machines enercon 2.pdf

8/14/2019 machines enercon 2.pdf

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Electric Motors andGenerators


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DIRECT CURRENT (DC) MACHINES Fundamentals

Generator action:An emf(voltage) is induced in a

conductor if it moves through a magnetic field.

Motor action:A force is induced in a conductor thathas a current going through it and placed in a

magnetic field

Any DC machine can act either as a generator or as

a motor.


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For emf to be induced, the conductors must cut the

flux lines as they move.

Otherwise, ( v!B)= 0.

eloop= eab+ ebc+ ecd+ edaeloop = Blv+ 0 + Blv+ 0

eloop = 2 B l v


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Introduction

In this lecture we consider various forms of rotating

electrical machines

These can be divided into:

generators which convert mechanical energy into

electrical energy

motors which convert electrical energy into

mechanical energy Both types operate through the interaction between a

magnetic field and a set of windings


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A Simple AC Generator

We noted earlier that Faradays law dictates that if a

coil of Nturns experiences a change in magnetic flux,

then the induced voltage Vis given by

If a coil of areaA rotates with respect to a field B, and

if at a particular time it is at an angle ! to the field,

then the flux linking the coil is BAcos!, and the rate ofchange of flux is given by

t

!NVd

d=

!"!!!

coscosd

d

d

sind===

ttBA

dt

d!


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Thus for the arrangement shown below

t

!NV

d

d=

!"!

cosd

sind

d

dNBA

tNBA

t

!NV ===


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Therefore this arrangement produces a sinusoidaloutput as shown below


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Wires connected tothe rotating coil

would get twisted

Therefore we use

circular slip rings

with sliding

contacts called

brushes


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A Simple DC Generator

The alternating signal from the earlier AC generator

couldbe converted to DC using a rectifier

A more efficient approach is to replace the two slip

rings with a single split slip ring called a commutator

this is arranged so that connections to the coil are

reversed as the voltage from the coil changes polarity

hence the voltage across the brushes is of a singlepolarity

adding additional coils produces a more constant output

23.3


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Use of a commutator


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A simple generator with two coils


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The ripple can be further reduced by the use of acylindrical iron core and by shaping the pole pieces

this produces anapproximately

uniform field in thenarrow air gap

the arrangementof coils and core

is known as thearmature


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DC Generators or Dynamos

Practical DC generatorsor dynamoscan take a

number of forms depending on how the magnetic field

is produced

can use a permanent magnet more often it is generated electrically using field coils

current in the field coilscan come from an external supply

this is known as a separately excited generator

but usually the field coils are driven from the generator output this is called a self-excited generator

often use multiple poles held in place by a steel tubecalled the stator

23.4


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A four-pole DC generator


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DC machine Construction

Stator:Stationary part of themachine. The stator carries afield winding that is used toproduce the required magnetic

field by DC excitation. Oftenknow as the field.

Rotor:The rotor is the rotatingpart of the machine. The rotorcarries a distributed winding,and is the winding where theemfis induced. Also known asthe armature.


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The picture shows the stator of a large DC machinewith several poles.

The iron core is supported by a cast iron frame.



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The rotor iron core is mountedon the shaft.

Coils are placed in the slots.

The end of the coils are bentand tied together to assuremechanical strength.

Note the commutator mounted

on the shaft. It consists ofseveral copper segments,separated by insulation.


DC Rotor construction


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Generated EMF in a DC Generator

WhereZ= total number of conductors,

P = total number of poles

a= P for lap winding, a = 2 for wave winding, "= flux,

#= speed in rad/s

n = speed in rpm.


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Example


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Example


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Types of Generators

Field coil excitation

sometimes the field coils are connected in serieswiththe armature, sometimes in parallel (shunt) and

sometimes a combination of the two (compound) these different formsproduce slightlydifferentcharacteristics

diagram hereshows ashunt-woundgenerator


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DC generator characteristics

vary slightly between forms

examples shown here are for a shunt-wound generator


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DC GENERATOR TYPES according to EXCITATION

There are four basic kinds of dc generators,namely separately excited, shunt, series andcompound generators

DC Separatel E cited Generators


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DC Separately Excited Generators

The required fieldcurrent is a very smallfraction of the rated

armature current ofthe order of 1 to 3percent in the averagegenerator.

A small amount ofpower in the fieldcircuit may control a

relatively large amountof power in thearmature circuit, i.e.,the generator is apower amplifier.

Separately excitedgenerators are oftenused in feedbackcontrol systems whencontrol of the armaturevoltage over a widerange is required.


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DC Shunt Generators

The field is a function of the terminal voltage, Vt. Hence, voltage decrease at the armature

terminals cause a corresponding decrease in the field flux and hence in the induced emf,E

gin return a decrease in V

tagain. This continues until an equilibrium is found.

The reverse of the above process is followed in building up Vt.


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DC Shunt Generators (Cont.)

