electrical machinery project

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IntroductionSingle-phase induction motors are the most familiar of all electric motors because they are

used in home appliances, businesses, and small industries. In general, they are employed

when three-phase power is not available. Single-phase induction motors are usually two-

pole or four-pole, rated at 2 hp or less, while slower and larger motor can be manufactured

for special purposes. They are widely used in domestic appliances and for a very large

number of low power drives in industry. The single phase induction motor resembles, three-

phase, squirrel-cage motor except that, at full speed, only a single winding in the stator is

excited.

In a single-phase motor we have only a single field winding excited with alternating current

therefore, it does not have a revolving field li!e three-phase motors. Thus, it does not self-

starting. Several methods have been devised to initiate rotation of the squirrel-cage rotor

and the particular method employed to start the motor will designate the specific type.


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Single Phase Induction Motor

The current in each winding produces a magnetic feld that is alsoincreasing in the upward direction. Since two rotor conductors thatare 180" electrical apart orm a closed loop, the rotor conductors canthen be paired as shown. Examine one o the closed loops, sa theloop ormed b conductors ! and !. The #ux passing through thisloop induces an em and thereb a current in this loop. The directiono the current in the loop is such that itproduces a magnetic fux which tends to oppose the increase in the

magnetic fuxset up in the windings. $or this to happen, the current must #ow outo conductor! and into conductor !, as shown b the dot and the cross,respecti%el. In thesame wa, we can determine the currents in the other conductors.Each currentcarringconductor must experience a orce in accordance with the &orent'orce e(uation. The direction o the orce acting on each conductor is

also indicated in $igure .The orces experienced b conductors 1, !,


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3, and 4 in unison withconductors l, !, 3', and 4' tend to rotate therotor in the countercloc)wise direction.*owe%er, the rotation is opposed b the orces acting on theremaining conductors.

The smmetric placement o the rotor conductors ensures that themotor conductors l, !, +, and tend to rotate the rotor in thecountercloc)wise direction.*owe%er, the rotation is opposed b the orces acting on theremaining conductors.

The smmetric placement o the rotor conductors ensures that themotorde%elops e(ual tor(ue in both directions and the net tor(uede%eloped b it is

'ero. *ence the rotor remains in its standstill position.-s mentioned earlier, i the rotor is made to rotate in an directionwhile thesinglephase winding is excited, the motor de%elops tor(ue in thatdirection.de%elops e(ual tor(ue in both directions and the net tor(uede%eloped b it is 'ero. *ence the rotor remains in its standstillposition.-s mentioned earlier, i the rotor is made to rotate in an directionwhile the singlephase winding is excited, the motor de%elops tor(ue

in that direction.

Double Revolving-Field Theory

-ccording to this theor, a magnetic feld that pulsates in time but is

stationarin space can be resol%ed into two re%ol%ing magnetic felds that aree(ual inmagnitude but re%ol%e in opposite directions. &et us consider thestandstill conditiono the rotor again. The magnetic feld produced b the motorpulsates upand down with time, and at an instant its magnitude ma be gi%enas

B = B, cos of (10.1)


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where /, is the maximum #ux densit in the motor.The #ux densit B can be resol%ed into two components B, and B,such that

the magnitude o B , is e(ual to the magnitude o B,.Thus, /, /, 0. /. I weassume that B, rotates in the cloc)wise direction, the direction orotation o /, iscountercloc)wise-n em is induced in the rotor circuit owingto each re%ol%ing feld. The polarit o the induced em in the rotordue to onere%ol%ing feld is in opposition to the other. Thus, the rotor currentsinduced b

the two re%ol%ing felds circulate in opposite directions. *owe%er, atstandstill,the slip in either direction is the same 2s 13 and so is the rotorimpedance.

Thereore, the starting currents in the rotor conductors are e(ual andopposite. 4nother words, the starting tor(ue de%eloped b each re%ol%ing feld isthe same.Since the direction o the starting tor(ue de%eloped b one re%ol%ing

feld opposesthe other, the net tor(ue de%eloped b the motor is 'ero. This is thesame conclusionwe arri%ed at beore. *owe%er, we ha%e gained some insight into theoperationo a singlephase induction motor according to the double re%ol%ingfeldtheor. 5e can loo) upon a singlephase induction motor as i itconsists o twomotors with a common stator winding but with rotors re%ol%ing inopposite directions.


