induction motor main document.doc

8/11/2019 induction motor Main Document.doc

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INTRODUCTION:

1.1 General:Induction machines are the most widely used machines in fixed-speed

applications due to reasons of cost, size, weight, reliability, ruggedness, simplicity,

efficiency and ease of manufacture. For variable speed, high-performance drives,

the dc machine is better than the induction machine, since an induction machine

requires more complex methods of control. he complexity arises because of the

variable frequency power supply! ac signals processing and complex dynamics of

the ac machine.

"lso it requires more expensive, higher-rated inverters. he

disadvantages of the induction machine are being eroded by the increasing #ower

of microprocessors and digital signal processors $%'&( with reducing cost and

size, together with improvements in inverter technology. )ence it would be

advantages to use induction machines as a basis for electrical to mechanical

power conversion.

In many applications, the dynamic behavior of induction machine has

an important effect up on the overall performance of the drive system. he

realization of this requires a suitable mathematical model of the induction machine

representation, which can be conveniently altered to simulate the &ymmetrical

induction machine in any reference frame.

"* induction motors, which contain a cage, are very popular in

variable speed drives. In many industries, we need to speed control of "*

induction motor. his drive application allows vector control of the "* induction

motor running in closed-loop with the speed+position sensor coupled to the shaft.

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1.2Vector control:

he fast torque response obtained using vector control is achieved by

estimating, measuring, calculating the magnitude and position of the motor flux in

the machine. if this flux is nown, the stator current phasor can be aligned to

maintain the field at he desired level and to produce torque as desired. "

reference a frame conversion is used to transform the thee- phase stator currents

into two orthogonal components, one to control the flux magnitude and the other

current to control the developed torque. he main difficulty lies in the

measurements or estimation of the flux position. he rotor flux position is required

to calculate the stator current vector position in the stationary reference frame thatis lined to the stator of the machine i.e. it is required to determine the orientation

of the motoring field flux vector. )ence these controls are also called field oriented

control.

here are two field orientation strategies to detect the rotor flux

position. %irect vector control method uses sensors to directly trac the flux

position. )all sensors are seldom used because of the high temperature inside the

induction machine. ypical flux in a stationary reference frame and not the rotorflux, which is used in the decoupling networ. &o, flux linage equations are

necessary to derive the rotor flux from the flux sensor measurements. he required

calculations introduce estimated machine parameters into the disturbance feed

forward path causing detuning errors.

he second category is called indirect vector control. )ere, the flux

position is derived using a calculated or estimated value of the angle between the

flux and the rotor position measurement gives the rotor fluxes position.

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2.1 INDUCTION MOTOR THEORY:

" polyphase induction motor is single excited ac machine. Its stator

winding is directly connected to a.c source , where its rotor winding receive its

energy from stator by means of induction $ i.e. transformer action ( balanced poly

phase currents in polyphase windings produce a constant /amplitude rotating

m.m.f wave both rotate in the air gap in the same direction at synchrnous speed .

these two m.m.f waves are thus stationary with respect to each other consequently

the development of steady electro magnetic torque is possible at all speeds but not

at synchronous speed. he stator and rotor mmf waves combine to give the

resultant air-gap flux density wave of constant amplitude and rotating at syncronusspeed. &ince an induction motor can't run at syncronus speed, it is called

syncronus machine.

0hen polyphase voltages are applied to the poly phase winding of

induction motor, constant amplitude rotating magnetic field is produced. he speed

of this rotating field is called the syncronus field and it is determined by number of

stator poles and applied stator frequency. he mmf produced by both stator and

rotor level in the same direction at syncronus speed. he combination of these twom.m.fs. iven rise to resultant air /gap m.m.f or flux-density wave rotating at

synchronous speed. &ince the relative speed between rotor m.m.f. and the

resultant flux density wave is zero, a steady torque is developed by their

interaction.

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2.2 Target Motor Theory:

he "* induction motor is a rotating electric machine designed to

operate from a 2-phase source of alternating voltage. For variable speed drives,the source is normally an inverter that uses power switches to produce

approximately sinusoidal voltages and currents of controllable magnitude and

frequency.

