Types of DC generators

Types of DC generators-

The magnetic field of a DC generator normally produced by electromagnets rather than permanent magnets. Generators are generally classified according to their methods of field excitation.

On the basis, DC generators are divided into the following two classes:

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

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

 Separately excited DC generators

A Dc generator whose field winding is supplied from an independent external DC source (e.g. a battery etc.) is called a separately excited generator.

The following figure shows the connection of a separately excited DC generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg=PΦZN/60A). If the speed and field current is greater then the generated e.m.f. will be greater. It may be noted that separately excited DC generators are rarely used in practice. The DC is normally of self-excited type.

Types of DC generators

 

Armature current Ia=IL

Terminal voltage, V=Eg-IaRa

Electrical power developed= EgIa

Power delivered to load= Egla-Ia2Ra=Ia(Eg-IaRa)=VIa

Self-Excited DC Generators

A DC 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 basis of the field winding is connected to the armature, namely:

  1. Series generator
  2. Shunt generator
  3. Compound generator

Series generators-

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 load. The Fig. shows the connection of a series wound generator. Field winding carries the whole load current so that it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purpose e.g. as boosters.

Armature current, Ia=Ise=IL=I(say)

Terminal voltage, V=Eg-I(Ra-Rse)

Power developed in armature=EgIa

Power delivered to load=EgIa-Ia2(Ra+Rse)=Ia(Eg-Ia(Ra+Rse))=VIa

Types of DC generators

Shunt generator-

In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it. The shunt field winding has many turns of high resistance wires. Therefore, only a part of armature current flow through shunt field winding and the rest flows through load. The Fig. shows the connections of a shunt wound generator.

Types of DC generators

Shunt field current, Ish=V/Rsh

Armature current, Ia=IL+Ish

Terminal voltage, V=Eg-IaRa

Power developed in armature= EgIa

Power delivered to load= VIL

Compound generator-

In a compound wound generator, there are two sets of field windings on each pole: one is in series and 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 (Fig. A).
  2. Long shunt in which shunt field winding is in parallel with both series field and armature winding (Fig. B).

Types of DC generators

Short shunt:

Series field current, Ise=IL

Shunt field current, Ish= V+IseRse/Rsh

Terminal voltage, V=Eg-IaRa-IseRse

Power developed in armature= EgIa

Power delivered to load= VIL

Long shunt:

Series field current, Ise=Ia=IL+Ish

Shunt field current, Ish=V/Rsh

Terminal voltage, V=Eg-Ia(Ra+Rse)

Power developed in armature= EgIa

Power deliver to load= VIL

 

Also Read- Construction of DC machines

Also read- Principle of operation of DC Generator

 

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