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Equivalent circuit of the single phase induction motor

6ne section o the rotor circuit is usuall reerred to as the orwardbranch,and the other is called the backward branch. 5hen the motorrotates, sa in thecloc)wise direction, the orward branch represents the e7ect o there%ol%ing feldin that direction. 4n this case, the bac)ward branch corresponds tothe rotor circuitassociated with the countercloc)wise re%ol%ing feld. -t standstill,both branchesha%e the same impedance. The rotor circuit currents are also the

same, and thesame is true or the tor(ues de%eloped. Thus, when the rotor is atrest, the nettor(ue de%eloped b it is 'ero. 5e usuall spea) o tor(ue de%elopedb a branch.5hat we reall mean is the tor(ue de%eloped b the rotor resistancein that particularbranch.the rotor is rotating in the cloc)wise direction with


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a speed Nm.The magnetic feld re%ol%ing in the cloc)wise directionhas a snchronousspeed o Ns, (Ns, 120/).The snchronous speed o there%ol%ing feld

in the countercloc)wise direction is then -Ns.The perunit slip in theorward2cloc)wise3 direction is

$49:E; E(ui%alent circuit o induction motor at an slip s

ANALS!S "# S!N$LE %&ASE !N'()*!"N +"*"R

$rom the e(ui%alent circuit we obtain the e7ecti%e impedance oorward and bac)ward branch as ;


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3

4!$ ; :otor current in orward branch

4!$ 412?2r!>s3@!s3@wS

The power a%ailable at the shat isC6CDC:


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C: ;rotational loss o motor

The load tor(ue is ;TsCo>wm

The tor(ue de%eloped b the orward and re%ol%ing felds is ;TdCag>wSTdbCagb>wS

The net tor(ue de%eloped b the motor is

Td Td Tdb

Speed tor(ue characteristic o a single phase induction motor

TYPES OF SIN!E P"#SE IND$%TION MOTOR

Each singlephase induction motor deri%es its name rom the methodused toma)e it selstarting. Some o the motors discussed in this section

are splitphase


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motor, capacitorstart motor, capacitorstart capacitorrun motor,and permanentsplitcapacitor motor. The currents in the two phase windings are 0"electrical

out o phase with each other. The placement o the two phasewindings in space(uadrature in a singlephase motor is no problem. *owe%er, theartifcial creationo a second phase re(uires some basic understanding o resisti%e,inducti%e, andcapaciti%e networ)s.

Split,%hase +otor

This is one o the most widel used induction motors ormechanical applications in the ractional horsepowerrange. The motor emplos two separate windings that areplaced in space (uadrature and are connected in parallelto a singlephase source. 6ne winding, )nown as the mainwinding, has a low resistance and high inductance. Thiswinding carries current and establishes the needed #ux atthe rated speed. The second winding, called the auxiliarwinding, has a high resistance and low inductance. This

winding is disconnected rom the suppl when the motorattains a speed o nearl FG o its snchronous speed. !centriugal switch is commonl used to disconnect theauxiliar winding rom the source at a predeterminedspeed. The disconnection is necessar to a%oid theexcessi%e power loss in the auxiliar winding at ull load .

)apacitor Start +otor

4n a capacitorstart motor a capacitor is included in series withthe auxiliar winding.4 the capacitor %alue is properl chosen, it is possible to designa capacitorstartmotor such that the mainwinding current lags the auxiliarwinding current


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Hapacitor start motor

b exactl 0". Thereore, the starting tor(ue de%eloped b acapacitor motor canbe as good as that o an polphase motor.6nce again, the auxiliar winding and the capacitor aredisconnected at about"#$ o the snchronous speed. Thereore, at the rated speed

the capacitorstartmotor operates onl on the main winding li)e a splitphaseinduction motor. Theneed or an external capacitor ma)es the capacitorstart motorsomewhat moreexpensi%e than a splitphase motor.

)apacitor,Start )apacitor,Run +otor

-lthough the splitphase and capacitorstart motors aredesigned to satis the rated load re(uirements, the ha%e lowpower actor at the rated speed. The lower the power actor,the higher the power input or the same power output. Thus,the eIcienc o a singlephase motor is lower than that o apolphase induction motor o the same si'e. $or example, theeIcienc o a capacitorstart or a splitphase singlephasemotor is usuall 0G to J0G in the ractional horsepower

range. 6n theother hand, or the same application, a threephase induction motor ma ha%e an eIcienc o "0$) to 80G.


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%ermanent Split,)apacitor +otor

! less expensi%e %ersion o a HSH: motor is called a permanentsplitcapacitor(%&) motor. - CSH motor uses the same capacitor or bothstarting and ull load.

Since the auxiliar winding and the capacitor sta in the circuitas long as themotor operates, there is no need or a centriugal switch. $orthis reason, the motorlength is smaller than or the other tpes discussed abo%e. Thecapacitor is usuallselected to obtain high eIcienc at the rated load. Since thecapacitor is not properl matched to de%elop optimal startingtor(ue, the starting tor(ue o a CSH

motor is lower than that o a HSH: motor. CSH motors are,thereore, suitable orblower applications with minimal starting tor(ue re(uirements.

These motors arealso good candidates or applications that re(uire re(uentstarts. 6ther tpes omotors discussed abo%e tend to o%erheat when startedre(uentl, and this mabadl a7ect the reliabilit o the entire sstem.