" cross-section of a two-pole in induction motor is shown in Fg!re " 2.2

lots in the inner periphery of the stator accommodate 2-phase winding a, b, c. he

turns in each winding are distributed so that a current in a stator winding produces

an approximately sinusoid ally-distributed flux density around the periphery of the

air gap. 0hen three currents that is sinusoid ally varying in time, but displaced in

phase by 1345 from each other, flow through the three symmetrically-placed

windings, a radically-directed air gap flux density is produced that is also sinusoid

ally distributed around the gap and rotates at an angular velocity equal to the

angular frequency, s, of the stator currents. he most common type of induction

motor has a squirrel cage rotor in which aluminum conductors or bars are cast into

slots in the outer periphery of the rotor. hese conductors or bars are shortedtogether at both ends of the rotor by cast aluminum end rings, which also can be

shaped to act as fans. In larger induction motors, copper or copper-alloy bars are

used to fabricate the rotor.

Fg!re " 2.2.1 2#$ha%e &C In'!cton Motor

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2.( In'!cton )otor a% a tran%*or)er:

"n induction motor is similar to a transformer in many respects .in this

connection consider an induction motor with both its star. the rotor winding isassumed open-circuited so that rotor current is zero and no electromagnetic torque

is developed. "pplication of 2-phase balanced voltages at line frequency to the

stator winding causes the production of a rotating magnetic field. his rotating flux

cuts both the stator and stationary rotor conductors at synchronous speed,

consequently emfs of line frequency f1are induced in them. he induction motor at

stand still is similar to a transformer at no load in induction machines ,

synchronously rotating air gap flux $or mutual flux ( is due to the combined actionof both stator and rotor m.m.f s.

he difference between induction motors and transformers is that the

no load current in induction motors varies from about 246 to 746 of full load

current, where as in transformers, no load current, where as in 36to 86 of full load

current .in induction motors, the magnetizing current $lagging nearly 94 obehind the

applied voltage( forms a considerable portion of no load current that is why

induction motor operate at low power factors at no loads induction motor with bothstator and rotor in star.

" "

:1 :3

;&tator


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" 3 / pole machine, the rotating field travels a distance covered by 3-

poles in 1-cycle. For a = pole machine, the rotating will travel a distance covered by

3 poles, i.e. half revolution in one cycle. For a 8-pole machine, the rotating field willtravel a distance covered by 3-poles i.e. 1+2rdrevolution in one cycle and so on. his

thought process reveals that the rotating field speed , for p-pole machine, is 1+$p+3(

revolution in 1-cycle and therefore f+$p+3( revolutions in f-cycles-in other words f+

$p+3( revolutions in one second, because f / cycles are completed in one second .

)ere f >frequency of the 2-phase currents. If ns denotes the rotating field speed in

rps.

ns>f+$p+3(>3f+p

?s>134f+prpm

2.+ Three ,ha%e %!,,ly:

It will now be shown that when three phase winding displaced in

space by 1344

are fed by three phase currents, displaced in time by 1344

, theyproduce a resultant magnetic flux, which rotates space as if actual magnetic poles

were being rotated mechanically.

he principle of a three phase, two poles stator having three identical

winding placed 134 space degrees apart as shown in fig. the flux $assume

sinusoidal( due to three phase windings is shown in fig.

he assumed positive directions of the fluxes are shown in fig. let themaximum value of flux due to any one of the three phases be @ m. he resultant

flux @r at any instant, is given by the vector sum of the individual fluxes @ 1, @3, and

@2due to three phases. 0e will consider the values of @rat four instants of 1+8th

time period apart corresponding two points mar 4, 1, 3 and 2 in fig3.=.1.

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2-phase power supplyA

Fg 2. +.1

$1( 0henB>44i.e corresponding in fig.

)ere @1 > 4, @3 > -C2 @m,@2 >C2 @mthe vector for @3in fig is

3 3drawn in direction opposite to the direction assumed positive in fig3.=.1.

@r > 3DC2 @m cos$84+3( > C2DC2 @m> 2 @m 3 3 3

7

F2 S

F2

S S2

S22

S3

S1


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(#,ha%e *l!-e% co),onent%:

Phase1 Phase2 Phase3

m

0 1 2 3 4

Fg 2.+.2

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(#,ha%e oltage ,ha%or 'agra):

1344

Fg2.+.(

$3(0hen B>844i.e corresponding to point 1 in fig

)ere @1 >C2 @m

3

@3 > -C2@m 3

@2 > 4

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@r > 3DC2 @mD cos244> 2 @m3 3

It is found that the resultant flux is again 2+3@mbut as rotating cloc

wise through an angle of 844.

