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CHAPTER 3 PERFORMANCES OF DC GENERATOR

1. OPEN-CIRCUIT (NO-LOAD) CHARACTERISTICS

Conditions:

0I

constn

No-load characteristic : )(0 fIfU

ff FI

U0)(

0 fIfU )( fFf

No-load characteristic is essentially the magnetizing curve

Function --- checking rated operating point --- knee point

No-load curve primarily depends on the magnetic system

and less relates to the excitation modes.

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2. EXTERNAL (TERMINAL) CHARACTERISTIC

Conditions:

),( constRgeneratorshuntForconstIconstn

ff

External characteristic : )(IfU

1). Separately Excited Generator

UEU

IIR

a

reactionarmaturesaturation

IREU

aL

aaa

).2().1(

Since Ra and the effect of AR are all small, is an

approximately constant curve.

)(IfU

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2). Shunt Generator

UEIUWhen

UE

U

IIR

af

I

a

reactionarmaturesaturation

IREU

aL

fRUf

aaa

).3(

).2(

).1(

The third reason is a primary reason to make U changed . It

could cause the terminal voltage U to decrease greater than

separately excited generator.

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UEII

UE

U

IIR

asa

a

reactionarmaturesaturation

IREU

aL

aaa

).3(

).2(

).1(

The third reason is also primary. It could cause the terminal

voltage U to increase. So the terminal characteristic of series

generator is a rising curve.

3). Series Generator

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4). Compound Generator

There are two field windings, shunt winding and series

winding, mounded on the stator.

Shunt winding plays main role and series winding

provides compensating mmf.

Cumulative compound generator --- shunt mmf and series

mmf have same direction and aid each other

0

0

0

UUundingundercompo

UUgcompoundinflat

UUndingovercompou

N

N

N

Differential compound generator --- shunt mmf and series

mmf have reverse direction and oppose each other

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3. VOLTAGE BUILDUP IN SELF-EXCITED DC

GENERATORS`++

Example: shunt dc generator

Three conditions for buildup:

(1). Residual magnetism exists in stator core.

(2). The mmf from shunt current aids the residual magnetism.

(3). Shunt circuit resistance is less than critical resistance.

Process of buildup

Two curves: magnetization curve and field resistance line

0)3(

)2(

1

)1(

).3

).2

).1

Ustable

UIcurrentshuntinitialU

Uvoltageresidualfluxresidual

condition

condition

fr

rr

condition