INDUCTION MACHINE_construction and principle

Induction machine basic construction, working principle, types ,

1. By: Prof. BijalMehta
Electrical Dept._ SCET

3. Generally easy to build and cheaper than corresponding
dc or synchronous motors
Induction motor is robust
The motor is driven by the rotational magnetic field
produced by 3phase currents, hence no commutatoror
blush is required
Maintenance is relatively easy and at low cost
Satisfactory efficiency and reasonable power factor
A manageable torque-speed curve
Stable operation under load
Range in size from few Watts to several MW

4. Inductionmotorhaslowinherentstartingtorque
Drawlargestartingcurrents,typically6-8xtheirfull
loadvalues
SpeedsnoteasilycontrolledasDCmotors
Operatewithapoorlaggingpowerfactorwhenlightly
loaded

15. Almost 90% of
the three-phase
AC Induction
motors are of this
type.

25. Theinductionmotorrotatesduetotherotatingmagnetic
fieldin3phaseinductionmotor,whichisproducedbythe
statorwindingintheairgapbetweeninthestatorandthe
rotor.
Thestatorhasathreephasestationarywindingwhichcanbe
eitherstarconnectedordeltaconnected.
Whenever the AC supply is connected to the stator windings,
line currentsI
R,I
Y, andI
Bstart flowing.
Theselinecurrentshavephasedifferenceof120
o
with
respecttoeachother.
Duetoeachlinecurrent,asinusoidalfluxisproducedinthe
airgap.
Thesefluxeshavethesamefrequencyasthatoftheline
currents,andtheyalsohavethesamephasedifferenceof
120
o
withrespecttoeachother.

26. Let the flux produced by the line currents I
R,I
B,I
Ybe
φ
R, φ
B, φ
Yrespectively.

27. Mathematically, they are represented as follows:
φ
R= φ
msin ωt = φ
msin θ
φ
B= φ
msin (ωt –120
o
) = φ
msin (θ –120
o
)
φ
Y= φ
msin (ωt –240
o
) = φ
msin (θ –240
o
)
The effective or total flux (ɸ
T) in the air gap is equal to
the phasorsum of the three components of fluxes ɸ
R,
ɸ
Yand, ɸ
B.
Therefore, ɸ
T= ɸ
R+ ɸ
Y+ ɸ
B

28. To prove that this RMF is rotating we will:
Step 1: We will find the values of total flux ɸ
Tfor different
values of θ such as 0, 60, 120 , 180 ….. 360
o
.
Step 2: For every value of θ in step 1, we will draw the phasor
diagrams.
By observing these phasordiagrams you can understand easily
that ɸ
Tkeep shifting its position from 90
o
to 30
o
to –30
o
to –
90
o
and so on. In other words ɸ
Trotates in the clock wise
direction.
We will use the phasorɸ
Ras the reference phasori.e. all the
angles are drawn with respect to this phasor.

29. Thefluxisassumedsinusoidaldueto3ɸwindings.Let
Maximumvalueoffluxduetoanyoneofthethreephasesbe
ɸm.theresultantfluxɸratanyinstantisgivenbythevector
sumoftheindividualfluxes,ɸ1,ɸ2andɸ3dueto3phases.

34. The magnetic field in three phase induction motor
(RMF) rotates at a constant speed calledsynchronous
speed (Ns), which is given by,
N
S= 120f/P RPM
wheref= Frequency of stator supply
P = Number of poles of motor.

35. DirectionofRotatingMagneticField
Thedirectionofrotatingmagneticfielddependsupon
thephasesequenceoftheACsupplyconnectedacross
thestatorwinding.Ifweinterchangeanytwophasesof
theACsupply,wewillgetnewphasesequence,then
thedirectionofrotatingmagneticfieldinthreephase
inductionmotorwillreverse.Itwillstartrotatingin
reversedirectionandsodoestherotor.

36. Themotorwhichworksontheprincipleof
electromagneticinductionisknownastheinduction
motor.
Theelectromagneticinductionisthephenomenonin
whichtheelectromotiveforceinducesacrossthe
electricalconductorwhenitisplacedinarotating
magneticfield.
Theinductionmotoristhesingleexcitedmotor,i.e.,
thesupplyisappliedonlytotheonepart,i.e.,stator.
Thetermexcitationmeanstheprocessofinducingthe
magneticfieldonthepartsofthemotor.

37. When the three
phasesupplyisgivento
thestator,therotating
magnetic field
producedonit.Thefigure
shows therotating
magneticfieldsetupinthe
stator.
Considerthattherotating
magneticfieldinducesin
the anticlockwise
direction.Therotating
magneticfieldhasthe
movingpolarities.

38. Theconductorsoftherotorarestationary.Thisstationary
conductorcuttherotatingmagneticfieldofthestator,and
becauseoftheelectromagneticinduction,theEMFinducesin
therotor.
ThisEMFisknownastherotorinducedEMF,anditis
becauseoftheelectromagneticinductionphenomenon.
Theconductorsoftherotorareshort-circuitedeitherbythe
endringsorbythehelpoftheexternalresistance.
Therelativemotionbetweentherotatingmagneticfieldand
therotorconductorinducesthecurrentintherotor
conductors.
Asthecurrentflowsthroughtheconductor,thefluxinduces
onit.Thedirectionofrotorfluxissameasthatoftherotor
current.