$2( 0hen B>1344 i.e corresponding to point 3 in fig3.=.3.

)ere @1 >C2 @m 3 @3 >4

@2 >-C2 @m 3

It can be again proved that@r > 2 @m3

&o the resultant is again of the same values, but has further rotated

cloc wise through an angle of 844 .

$=( 0hen B>1E44i.e corresponding to point 2 in fig

)ere@1 >4,

@3 >C2 @m 3

@2 >-C2@m

3

he resultant 2+3 @m and as rotated cloc wise through an additional angle

844through an angle 1E44 from the start.

)ence we conclude that

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$1( the resultant flux is of constant value 2+3 @mi.e 1.7 times the maximum

value of the flux due to any phase

$3( the resultant flux rotates around the stator at synchronous speed given by

?s > 134f #

2./ 0hy 'oe% the rotor rotate

0hen three phase stator windings, are fed by three phase supply then,

as &een from above a magnetic flux of constant magnitude rotating at syncronus

&peed is set up. In the flux passes through the air gap, sweeps past the rotor

surface and so cuts to the rotor conductors which, as yet are stationary. %ue to the

relative speed between the rotating flux and the stationary conductors, and emf is

induced in the later according to faradays laws of electro magnetic induction, the

frequency of the induced emf is as same as the supply frequency. Its magnitude is

proportional to the relative velocity between flux and the conductors and directionis given by Flemings right hand rule sine the rotor bars or conductors forms a

closed circuit, the rotor current is produced whose direction, as given by lenzs law

is such as to oppose the very cause producing It. In this case, the cause which

produces the rotor current is the relative velocity the rotating flux of the stator and

the stationary rotor conductors. )ence to reduce the relative speed, the rotor starts

running in the same direction as that of the flux and tries to catch up with the

rotating flux.

he setting up of the torque for rotating is explained belowA he stator

field which is assumed to be rotating clocwise. otion of he rotor with respect to

the stator is anticlocwise. ;y applying right hand rule. he direction of the of

induced emf in the rotor is found to be out wards. )ence the direction of flux due to

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rotor current alone is as shown in fig. now, by applying the left hand rule, or by the

effect of combined field it is clear that the rotor conductors experience a force

tending to rotate them in clocwise direction. )ence the rotor is set into rotation in

the same direction as that of the stator flux. %tator

rotor

Fg 2./.1

2. 3l,:

In practice, the rotor never succeeds in catching up with the stator

field .if it really did so, then there would be relative speed between the two, hence

no rotor emf, no current, and so torque to maintain rotation. hat is why the runs at

a speed which always is less than the speed of the stator fielded. he difference in

speeds depends upon the load on the motor.

he difference between the synchronous speed ?sand the actually

speed ? of the rotor is nown as slip. hrough it may be expressed in so many

revolutions +second, yet it is usual to express it as a percentage of the

synchronous speed. "ctually the term slip is descriptive of the way in which the

Grotor slip bac From synchronism.

&ometimes, ?s-? is called the slip speed.

Hbviously, rotor $or motor( speed is?>?s $1-s(.

It may be ept in mind that revolving flux is rotating synchronously, relative o

stator but at slip speed relative to the rotor.

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2.4 ROT&R FRE5UENCY:

It has been show that the rotor running in the direction of rotatingmagnetic field. "t stand still, rotor conduction are being cut by rotating flux wave at

synchrnous speed ns, there fore frequency f3of the rotor emf and current is equal

to the line frequency f1 . when rotor revolves at a speed of rps in direction of

rotating flux wave , the relative speed between synchrnous by rotating flux wave,

the relative speed between synchronous by- rotating flux and rotor conduction

becomes $ns/ nr (rps

here fore frequency of rotor emf > poles+3

> p $ns/ nr ( +3

;ut s > ns/ nr+ ns

here fore rotor frequency, f3 > ps ns+3 > sf1

2.6 Fre7!ency o* rotor c!rrentA

0hen the rotor is stationary, the frequency of rotor current is the sameas the supply frequency but when rotor starts revolving, and then the frequency

depends upon the relative speed or on slip speed. et at any slip speed, the

frequency of the rotor current is f. then

?s-?>134f' also ?s>134f # #

%ividing one by other, we get,

f'>?s-?>& f ?s

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"s seen, rotor currents have a frequency of f>sf and when flowing

through the individual phases of rotor winding, give raise to rotor magnetic fields.

hese individuals rotor magnetic fields produces a combined rotating magnetic

field, whose speed relative to rotor is

>134f'>134sf>s?s # #

)owever, the rotor itself is running at speed ? with respect to space.

