chap 2(1)-dc generator

8/12/2019 Chap 2(1)-Dc Generator

1/31

DC GENERATORSDC GENERATORS


2/31

DC generators : dc machines used as generator.

Five major types of dc generators, classified according to the manner in

which their field fluxis produced:

Separately excited generator:In separately excited generator, the field

flux is deried fro! a separately po"ersource independent of the

generator itself#

Shunt generator:In a shunt generator, the field flux is deried $yconnecting the field circuitdirectly across the ter!inals of the generators#

Series generator:In a series generator, the field flux is produced $y

connecting the field circuit inseries "ith the ar!ature of the generator#

Cumulatively compounded generator:In a cu!ulatiely co!poundedgenerator, $oth a shunt and series field is present, and their effects are

additie#

Differentially compounded generator:In differentially co!pounded

generator% In a differentially co!pounded generator, $oth a shunt and a

series field are present, $ut their effects are su$tractie#


3/31

DC Generators

These various types of dc generator differ in their terminal voltage!current"

characteristic, and the application is depending to which is suited.

DC generators are compared #y their voltages, power ratings, efficiencies and

voltage regulations:

%100

=

fl

flnl

V

VVVR

$%& ' Drooping characteristics

!%& ' &ising characteristic


4/31

E&uialent Circuit of DC Generators

The e(uivalent circuit of a DC

generator

) simplified e(uivalent circuit

of a DC generator, with &Fcom#iningthe resistances of the field coils and

the varia#le control resistor


5/31

Separately Excited Generator

Fig : *eparately excited DC generator

) separately excited DC generator is a generator whose field current is supplied #ya separately external DC voltage source

%T' )ctual voltage measured at the terminals of the generator

+' current flowing in the lines connected to the terminals.

-)' +nternal generated voltage.

+) ' )rmature current.

AL II =


6/31

The Ter!inal Characteristic of A Separately

Excited DC Generator

The terminal characteristic of a separately excited dc generator a" with and #"

without compensating windings -)' "

For DC generator, the output (uantities are its terminal voltage and line

current. The terminal voltage is %T' -)/ +)&) +)' +"

*ince the internal generated voltage -)is independent of +

), the terminal

characteristic of the separately excited generator is a straight line.

Ta'e note a$out the axes $et"een !otors ( and ind) and generators (*Tand I+)


7/31

The Ter!inal Characteristic of A Separately

Excited DC Generator

0hen the loadis supplied #y the generator is increased, +and therefore IA"

increase. )s the armature current increase, the +)&)drop increase, so the

terminal voltage of the generator falls. Figure a" 1&-%+23* *+D-"

This terminal characteristic is not always entirely accurate. +n the generators

without compensating windings, an increase in +)causes an increase in the

armature reaction, and armature reaction causes flux wea4ening. This flux

wea4ening causes a decrease in -) ' 5which further decreases the terminalvoltage of the generator. The resulting terminal characteristic is shown in Figure

#" 1&-%+23* *+D-"


8/31

Control of Ter!inal *oltageIf DC otors "e control tor&ue-speed, in DC Generator "e control *

T

The terminal voltage of a separately excited DC generator can #e controlled #y

changing the internal generated voltage -)of the machine.

*T. EA/ IARA

+f -)increases, %Twill increase, and if -)decreases, %Twill decreases. *ince

the internal generated voltage, -)' 65, there are two possi#le ways to

control the voltage of this generator:

0# Change the speed of rotation. +f 5 increases, then -)' 65 increases, so%

T' -

)! +

)&

)increasestoo.

1# Change the field current. +f &Fis decreased, then the field current increases

+F'%F7&F".Therefore, the flux 6 in the machine increases. )s the flux rises,

-)'

5must rise too, so %T' -)/ +)&)increases.


9/31

The Shunt DC Generator

A shunt DC generator: DC generator that supplies its o"n field current#yhaving its field connected directly across the terminals of the machine.

Figure : The e(uivalent circuit of a

shunt DC generator.

=

=

+=

F

T

F

AAAT

LFA

R

VI

RIEV

III

8ecause of generator supply it own

field current, it re(uired voltage

#uildup


10/31

*oltage 2uildup in A Shunt

Generator

)ssume the DC generator has no loadconnected to it and that the prime mover

starts to turn the shaftof the generator. The voltage #uildupin a DC generator

depends on the presence of a residual flux in the poles of the generator.

