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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.
DC machine Construction
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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 machine Construction
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.