)ence, &peed of rotor field in space >speed of rotor magnetic field relative to rotor

&peed of rotor relative to space

>s?s J ? > s?s J ?s$1-s( > ?s

It means that no matter what the value of the slip, rotor currents and

stator currents each produce a sinusoid ally distributed magnetic field of constant

magnitude and constant space speed of ?s. In other words, both the rotor and

stator field rotate synchronously, which means that the are stationary with respect

to each other. hese two synchronously rotating magnetic fields, in fact

superimpose on each and give rise to the actually existing rotating field, whichcorresponding to the magnetizing current of the stator winding.

2.8Relaton 9eteen tor7!e an' %l,;

" family of torque+slip curves is shown in fig. for range of s>4 to s>1

with 4 >4, hence the

curve starts from point 4. "t normal speed, close to synchronism $sx3( is and

hence negligible w.r.t


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increase in motor load, then Ls :3 4, hence the curve starts from point 4.

"t normal speed the term $sK3( is small and hence negligiblew.r.t


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Fg!re#2.8.1 &C In'!cton Motor 3,ee'#Tor7!e Character%tc

&quirrel-cage "* induction motors are popular for their simple

construction, low cost per horsepower, and low maintenance $they contain no

brushes, as do %* motors(. hey are available in a wide range of power ratings.

0ith field-oriented vector control methods, "* induction motors can fully replace

standard %* motors, even in high-performance applications.

2.1< Voltage E7!aton% n Machne Vara9le%:

"n induction machine consists of two essential partsA stator and rotor

windings. " typical 2 - ac machine has a symmetrical three phase winding in the

stator and it can be well assumed that its rotor has a symmetrical three phase

windings as shown in Fig3.2.1.

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For the formulation of machine equations, the following assumptions are

made

1. he stator and rotor windings are of balanced three phase windings3. he air gap flux distribution is radial and sinusoidal

2. he machine is of cylindrical rotor construction and is not saturated

et stator windings has ?s equivalent turns and resistance rs, and rotor

windings has ?requivalent turns and resistance rr.

2.11 E**ect o* change% n %!,,ly *re7!ency on tor7!e an' %,ee':

)ardly any important changes in frequency tae place on a largedistribution &ystem except during a maNor disturbance. )owever, large frequency

change taes place on isolated low power system in which electric energy is

generated by means of diesel engines are gas turbines. :xamples of such system

are! emergency supply in a hospital and electrical system on a ship etc.

he maNor effect of change in supply frequency is on motor speed if

frequency drops by 146, then motor speed also drops by 146. achine tools and

other motor driven-equipment meant for 74 )z cause problem when connected to

84 )z supply. :verything runs $84-74(D144+74>346 faster then normal and this

may not be acceptable in all applications. In fact case, we have to use either gears

to reduce motor speed or expensive 74)z source.

" 74)z motor operate well on a 84)z lion provide its terminal voltage is

raised to 84+74>8+79$i.e. 1346( of the name plate rating. In that case, the new

breadown torque becomes equal to the original breadown torque and starting

torque is only slightly reduced. )owever, power factor, efficiency and temperature

rise remain satisfactory.

&imilarly, a 84)z motor can operate satisfactorily well on 74)z supply

#rovided its terminal voltage is reduced to 7+8$I.e. E46( of its name plate rating.

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2.12 Tor7!e =%,ee' c!re:

he torque developed by a conventional 2-phase motor depends on its

speed two cannot be represented by a simple equation. It is easier to show the

relationship in form of a curve. In this diagram, represents the nominal full oad

torque of the motor. "s seen, the starting torque is 1.7 and the maximum orque

is 3.7

"t full load, the motor runs at a speed of ?. when mechanical load

increases, motor speed till the motor torque again becomes equal to the load

torque. "s long as the two torques are balanced, the motor will run at constant

speed. )owever, if the load torque exceeds 3.7, the motor will suddenly stop.