This voltage is given #yresA KE =

This voltage, -)a volt of two appears at terminal of generators",and it causes a

current +Fto flow in the field coils. This field current produces a magnetomotive force

in the poles, which increases the fluxin them.

-), then %Tincreaseand cause further increase +F, which further increasing

the flux and so on.

The final operating voltageis determined #y intersection of the field resistanceline

and saturation curve. This voltage #uildup process is depicted in the next slide


11/31

EAmay be a volt or

two appear at the

terminal during start-up

Voltage buildup

occurred in discrete

steps


12/31

*everal causes for the voltage to fail to #uild up during starting which are : Residual !agnetis!. +f there is no residual fluxin the poles, there is no

+nternal generated voltage, -)' 9%and the voltage will never #uild up.

Critical resistance. ormally, the shunt generator #uilds up to a voltagedetermined #y the intersection of the field resistance line and the saturationcurve. +f the field resistance is greater than critical resistance, the generatorfails to #uild up and the voltage remains at the residual level. To solve this

pro#lem, the field resistance is reduced to a value less than criticalresistance.

Refer 3igure 4-50 page 675 (Chap!an)

Critical resistance


13/31

The direction of rotation of the generator may have #een reversed, or the

connections of the field may have #een reversed. +n either case, the

residual flux produces an internal generated voltage -). The voltage -)

produce a field current which produces a flux opposing the residual flux,

instead of adding to it.3nder these conditions, the flux actually decreases #elow resand no

voltage can ever #uild up.


14/31

The Ter!inal Characteristic of a Shunt DC

Generator

Figure : The terminal characteristic of a shunt dc generator

)s the loadon the generator is increased, I+increasesand so IA. I38 I+alsoincrease. )n increase in +)increases the armature resistance voltage drop +)&),

causing *T' -)!+)&)to decrease.

;owever, when *Tdecreases, the field current I3in the machine decreases withit. This causes the fluxin the machine to decrease< decreasing E

A

. Decreasing -)

causes a further decreasein the ter!inal oltage, %T' -)! +)&)


15/31

*oltage Control for Shunt DC Generator

There are two ways to control the voltage of a shunt generator:

=. Change the shaft speed, 9!of the generator.

>. Change the field resistorof the generator, thus changing the field current.

Changing the field resistoris the principal !ethodused to control terminal

voltage in real shunt generators. +f the field resistor &F is decreased, then the

field current +F ' %T7&Fincreases.

0hen +F, the machine?s flux , causing the internal generated voltage

-). -)causes the terminal voltage of the generator to increase as well.


16/31

The Series DC Generator

Figure : The e(uivalent circuit of a

series dc generator

) series DC generator is a generator whose field is connected in series "ith its

ar!ature. 8ecause the field "indinghas to carry the rated load current, it usuallyhave fe" turns of heay "ire.

Clear distinction, shunt generatortends to maintain a constant ter!inal oltage

while the series generatorhas tendency to supply a constant load current#

The irchhoff?s voltage law for this e(uation : )(SAAAT

RRIEV +=


17/31

Ter!inal Characteristic of a Series

Generator

The magneti@ation curve of a series DC generator loo4s very much

li4e the magneti@ation curve of any other generator. )t no load,

however, there is no field current, so *Tis reduced to a ery s!all

leelgiven #y the residual flux in the machine. )s the load increases,the field current rises, so EArises rapidly. The +)&)$ &*" drop goes

up too, #ut at the first the increase in EAgoes up !ore rapidly than

the IA(RA8 RS) drop rises, so *Tincreases. After a "hile, the

!achine approaches saturation, and EA $eco!es al!ost

constant# At that point, the resistie drop is the predo!inanteffect, and *Tstarts to fall#

Figure : ) series generator terminal

characteristic with large armature

reaction effects


18/31

The Cu!ulatiely Co!pounded DC

Generator

Figure : The e(uivalent circuit

of a cumulatively compounded

DC generator with a long shunt

connection

) cumulatively compounded DC generator is a DC generator with #othseries and

shunt fields,connected so that the !agneto!otie forcesfrom the two fields are

additie.


