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Characteristics Of DC Generators

By Kiran Daware DC Machines Leave A Comment

Generally, following three characteristics of DC generators are taken into considerations: (i !"en

Circ#it Characteristic (!$C$C$, (ii %nternal or &otal Characteristic and (iii 'ternal

Characteristic$ &hese characteristics of DC generators are e"lained )elow$

1. Open Circuit Characteristic (O.C.C.) (E0 /If )!"en circ#it characteristic is also known as magnetic characteristic or no-load saturation

characteristic$ &his characteristic shows the relation )etween generated emf at no load (' * and

the field c#rrent (%f  at the given fied s"eed$ &he !$C$C$ c#rve is +#st the magnetiation c#rve

and it is "ractically similar for all ty"e of generators$ &he data for !$C$C$ c#rve is o)tained )y

o"erating the generator at no load and kee"ing s"eed constant$ -ield c#rrent is varied and the

corres"onding terminal voltage is recorded$

&he a)ove fig#re shows a ty"ical no.load sat#ration c#rve or o"en circ#it characteristics forall ty"es of DC generators$

2. Internal Or Total Characteristic (E/Ia)&he internal characteristic c#rve shows the relation )etween the on.load generated emf ('g and

the armat#re c#rrent (%a$ &he on.load generated emf 'g is always less than '* d#e to armat#re

reaction$ 'g can )e determined )y s#)tracting the dro" d#e to demagnetiing effect of armat#re

reaction from no.load voltage '*$ &herefore, internal characteristic c#rve lies )elow !$C$C$ c#rve$

3. External Characteristic (/I!)

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&he eternal characteristic c#rve shows the relation )etween the terminal voltage (/ and load

c#rrent (%L$ &he terminal voltage / is less than generated emf 'g d#e to voltage dro" in the

armat#re circ#it$ &herefore the eternal characteristic c#rve lies )elow the internal characteristic

c#rve$ 'ternal characteristics are very im"ortant to determine the s#ita)ility of a generator for a

given "#r"ose$

%nternal and eternal characteristic c#rves are shown )elow for each ty"e of generator $

Characteristics Of "eparatel# Excite$ DC Generator 

%f there is no armat#re reaction and armat#re voltage dro", voltage will remain constant for any

load c#rrent$ &h#s the straight line AB in a)ove fig#re re"resents the no.load voltage vs$ load

c#rrent %L$ D#e to demagnetiing effect of armat#re reaction the on.load generated emf is less

than the no.load voltage$ &he c#rve AC re"resents the on.load generated emf 'g vs$ load c#rrent

%L i$e$ internal characteristic$ &he c#rve AD re"resents the terminal voltage vs$ load c#rrent i$e$

eternal characteristic$

Characteristics Of DC "eries Generator 

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&he c#rve AB in a)ove fig#re identical to o"en circ#it characteristic (!$C$C$ c#rve$ &his is

 )eca#se, in DC series generators field winding is connected in series with armat#re and load$

0ence, here load c#rrent is similar to field c#rrent$ &he c#rve !C and !D re"resents internal and

eternal characteristic res"ectively$

Characteristics Of DC "hunt Generator 

When load resistance is decreased in DC shunt generator, the load current increases.But, load resistance can be decreased upto a certain limit, beyond this limit any further

decrease in load resistance results in decreasing load current and terminal voltage.

Consequently, the external characteristic curve turns back as shown by dotted line in

above figure.

Characteristics Of DC Co%poun$ Generator 

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he above figure shows the external characteristic of DC compound generators. !f series

winding is ad"usted so that, increase in load current causes increase in terminal voltage

then the generator is called to be over compounded. he external characteristic for over

compounded generator is shown by the curve #B in above figure.

!f series winding is ad"usted so that, terminal voltage remains constant even the load

current is increased, then the generator is called to be flat compounded. he external

characteristic for a flat compounded generator is shown by the curve #C.

!f the series winding has lesser number of turns than that would be required to be flat

compounded, then the generator is called to be under compounded. he external

characteristics for an under compounded generator is shown by the curve #D.