2.1( C!rrent=%,ee' c!re o* an n'!cton )otor:

It is a v-shaped curve having a maximum value at synchronous speed.

his maximum is equals to the magnetizing current which is need to create flux In

the machine. &ince flux is purposely ept constant, it means that magnetizing

*urrent is the same at all synchronous speeds. &hows the current+speed curve of

induction motor discussed in art. "s, seen loced rotor current is144" and the

corresponding torque is O7 ?-m. If stator voltage and frequency are varied in the

same proportion current+speed curve has the same shape, but shift along the

speed axis. &uppose that voltage and frequency reduced to one fourth of their

previous values to 114v to 17)z respectively. hen loced rotor current decreases

to O7 a but corresponding torque increases to 174 n-m which is equal to full

breadown torque. It means that reducing frequency! we can obtain a larger torque

with a reduced current. his is one of the big advantages of frequency control

method. ;y progressively increasing the voltage and current during the start-up

period, a &*I can be made to develop close to its breadown torque all way from

zero to rated speed.

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"nother advantage of frequency control is that it permits regenerative

braing of the motor. In fact, the main reason for the popularity of frequency-

controlled induction motor drives is their ability to develop high torque from zero tofull speed together with the economy of regenerative braing.

*urrent

174

144

O7

4 =74 944 1274 1E44

&peed

Fg2.1(.1 C!rrent " %,ee' c!re%.

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&tator current orque

174 17 84

144

I84

O7

I17

4 =74 944 1274 1E44 3374

&peed

Fg 2.1(.2 3tator c!rrent=to7!e an' %,ee' c!re%

2.1+ Mathe)atcal De%cr,ton o* &C In'!cton Motor%:

here are a number of "* induction motor models. he model used for

vector control design can be obtained by using the space vector theory. he 2-

phase motor quantities $such as voltages, currents, magnetic flux, etc.( are

expressed in terms of complex space vectors. &uch a model is valid for any

instantaneous variation of voltage and current and adequately describes the

performance of the machine under both steady-state and transient operation.

*omplex space vectors can be described using only two orthogonal axes. he

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motor can be considered a 3-phase machine. he utilization of the 3-phase motor

model reduces the number of equations and simplifies the control design.

2.1/ O9>ecte%:

he obNective of this proNect is to become familiar with most aspects of

a vector controlled induction motor in a simulation environment. "fter completing

the proNect, you should be able toA

Identify the equivalent parameters of an induction machine.

"dapt the machine model to different reference systems $ransformation

between two and three phase systems! transformation between stator

reference frame and synchronous reference frame(.

Implement current and speed regulation loops and calculate #I-controllers.

Implement position estimation $sensor less control( and analyze itslimitations.

Implement a #0 inverter.

Implement the &P technique.

&imulate the blocs.

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VECTOR CONTRO?:

(.1 Re*erence Fra)e Theory:


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by simple assigning the appropriate speed of rotation to this so-called arbitrary

reference frame. his is explained below.

(.23,ace Vector De*nton:

"ssume that isa, isb, and isc are the instantaneous balanced 2-phase

stator currentsA

isaJisbJisc> 4 2.31

he stator current space vector can then be defined as followsA

is> $ isaJaisbJa3isc( 2.3.3

0hereA

a and a2 > he spatial operators, a = e j2/3 , a3> ej4/3

k > the transformation constant and is chosen k=2/3

YPhase B

IsIsb isy

IsaIsx X, phase A

IscPhase C

Fg (.2.1 (#,ha%e to 2#,ha%e coner%on.

(.( 3tator C!rrent 3,ace Vector an' It% $ro>ecton:

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he space vector defined by E5. 2can be expressed utilizing the two-

axis theory. he real part of the space vector is equal to the instantaneous value of

the direct-axis stator current component, isx, and whose imaginary part is equal to

the quadrature-axis stator current component, isy. hus, the stator current spacevector in the stationary reference frame attached to the stator can be expressed

asA

I% @ %-A>%y 2.2.1

In symmetrical 2-phase machines, the direct and quadrature axis stator

currents isx, isy fictitious quadrature-phase $3-phase( current components, which

are related to the actual 2-phase stator currents as followsR

isx> $ isa-1 isb-1 isc( 2.2.33 3

isy> 2 $isb- isc( 2.2.23

0hereA

k=2/3 is a transformation constant

he space vectors of other motor quantities $voltages, currents, magnetic fluxes,

etc.( can be defined in the same way as the stator current space vector.