19/31

The Cu!ulatiely Co!pounded DC

GeneratorThe total !agneto!otie forceon this machine is given #y

3net . 338 3SE- 3AR

where FF' the shunt field magnetomotive force

F*-' the series field magnetomotive force

F)& ' the armature reaction magnetomotive force

NFI*F= NFIF+ NSI! " F!#

F

ARA

F

SEFF

N

FI

N

NII +=

*


20/31

The other voltage and current relationships forthis generator are

F

TF

SAAAT

LFA

R

VI

RRIEV

III

=

+=

+=

)(


21/31

)nother way to hoo4 up a cumulatively compounded generator. +t is the

:short-shunt; connection, where series field is outside the shunt fieldcircuit and has current +flowing through it instead of +).

Figure : The e(uivalent circuit of a cumulatively DC generator

with a short shunt connection


22/31

The Ter!inal Characteristic of a

Cu!ulatiely DC Generator0hen the load on the generator is increased, the load current I+also

increases.

*ince +) ' +F$ +, the armature current IA increasestoo. )t this point two

effects

occur in the generator:

=. )s +)increases, the +) &)$ &*" voltage drop increases as well. This

tends to cause a decreasein the terminal voltage, *T' -)/+)&)$

&*".

>. )s IAincreases, the series field magnetomotive force 3SE. NSEIA

increasestoo. This increases the total !agneto!otie force 3tot .

N3I38 NSEIAwhich increases the fluxin the generator. The increased

flux in the generator increases EA, which in turn tends to ma4e *T'

-)/ +)&)$ &*" rise#


23/31

*oltage Control of Cu!ulatiely

Co!pounded DC GeneratorThe techni(ues availa#le for controlling the terminal voltage of acumulatively

compounded DC generator are exactly the same as the techni(ue for

controlling the

voltage of a shunt DC generator:

=. Change the speed of rotation. )n increase in causes -)' to

increase, increasing the terminal voltage %T' -)/ +)&)$ &*".

>. Change the field current. ) decrease in &Fcauses +F' %T7&Fto

increase, which increase the total magnetomotive force in the generator.

)s Ftotincreases, the flux in the machine increases, and -)' increases. Finally, an increase in -)raises %T.


24/31

Analysis of Cu!ulatiely Co!pounded DC

Generators

The e&uialent shunt field current Ie&due to the effects of

the series field and armature reaction is given #y

F

ARA

F

SEeq

NI

NNI F=

The total effectie shunt field currentis

eqFF III +=*


25/31


26/31

The Differentially Co!pounded DC

Generator

)( FAAAT

F

TF

FLA

RRIEV

R

VI

III

+=

=

+=

) differentially compounded DC generator is a generator "ith $oth shunt

and series fields, $ut this ti!e their !agneto!otie forces su$tract

fro! each other#

The e(uivalent circuit of a differentially

compounded DC generator


27/31

The Differentially Co!pounded DC

Generator

The net magnetomotive force is

3net. 33/ 3SE/ 3AR 3net. N3I3/ NSEIA - 3AR

)nd the e(uivalent shunt field current due to the series field and armaturereaction is given #y :

F

ARA

F

SEeq

NI

N

NI

F=

The total effective shunt field current in this machine is

eqFF III +=*

or

F

ARA

F

SEFF

N

I

N

NII

F=

*


28/31

*oltage Control of Differentially

Co!pounded DC Generator

Two effects occur in the terminal characteristic of a differentiallycompounded

DC generator are

=. )s IAincreases, the IA(RA8 RS) oltage drop increasesas well.

This increase tends to cause the terminal voltage to decrease *T#

>. )s IAincreases, the series field !agneto!otie 3SE. NSEIA

increasestoo. This increases in series field magnetomotive force

reduces the net !agneto!otie forceon the generator, 3tot. N3I3

/ NSEIA", which in turn reduces the net fluxin the generator. )decrease in flux decreases EA, which in turn decreases *T.

*ince #oth effects tend to decrease %T, the voltage drop drastically as

the load is increased on the generator as sho"n in next slide


29/31


30/31

*oltage Control of Differentially

Co!pounded DC Generator

The techni(ues availa#le for adjusting terminal voltage are exactly the

same as

those for shunt and cumulatively compounded DC generator:

=. Change the speed of rotation, !#

>. Change the field current, I3.


31/31

-D 2F C;)1T-& >

chap 2(1)-dc generator

Documents