39. Nowwehavetwofluxesonebecauseoftherotorand
anotherbecauseofthestator.
Thesefluxesinteracteachother.Ononeendofthe
conductorthefluxescanceleachother,andontheother
end,thedensityofthefluxisveryhigh.
Thus,thehigh-densityfluxtriestopushtheconductorof
rotortowardsthelow-densityfluxregion.
Thisphenomenoninducesthetorqueontheconductor,and
thistorqueisknownastheelectromagnetictorque.
Thedirectionofelectromagnetictorqueandrotating
magneticfieldissame.Thus,therotorstartsrotatinginthe
samedirectionasthatoftherotatingmagneticfield.

40. Thespeedoftherotorisalwayslessthantherotating
magneticfieldorsynchronousspeed.
Therotortriestotherunatthespeedoftherotor,butit
alwaysslipsaway.
Thus,themotorneverrunsatthespeedoftherotating
magneticfield,andthisisthereasonbecauseofwhichthe
inductionmotorisalsoknownastheasynchronousmotor.

41. Therotatingmagneticfield
rotatesintheclockwise
direction.
Considerthesingle
conductoronthestationary
rotor.
AccordingtoFaraday’s
Lawofelectromagnetic
induction,theEMF
inducesintheconductor.

42. TherotorinducesanEMF
whichcausesthecurrentin
thecircuit.
Thedirectionoftherotor
inducescurrentisopposite
tothatoftherotating
magneticfield.
Therotorcurrentinduces
thefluxintherotor.
Thedirectionoftherotor
fluxissameasthatofthe
current.

43. Theinteractionofrotorand
statorfluxesdevelopsaforce
whichactsontheconductorsof
therotor.
Theforceactstangentiallyonthe
rotorandhenceinducesatorque.
Thetorquepushesthe
conductorsoftherotor,andthus
therotorstartsmovinginthe
directionoftherotating
magneticfield.

44. Therotorstartsmovingwithoutanyadditional
excitationsystemandbecauseofthisreasonthe
motoriscalledtheself-startingmotor.
The operation of the motor depends on the voltage
induced on the rotor, and hence it is called the
induction motor.


46. Ifthespeedoftherotorisequaltothesynchronousspeed,
norelativemotionoccursbetweentherotatingmagnetic
fieldofthestatorandtheconductorsoftherotor.
ThustheEMFisnotinducedontheconductor,andzero
currentdevelopsonit.Withoutcurrent,thetorqueisalso
notproduced.
Becauseoftheabovementionreasonstherotornever
rotatesatthesynchronousspeed.Thespeedoftherotoris
alwayslessthanthespeedoftherotatingmagneticfield.

47. The rotational speed of therotating magnetic fieldis
called as synchronous speed.
where, f = frequency of the supply
P = number of poles

48. InductionMotorRotortriestocatchupthe
synchronousspeedofthestatorfield,andhenceit
rotates.
ButItneverrunsatsynchronousspeed,rotoralways
runslessthesynchronousspeed.
Thedifferencebetweenthesynchronousspeed(Ns)
andactualspeed(N)oftherotoriscalledasslip.

49. Thedifferencebetweenthesynchronousspeedandthe
actualspeedoftherotorisknownastheslipspeed.
Thespeedoftherotorisslightlylessthanthe
synchronousspeed.Thus,theslipspeedexpressesthe
speedoftherotorrelativetothefield.
Theslipspeedoftheinductionmotorisgivenas
Therefore,therotorspeedisgivenbytheequation
shownbelow.

50. SlipplaysanessentialroleinInductionmotor.
Asweknow,thedifferencebetweenthesynchronous
speedoffluxandrotorspeeddeterminestherateatwhich
fluxiscutbytherotorconductor.
Hencemagnitudeoftheemfinducesintherotordepends
ontherelativemotion,orwecansaytheslipspeedofthe
motor.So,
The rotor current is directly proportional to the induced
emf.

51. Thetorqueisdirectlyproportionaltotherotorcurrent.
Therefore,
Theaboveequationshowsthatthetorqueinducesontherotor
isdirectlyproportionaltotheslipoftheinductionmotor.
Thehighvalueofslipinducesthehighemfintherotor.This
EMFdevelopstheheavytorqueontherotorconductors.

52. Thevalueoftheslipisadjustedbyconsideringtheload
onthemotor.Forfull-load,thehighvalueoftorqueis
required.
Thiscanbeachievedbyincreasingtheamountofthe
slipandreducingthespeedoftherotor.
Theslipofthemotoriskeptlowwhentheinduction
motorisrunningatno-load.Thesmallslipproduces
thesmalltorqueonthemotor.
Thevalueoftheinductionmotorslipisadjusted
accordingtotherequirementofthedrivingtorqueat
thenormalworkingcondition.