(.+ &C In'!cton Motor Mo'el:

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he "* induction motor model is given by the space vector form of the

voltage equations. he system model defined in the stationary K, S-coordinate

system attached to the stator is expressed by the following equations. Ideally, the

motor model is symmetrical, with a linear magnetic circuit characteristic.

a. he stator voltage differential equationsA

Psx> 4> 4> sisxJmirx 2.=.7

Vsy> sisyJmiry 2.=.8

Vrx> rirxJmisx 2.=.O

Vry> riryJmisy 3.4.8

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d. :lectromagnetic torque expressed by utilizing space vector quantitiesA

te=3Pp(Tsxisy-TsSisx) .3.4.9 2

0hereA

K, y> &tator orthogonal coordinate system

Ps> &tator voltages WPX,y,x

Isx,y=&tator currents W"X

Prx, y>


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general reference frame, then the following equation defines the stator current

space vector in general reference frame.

isg> ise/ Ng> isxJNisy 2.=.14

SS

Ug is,isg x

isS Bg K

isK

Fg!re(.+.1 &,,lcaton o* the General Re*erence Fra)e

he stator voltage and flux-linage space vectors can be similarly

obtained in the general reference frame. &imilar considerations hold for the space

vectors of the rotor voltages, currents and flux linages. he real axis $r x( of the

reference frame attached to the rotor is displaced from the direct axis of the stator

reference frame by the rotor angle, r."s shown, the angle between the real axis

$x( of the general reference is frame and the real axis of the reference frame

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rotating with the rotor $r x( g -r. In the general reference frame, the space vector

of the rotor currents can be expressed asA

irg > ire/N$g-r(> irxJNiry 2.=.11

0hereA

ir > he space vector of the rotor current in the rotor reference frame

he space vectors of the rotor voltages and rotor flux linages in the

general reference frame can be expressed similarly.

he motor model voltage equations in the general reference frame can

be expressed by using the transformations of the motor quantities from one

reference frame to the general reference frame introduced. he "* induction motor

model is often used in vector control algorithms. he aim of vector control is to

implement control schemes which produce high-dynamic performance and are

similar to those used to control %* machines. o achieve this, the reference

frames may be aligned with the stator flux-linage space vector, the rotor flux-linage space vector or the magnetizing space vector. he most popular reference

frame is the reference frame attached to the rotor flux linage space vector with

direct axis $d( and quadrature axis $q(. "fter transformation into d-q coordinates the

motor model followsA

Psd > 4=


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Psq > 4=


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Fg!re #(./.1. (#$ha%e Inerter

he inverter consists of three half-bridge units where the upper and lower switchare controlled complimentarily, meaning when the upper one is turned on, the lower

one must be turned off, and vice versa. "s the power device's turn-off time is longer

than its turn-on time, some dead time must be inserted between the time one

transistor of the half-bridge is turned off and its complementary device is turned on.

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he output voltage is mostly created by a #ulse 0idth odulation $#0( technique,

where an isosceles triangle carrier wave is compared with a fundamental-frequency

sine modulating wave. he natural points of intersection determine the switching

points of the power devices of a half-bridge inverter. his technique is shown in

*g(./.1he 2 -phase voltage waves are shifted 1344 to one another and thus a 2-

phase motor can be supplied.

(. INTERN&? CONTRO? OF INVERTER:

Hutput voltage from an inverter can also be adNusted by exercising a control

with in the inverter itself. he most efficient method of doing this is by pulse widthmodulation control used with in an inverter.

(.4$U?3E 0IDTH MODU?&TION COTRO?:

In this method, a fixed dc input voltage is given to inverter and a controlled ac

output Poltage is obtained by adNusting the on and off periods of the inverter

components. his is most popular method of controlling the output voltage and this

method is termed as pulse width modulation $#0( control.

(.6 The a'antage% ,o%%e%%e' 9y $0M techn7!e are a% !n'erA

he output voltage control with method can be obtained with out any

additional components.

