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Generator Efficiency

Various power stages in the case of a d.c

generator are shown below

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The three generator efficiencies

1. Mechanical Efficiency

2. Electrical Efficiency

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3.Overall or Commercial Efficiency

It is obvious that overall efficiency is the

product of mechanical and electrical

efficiencies.

For good generators, its value may be as highas 95%.

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Generator Losses

In dc generators, as in most electrical devices,

certain forces act to decrease the efficiency.

These forces, as they affect the armature, are

considered as losses and may be defined asfollows:

1. Copper loss in the winding

2. Magnetic Losses3. Mechanical Losses

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Copper loss

The power lost in the form of heatin the armature

winding of a generator is known as Copper loss.Heatis generated any time current flows in a

conductor.

I2Rloss is the Copper loss, which increases as

current increases.

The amount of heat generated is also proportional

to the resistance of the conductor.

The resistance of the conductor varies directly with

its length and inversely with its cross- sectional area.

Copper loss is minimized in armature windings by

using large diameter wire.

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Copper loss is again divided as(i) Armature copper loss

Ia2R

a = Armature copper loss.

WhereRa =resistance of armature and interpoles and series field

winding etc.

This loss is about 30 to 40% of full -load losses.

(ii) Field copper loss: It is the loss in series or shunt field ofgenerator.

Is2Rsis the field copper loss in case of series generators,

where Rsis the resistance of the series field winding.

If2Rf is the field copper loss in case of shunt generators.

This loss is about 20 to 30% of F.L losses.

(iii)The loss due to brush contact resistance. It is usually included in the

armature copper loss.

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Magnetic Losses(also known as iron or core losses)

(i) Hysteresis loss (Wh)

Hysteresis loss is a heat loss caused by the

magnetic properties of the armature.

When an armature core is in a magnetic field, the

magnetic particles of the core tend to line up with the

magnetic field.

When the armature core is rotating, its magnetic

field keepschanging direction. The continuous movement of the magnetic

particles, as they try to align themselves with the

magnetic field, produces molecular friction.

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This, in turn, producesheat.

This heat is transmittedto the armature windings.

The heat causes armature resistances to increase.

To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most

dc generator armatures.

After the steel has been formed to the proper

shape, the laminations are heated and allowed tocool.

This annealing process reduces the hysteresis

loss to a low value.

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(ii) Eddy Current Loss (We)

The core of a generator armature is made from softiron, which is a conducting materialwith desirable

magnetic characteristics.

Any conductor will have currents induced in itwhen

it is rotated in a magnetic field.

These currents that are induced in the generator

armature core are called EDDY CURRENTS.

The power dissipated in the form of heat, as aresult of the eddy currents, is considered a loss.

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Eddy currents, just like any other electrical currents, are affected

by the resistance of the material in which the currents flow.

The currents in each piece of the laminated core are considerably

less than in the solid core, because the resistance of the pieces is

much higher. (Resistance is inversely proportional to cross-sectional area.)

The currents in the individual pieces of the laminated core are so

small that the sum of the individual currents is much less than the

total of eddy currents in the solid iron core.

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Most generators use armatures with laminated

cores to reduce eddy current losses.

These magnetic losses are practically constant for shuntand compound-wound generators,because in their

case, field current is constant.

http://4.bp.blogspot.com/_v2T5FtcWTMk/RgIwZlvlLCI/AAAAAAAAA9Q/DOX84AFouEM/s1600-h/eddy.PNG

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Mechanical or Rotational Losses

These consist of

(i) friction loss at bearings and comutator.

(ii) air-friction or windage loss of rotatingarmature

These are about 10 to 20% of F.L losses.

Careful maintenance can be instrumental in

keeping bearing friction to a minimum.

Clean bearings and proper lubrication are

essential to the reduction of bearing friction.

Brush friction is reduced by assuring properbrush seating, using proper brushes, and

maintaining proper brush tension.

A smooth and clean commutator also aids in the

reduction of brush friction.

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Usually, magnetic and mechanical lossesare

collectively known as Stray Losses. These are also

known as rotational lossesfor obvious reasons.Field Cu loss is constant for shunt and compound

generators.

Hence, stray losses and shunt Cu loss are constant.

These losses are together known as standing orconstant losses. Wc.

Hence, for shunt and compound generators,

Total loss = armature copper loss + Wc

Armature Cu lossis known as variable loss, because it

varies with the load current.

Total loss = Variable loss+ constant losses, Wc

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Condition for Maximum Efficiency

In general,

lossesoutput

outputefficiencyGenerator

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The efficiency will be maximum when the denominator of Eq.(i) is minimum i.e.

Hence, the efficiency of a d.c. generator will be maximumwhen the load current is such that variable loss is equal

to the constant loss.

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