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TESTIN OF SIN!E P"#SE IND$%TION MOTORSSimilar to a three phase induction motor, the %arious tests can beperormed on single phase induction motor. The results o thesetests can be used to obtain the e(ui%alent circuit parameters o asingle phase induction motor. The tests usuall conducted are ;1. Ko load test or open circuit test!. /loc)ed rotor test or short circuit test

1. No load test

The test is conducted b rotating the motor without load. Theinput current, %oltage and power are measured b connecting theammeter, %oltmeter and wattmeter in the circuit. These readings aredenoted as Lo , 4o and 5o .Kow 5o Lo 4o cosM

The motor speed on no load is almost e(ual to its snchronousspeed hence or practical purposes, the slip can be assumed 'ero.*ence r!>s becomes N and acts as open circuit in the e(ui%alentcircuit. *ence or orward rotor circuit, the branch r!>s @ < x!getseliminated.

5hile or a bac)ward rotor circuit, the term r!>2! s3 tends tor!>!. Thus xo is much higher then the impedance r!>! @ < x!. *ence itcan be assumed that no current can #ow through and that branchcan be eliminated. So circuit reduces to as shown in the $ig.1.


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. 1

Kow the %oltage across xo is L-/

/ut L-/ 4o xo... xo L-/ >4o/ut xo =o >!

Thus magnetising reactance =o can be determined.

The no load power 5o is nothing but the rotational losses.

2. *+ocked o-or Tes-

4n balanced rotor test, the rotor is held fxed so that it will notrotate. - reduced %oltage is applied to limit the short circuit current.

This %oltage is ad


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. 2

5sc Lsc 4sc cos Msc cos Msc 5sc >Lsc 4sc

bloc)ed rotor power actorKow Oe( Lsc> 4sc :e( 5sc >2 4sc3!/ut :e( :1 @ :!... :! :e( :1

rotor resistance reerred to stator =e( P2Oe( ! :e( !3 =1 =! we get,

The stator resistance is measured b %oltmeterammetermethod, b disconnecting the auxiliar winding and capacitorspresent i an. Due to s)in e7ect, the a.c. resistance is 1.! to 1.times more than the d.c. resistance. Qe point ; Thus with two tests, all the parameters o single

phase induction motor can be obtained.


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Shaded Pole Single Phase Induction

Motor

The stator o the shaded o+e sn+e hase nduc-on o-orhassalient or pro


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#ux induces em in the shaded coil. Since this shaded portion is

short circuited, the currentis produced in it in such a direction to

oppose the main #ux. The #ux in shaded pole lags behind the #ux inthe unshaded pole. The phase di7erence between these two #uxes

produces resultant rotating #ux.

5e )now that the stator winding currentis alternating in nature and

so is the #ux produced b the stator current. 4n order to clearl

understand the wor)ing o shaded pole induction motor consider

three regions

1. 5hen the #ux changes its %alue rom 'ero to nearl

maximum positi%e %alue.

!. 5hen the #ux remains almost constant at its

maximum %alue.

+. 5hen the #ux decreases rom maximum positi%e

%alue to 'ero.I 15hen the #ux changes its %alue rom 'ero to nearlmaximum positi%e %alue R 4n this region the rate o rise o #ux andhence currentis %er high. -ccording to $aradas lawwhene%erthere is change in #ux em gets induced. Since the copper band isshort circuit the currentstarts #owing in the copper band due to thisinduced em. This currentin copper band produces its own #ux. Kowaccording to &en's lawthe direction o this currentin copper band issuch that it opposes its own cause i.e rise in current. So the shaded

ring #ux opposes the main #ux, which leads to the crowding o #uxin non shaded part o stator and the #ux wea)en in shaded part. Thisnon uniorm distribution o #ux causes magnetic axis to shit in themiddle o the non shaded part.

I 25hen the #ux remains almost constant at its maximum

%alue R 4n this region the rate o rise o current and hence #ux

remains almost constant. *ence there is %er little induced em in

the shaded portion. The #ux produced b this induced em has no
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e7ect on the main #ux and hence distribution o #ux remains

uniorm and the magnetic axis lies at the center o the pole.

I 35hen the #ux decreases rom maximum positi%e %alueto 'ero 4n this region the rate o decrease in the #ux and hence

current is %er high. -ccording to $aradas lawwhene%er there is

change in #ux em gets induced. Since the copper band is short

circuit the current starts #owing in the copper band due to this

induced em. This currentin copper band produces its own #ux. Kow

according to &en's lawthe direction o the currentin copper band is

such that it opposes its own cause i.e decrease in current. So the

shaded ring #ux aids the main #ux, which leads to the crowding o

#ux in shaded part o stator and the #ux wea)en in non shaded part.

This non uniorm distribution o #ux causes magnetic axis to shit in

the middle o the shaded part o the pole.

This shiting o magnetic axis continues or negati%e ccle also and

leads to the production o rotating magnetic feld. The direction o

this feld is rom non shaded part o the pole to the shaded part o

the pole.
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