0ith this method, lower order harmonic can be eliminated or minimized

along with its output voltage control. "s higher order harmonics can be

filtered easily, the filtering requirements are minimized.

he main disadvantages of this method are that the &*


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(.8 $U?3E 0IDTH MODU?&TED INVERTER3:

#0 inverters are gradually taing over other types of inverters in

industrial applications. #0 techniques are characterized by constant amplitude

pulses. he width of these pulses is, however modulated to obtained inverter

output voltage *ontrol and to reduce its harmonic content.

%ifferent #0 techniques are as underA

$a(&ingle-pulse odulation

$b(ultiple-pulse odulation

$c(&inusoidal-pulse odulation

In #0 inverters, forced commutation is essential .he three #0

techniques isted above differ from each other in harmonic content in their

respective output voltages. hus, choice of a particular pwm technique depends

upon the permissible harmonic content in the inverter output voltage In industrialapplication, #0 inverter is supplied from a diode bridge rectifier and "n *

filter .?ow the devices are switched on and off several times with each half

*ycle to control the output voltage which has low harmonic content.

(.1


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(.11 MU?TI$U?E#$U?3E MODU?&TION:

his method of pulse modulation is an extinction of single-pulse modulation.In multiple /pulse $#(, several equidistant pulses per half cycle are wised. For

simplicity, the effect of using two symmetrically spaced pulses per half cycle, pulse

width is taen half of that, but their amplitudes are the same.

(.12 3IN3OID&?#$U?3E MODU?&TION:

In this method of modulation, several pulses per half cycle are used as in

the case of multiple /pulse modulation $#(. he pulse width is equal for all thepulses, but is sin ! the pulse width is a sinusoidal function of the angular position

of the pulse in a cycle.

For realizing sin m, high-frequency triangular carrier wave vcis compared

with a sinusoidal reference wave vrwaves determines the switching instants and

commutation of the modulated pulse . vcis the pea value of triangular carrier

wave and vrthat of the reference, or modulating, signal. he carrier and reference

waves are mixed in a comparator. 0hen sinusoidal wave has magnitude higher

than the triangular wave, the comparator output high, otherwise it is low. he

comparator output is processed in a trigger pulse generator in such a manner that

the output voltage wave of the inverter has a pulse width tn agreement width in

agreement with the comparator output pulse.

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Fg!re (.12.1. $!l%e 0'th Mo'!laton

he most popular power devices for motor control applications are #ower

osfet and I;s.

" #ower H&F: is a voltage-controlled transistor. It is designed for high-

frequency operation and has a low-voltage drop, so it has low power losses.

)owever, saturation temperature sensitivity limits the H&F:'s use in high-power

applications.

"n Insulated-ate ;ipolar ransistor $I;( is controlled by a H&F:

on its base. he I; requires low drive current, has fast switching time, and is

suitable for high switching frequencies. he disadvantage is the higher voltage

drop of the bipolar transistor, causing higher conduction losses.

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DE3IGN CONCE$T OF &CIM VECTOR CONTRO?:

+.1 3y%te) o!tlne:

he system is designed to drive a 2-phase ac induction motor $"*I(. he

application has the following specificationsA

Pector control technique used for"*Icontrol

&peed control loop of the"*I


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+.2 &c n'!cton )otor %,ec*caton%:

#>144 )p

Pline>=34P

Frequency>74 )z

#oles =

?s>1744rpm

m>4.4188=) r>4.41O)

4.4297Oohm 4.43317 ohm

+.( Vector Control o* &C In'!cton Machne%:

Pector control is the most popular control technique of "* induction motors.

In special reference frames, the expression for the electromagnetic torque of the

smooth-air-gap machine is similar to the expression for the torque of the separately

excited %* machine. In the case of induction machines, the control is usually

performed in the reference frame $d-q( attached to the rotor flux space vector.

hat's why the implementation of vector control requires information on the

modulus and the space angle $position( of the rotor flux space vector. he stator

currents of the induction machine are separated into flux- and torque-producing

components by utilizing transformation to the d-q coordinate system, whose direct

axis $d( is aligned with the rotor flux space vector. hat means that the q-axis

component of the rotor flux space vector is always zeroA

Vrq> 4 and d Trq>4 =.2.1 dt

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he rotor flux space vector calculation and transformation to the d-q coordinate

system require the high computational power of a microcontroller! a digital signal

processor is suitable for this tas. he following sections describe the space vector

transformations and the rotor flux space vector calculation.

induction motor main document.doc

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