chap 2(1)-dc generator.ppt

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DC GENERATORS



DC generators : dc machines used as generator.

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

which their field flux is produced:

• Separately excited generator : In separately excited generator, the field

flux is derived from a separately power source independent of the

generator itself .

• Shun t generator : In a shunt generator, the field flux is derived byconnecting the field circuit directly across the terminals of the generators.

• Series generator : In a series generator, the field flux is produced by

connecting the field circuit in series with the armature of the generator.

• Cumu lat ively com pounded generator : In a cumulatively compoundedgenerator, both a shunt and series field is present, and their effects are

additive.

• Dif ferent ial ly compounded generator : In differentially compounded

generator: In a differentially compounded generator, both a shunt and a

series field are present, but their effects are subtractive.



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 by their voltages, power ratings, efficiencies and

voltage regulations:

%100

fl

fl nl

V

V V VR

+VR = Drooping characteristics

-VR = Rising characteristic



Equivalent Circuit of DC Generators

The equivalent circuit of a DC

generator

A simplified equivalent circuit

of a DC generator, with RF combiningthe resistances of the field coils and

the variable control resistor



Separately Excited Generator

Fig : Separately excited DC generator

A separately excited DC generator is a generator whose field current is supplied bya separately external DC voltage source

VT = Actual voltage measured at the terminals of the generator

IL = current flowing in the lines connected to the terminals.

E A = Internal generated voltage.

I A = Armature current.

A L I I



The Terminal Characteristic of A Separately

Excited DC Generator

The terminal characteristic of a separately excited dc generator (a) with and (b)

without compensating windings (E A = K)

• For DC generator, the output quantities are its terminal voltage and line

current. The terminal voltage is VT = E A – I AR A (I A = IL)

• Since the internal generated voltage E A is independent of I A, the terminal

characteristic of the separately excited generator is a straight line.

Take note about the axes between motors ( and ind) and generators (VT and IL)



The Terminal Characteristic of A Separately

Excited DC Generator

• When the load is supplied by the generator is increased, IL (and therefore IA)

increase. As the armature current increase, the I AR A drop increase, so the

terminal voltage of the generator falls. (Figure (a) PREVIOUS SLIDE)

• This terminal characteristic is not always entirely accurate. In the generators

without compensating windings, an increase in I A causes an increase in the

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

weakening causes a decrease in E A = Kω which further decreases the terminal

voltage of the generator. The resulting terminal characteristic is shown in Figure

(b) PREVIOUS SLIDE)



Control of Terminal Voltage

If DC Motors we control torque-speed, in DC Generator we control VT

The terminal voltage of a separately excited DC generator can be controlled by

changing the internal generated voltage E A of the machine.

VT = EA – IARA

• If E A increases, VT will increase, and if E A decreases, VT will decreases. Since

the internal generated voltage, E A = KΦω, there are two possible ways to

control the voltage of this generator:

1. Change the speed of rotation. If ω increases, then E A

= KΦω increases, so

VT = E A - I AR A increases too.

2. Change the field current. If RF is decreased, then the field current increases

(IF =VF/RF ). Therefore, the flux Φ in the machine increases. As the flux rises,

E A= K ω must rise too, so VT = E A – I AR A increases.



The Shunt DC Generator

A shunt DC generator : DC generator that supplies its own field current byhaving its field connected directly across the terminals of the machine.

Figure : The equivalent circuit of a

shunt DC generator.

F

T

F

A A AT

L F A

R

V I

R I E V

I I I

Because of generator supply it own

field current, it required voltage

buildup



Voltage Buildup in A Shunt

Generator

• Assume the DC generator has no load connected to it and that the prime mover

starts to turn the shaft of the generator. The voltage buildup in a DC generator

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

This voltage is given by res A K E

• This voltage, E A (a volt of two appears at terminal of generators), and it causes a

current IF to flow in the field coils. This field current produces a magnetomotive force

in the poles, which increases the flux in them.

• E A, then VT increase and cause further increase IF, which further

increasing the flux and so on.

• The final operating voltage is determined by intersection of the field resistance line

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



EA may be a volt ortwo appear at theterminal duringstart-up

Voltage buildupoccurred in discrete

steps



Several causes for the voltage to fail to build up during starting which are :

• Residual magnetism. If there is no residual flux in the poles, there is noInternal generated voltage, E A = 0V and the voltage will never build up.

• Critical resistance. Normally, the shunt generator builds up to a voltagedetermined by the intersection of the field resistance line and the saturationcurve. If the field resistance is greater than critical resistance, the generator fails to build up and the voltage remains at the residual level. To solve thisproblem, the field resistance is reduced to a value less than criticalresistance.

Refer Figure 9-51 page 605 (Chapman)

Critical

resistance



• The direction of rotation of the generator may have been reversed, or the

connections of the field may have been reversed. In either case, the

residual flux produces an internal generated voltage E A. The voltage E A

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

instead of adding to it.Under these conditions, the flux actually decreases below res and no

voltage can ever build up.



The Terminal Characteristic of a Shunt DC

Generator

Figure : The terminal characteristic of a shunt dc generator

• As the load on the generator is increased, IL increases and so IA = IF + IL alsoincrease. An increase in I A increases the armature resistance voltage drop I AR A,

causing VT = E A -I AR A to decrease.

• However, when VT decreases, the field current IF in the machine decreases with

it. This causes the flux in the machine to decrease; decreasing EA. Decreasing E A

causes a further decrease in the terminal voltage, VT = E A - I AR A



Voltage Control for Shunt DC Generator

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

1. Change the shaft speed, ωm of the generator.

2. Change the field resistor of the generator, thus changing the field current.

Changing the field resistor is the principal method used to control terminalvoltage in real shunt generators. If the field resistor RF is decreased, then

the

field current IF = VT/RF increases.