For a field resistance rf, the operating point is the intersection point of the rfline and magnetization curve.

DC Sh t G t (C t )


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DC Shunt Generators (Cont.)

Its output voltage sharply decreases with the armature reaction as it isecpected. The effect of the armature reaction is more severe for this

kind of generator.

t g L f aV = e - (I + I ) r

DC Series Generators


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DC Series Generators

The field current of a series generator is the same as the load (orarmature) current, so that the air gap flux and hence the voltagevary widely with load.

As a consequence, series generators are not very often used.

DC S i G t (C t )


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DC Series Generators (Cont.) Example:An 8-pole, lap wound, dc machine has 60 armature conductors is

series between the brushes. The armature resistance, ra=0.04ohm and the

field resistance rs=0.02ohm. When connected as a series generator and run

at 1000rpm, it delivers a certain amount of current to a load at a terminalvoltage, Vt=126V. If it is run at 700rpm, the terminal voltage decreases to 86V

for the same load current. Calculate:

a) the direct-axis flux/pole

b) the power delivered to the load under consideration at a shaft speed of

1200rpm.

Soln: conductors and a=p=8 for this machine. SoZ

60a

a

= a

240K =

!

t g a a sV = e - I (r + r )

1g a126 = e - I (0.04 + 0.02)2g a

86 = e - I (0.04 + 0.02) 1 2g ge - e = 40 V!

1g a d 1e = K w!

2g a d 2

e = K w! 1 2g g a d 1 2

e - e = 40 = K (w - w )! "

DC Series Generators (Cont )


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DC Series Generators (Cont.)

3g

240 1 1200 960

e = 2 = = 160 V60 60 6!

!

d

40 10.0167 Web

240 2 60(1000 700) 60

!

!

" = = =

#

1g

240 1 1000 800e = 2 = = 133.3 V

60 60 6!

!

a

133.3 -126 7.33I = = = 122.2 A

0.06 0.06

tV = 160 -122.2 x 0.6 = 160 - 7.33 = 152.7 V

outP = 152.7 x122.2 = 18.5 kW

DC Compound Generators


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In most of theusualapplications of

the dcgenerators thedesired point isto have thesame voltage at

both no-loadand full-load.

A suitablecombination ofshunt andseries

generatorsgives thecompoundgenerator.

DC Compound Generators

DC C d G t (C t )


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DC Compound Generators (Cont.)

The required number of turns in the series field winding is relativelysmall since the contribution of this winding is just to compensate the

small terminal voltage drop with the loading. So this kind of compoundgenerator is called the cumulatively compound generator.

DC Compound Generators (Cont )


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DC Compound Generators (Cont.)

The series field winding can be purposely connected such that itsflux does not add to that of the shunt field, then the resultantairgap flux is the difference of the series field flux from the mainshunt field flux.


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DC Motors

When current flows in a conductor it produces a

magnetic field about it - as shown in (a) below

when the current-carrying conductor is within an

externally generated magnetic field, the fields interactand a force is exerted on the conductor - as in (b)

23.6


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Therefore if a conductor lies within a magnetic field:

motionof the conductor produces an electric current

an electric currentin the conductor will generate motion

The reciprocal nature of this relationship means that,for example, the DC generator above will function as a

DC motor

although machines designed as motors are more

efficient in this role

Thus the four-pole DC generator shown earlier could

equally well be a four-pole DC motor


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DC motor characteristics

many forms each with slightly different characteristics

again can be permanent magnet, or series-wound,

shunt-woundor compound wound figure below shows a shunt-wound DC motor


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Electrical Machines A Summary

Power generation is dominated by AC machines

range from automotive alternators to the synchronous

generators used in power stations

efficiency increases with size (up to 98%)

Both DC and AC motors are used

high-power motors are usually AC, three-phase

domestic applications often use single-phase inductionmotors

DC motors are useful in control applications

23.9


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AC Generators or Alternators

Alternators do not require commutation

this allows a simpler construction

the field coilsare made to rotate while the armature

windings are stationary

Note: the armature windings are those that produce the output

thus the large heavy armature windingsare in the

stator

the lighter field coilsare mounted on the rotorand

direct current is fed to these by a set of slip rings


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A four-pole alternator


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As with generators multiple poles and sets of windingsare used to improve efficiency

sometimes three sets of armature windingsare spaced 120 apart around the stator to form

a three-phase generator The e.m.f. produced is in sync with rotation of the

rotor so this is a synchronous generator

if the generator has a single set of poles the outputfrequency is equal to the rotation frequency

if additional pole-pairs are used the frequency isincreased accordingly


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Example see Example 23.2from course textA four-pole alternator is required to operate at 60 Hz.What is the required rotation speed?

A four-pole alternator has two pole pairs. Thereforethe output frequency is twice the rotation speed.Therefore to operate at 60Hz, the required speedmust be 60/2 = 30Hz. This is equivalent to 30 "60 =

1800 rpm.

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