Types of DC Generators – Series, Shunt and Compound

The mmf necessary to establish flux in the magnetic circuit of a dc generator can be obtained by means of

• i. Permanent magnets
• ii. Field coils excited from some external source and
• iii. Field coils excited by the generator itself. 

Generators are generally classified according to these methods of field excitation. On this basis, dc generators are divided into the following two classes:

1. Permanent magnet dc generators 
2. Separately excited dc generators 
3. Self-excited dc generators 

In permanent magnet dc machines, a permanent magnet is used to establish flux in the magnetic circuit. 

These generators are not found in industrial applications because of the low power generated from it. Such generators are employed only in small sizes like dynamos in motorcycles.

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

Separately Excited D.C. Generators

 

The figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). 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. The d.c. generators are normally of self-excited type.

Seperately Excited DC Generator

 Armature current, Ia = IL

 Terminal voltage, V = Eg – IaRa

 Electric power developed = EgIa

 Power delivered to load = EgIa – I R = I E – I R = VIa 

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;

 

• i. Series generator
• ii. Shunt generator
• iii. Compound generator

Series generator

In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.

 

The figure shows the connections of a series wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., as boosters.

Shunt generator

Compound generator

In a compound-wound generator, there are two sets of field windings on each pole – one is in series and the other in parallel with the armature. 

A compound-wound generator may be:

1. Short Shunt in which only shunt field winding is in parallel with the armature winding.
2. Long Shunt in which shunt field winding is in parallel with both series field and armature winding.