – excited generators are those whose field magnets are energized by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux is the poles. When the armature is rotated, some e.m.f. and hence some induced current is produced which is partly or fully passed through the coils, thereby strengthening the residual  There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

1. Series wound DC generators
2. Shunt wound DC generators
3. Compound wound DC generators
1. Series wound DC generators

In series wound DC generator, the field winding is connected in series with the armature winding forming a series circuit. Therefore, full line current IL or armature current Ia flows through it. As they carry full load current, they consists of relatively few turns of thick wire or strip.

Important Relation;

Here, Ia = IL = Ise

Ia = armature current

IL = Line current

Ra = armature resistance

Rse = series resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa – IseRse

Or

Terminal Voltage, V  = Eg – Ia (  Ra – Rse)- 2vb

Power developed = Eg Ia

Power output = VIL = VIa

2. Shunt wound DC generators

in a shunt wound DC generators, the field winding is connected across the armature winding forming a parallel or shunt circuit. Therefore, full terminal voltage is applied across them. As they carry very small load current, they consists of many turns of fine wire.

Important Relation;

Here, Ia = IL = Ish

Ia = armature current

IL = Line current

Ra = armature resistance

Rsh = series resistance

Vb = brush drop per brush

Ish = shunt current

Ish = V/Rsh

Ia = IL + Ish

Terminal Voltage, V  = Eg – IaRa

Or

Terminal Voltage, V  = Eg – Ia Ra– 2vb

Power developed = Eg Ia

Power output = VIL = VIa

3. Compound wound DC generators

In compound wound DC generator, there are two sets of field windings on each pole. One of them (having many turns of fine wire ) is connected across the armature and the other winding (having few turns of thick wires ) is connected in series with the armature winding. A compound wound DC generator may be; Long Shunt and Short Shunt.

Long shunt in which the shunt field winding is connected in parallel with combination of both armature and series field winding.

Short shunt in which the shunt field winding is connected in parallel with only armature winding.