When IF , the machine’s flux , causing the internal generated voltage

E A. E A causes the terminal voltage of the generator to increase as well.



The Series DC Generator

Figure : The equivalent circuit of a

series dc generator

• A series DC generator is a generator whose field is connected in series with its

armature. Because the field winding has to carry the rated load current, it usuallyhave few turns of heavy wire.

Clear distinction, shunt generator tends to maintain a constant terminal voltage

while the series generator has tendency to supply a constant load current.

The Kirchhoff’s voltage law for this equation : )(S A A AT

R R I E V



Terminal Characteristic of a Series

Generator

•The magnetization curve of a series DC generator looks very much

like the magnetization curve of any other generator. At no load,

however, there is no field current, so VT

is reduced to a very small

level given by the residual flux in the machine. As the load increases,

the field current rises, so EA rises rapidly. The I A (R A + RS) drop

goes up too, but at the first the increase in EA goes up more rapidly

than the IA(RA + RS) drop rises, so VT increases. After a while, the

machine approaches saturation, and EA becomes almost

constant. At that point, the resistive drop is the predominanteffect, and VT starts to fall.

Figure : A series generator terminal

characteristic with large armature

reaction effects



The Cumulatively Compounded DC

Generator

Figure : The equivalent circuit

of a cumulatively compounded

DC generator with a long shunt

connection

A cumulatively compounded DC generator is a DC generator with both series and

shunt fields, connected so that the magnetomotive forces from the two fields are

additive.



The Cumulatively Compounded DC

Generator

The total magnetomotive force on this machine is given by

Fnet = FF + FSE - FAR

where FF = the shunt field magnetomotive force

FSE = the series field magnetomotive forceF AR = the armature reaction magnetomotive force

N F I* F = N F I F + N SE I A - F AR

F

AR A

F

SE F F

N

F I

N

N I I

*



The other voltage and current relationships for this generator are

F

T F

S A A AT

L F A

R

V I

R R I E V

I I I

)(



Another way to hook up a cumulatively compounded generator. It is the

“short-shunt” connection, where series field is outside the shunt fieldcircuit and has current IL flowing through it instead of I A.

Figure : The equivalent circuit of a cumulatively DC generator

with a short shunt connection



The Terminal Characteristic of a

Cumulatively DC Generator

When the load on the generator is increased, the load current IL also

increases.

Since I A = IF + IL, the armature current IA increases too. At this point two

effects

occur in the generator:

1. As I A increases, the I A (R A + RS) voltage drop increases as well. This

tends to cause a decrease in the terminal voltage, VT = E A –I A (R A +

RS).

2. As IA increases, the series field magnetomotive force FSE = NSEIA

increases too. This increases the total magnetomotive force Ftot =

NFIF + NSEIA which increases the flux in the generator. The increased

flux in the generator increases EA, which in turn tends to make VT =

E A – I A (R A + RS) rise.



Voltage Control of Cumulatively

Compounded DC Generator The techniques available for controlling the terminal voltage of a cumulatively

compounded DC generator are exactly the same as the technique for

controlling the

voltage of a shunt DC generator:

1. Change the speed of rotation. An increase in causes E A = K to

increase, increasing the terminal voltage VT = E A – I A (R A + RS).

2. Change the field current. A decrease in RF causes IF = VT/RF to

increase, which increase the total magnetomotive force in the generator. As Ftot increases, the flux in the machine increases, and E A = K

increases. Finally, an increase in E A raises VT.



Analysis of Cumulatively Compounded DC

Generators

The equivalent shunt field current Ieq due to the effects of

the series field and armature reaction is given by

F

AR A

F

SE eq

N I

N N I F

The total effective shunt field current is

eq F F I I I *



Field Resistance

IA (RA + RS)

VT at no load condition will be the point at which the

resistor line and magnetization curve intersect.

As load is added to the field current Ieq and theresistive voltage drop [I A(R A + RF)].

The upper tip triangle represents the internal

generated voltage E A.

The lower line represents the terminal voltage VT



The Differentially Compounded DC

Generator

)( F A A AT

F

T F

F L A

R R I E V

R

V I

I I I

A differentially compounded DC generator is a generator with both shunt

and series fields, but this time their magnetomotive forces subtract

from each other.

The equivalent circuit of a differentially

compounded DC generator



The Differentially Compounded DC

Generator

The net magnetomotive force is

Fnet = FF – FSE – FAR Fnet = NFIF – NSEIA - FAR

And the equivalent shunt field current due to the series field and armaturereaction is given by :

F

AR A

F

SE eq

N I

N

N I

F

The total effective shunt field current in this machine is

eq F F I I I *

or

F

AR A

F

SE F F

N

I

N

N I I

F

*



Voltage Control of Differentially

Compounded DC Generator

Two effects occur in the terminal characteristic of a differentially

compounded

DC generator are

1. As IA increases, the IA (RA + RS) voltage drop increases as well.

This increase tends to cause the terminal voltage to decrease VT.

2. As IA increases, the series field magnetomotive FSE = NSEIA

increases too. This increases in series field magnetomotive force

reduces the net magnetomotive force on the generator, (Ftot = NFIF

– NSEIA), which in turn reduces the net flux in the generator. Adecrease in flux decreases EA, which in turn decreases VT.

Since both effects tend to decrease VT, the voltage drop drastically as

the load is increased on the generator as shown in next slide





Voltage Control of Differentially

Compounded DC Generator

The techniques available for adjusting terminal voltage are exactly the

same as

those for shunt and cumulatively compounded DC generator:

1. Change the speed of rotation, m.

2. Change the field current, IF.



END OF CHAPTER 2

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chap 2(1)-dc generator.ppt