Single Phase Induction Motors.ppt

A.C machines

1. Single-Phase Motors
Thesemotors,haveoutputlessthanonehorse-power
oronekilowatt,hencearecalledfractionalhorse-
powerorfractionalkilowattmotors.
ACsingle-phase,fractionalkilowattmotorsperform
varietyofservicesinthehomes,offices,business
concerns,factoriesetc.
Almostinallthedomesticappliancessuchas
refrigerators,fans,washingmachines,hairdriers,
mixergrindersetc.,only1-phaseinductionmotorsare
employed.

2. Classification of Single-phase Motors
1. Single-phase induction motors
(i) split-phase type (ii) capacitor type
(iii) Shaded-pole type
2. AC series motors or universal motors
3. Repulsion motors
(i) Repulsion-start induction-run motors
(ii) Repulsion-induction motors
4. Synchronous motors
(i) Reluctance motors
(ii) Hysteresis motors.

3. Single-phase Induction Motors
Asingle-phaseinductionmotorisverysimilartoa3-
phasesquirrelcageinductionmotorinconstruction.
•Similarto3-phaseinductionmotoritconsistsoftwo
mainpartsnamelystatorandrotor.
1.Stator:Itisthestationarypartofthemotor.Ithas
threemainparts,namely.
(i)Outerframe(ii)Statorcoreand
(iii)Statorwinding.
(i)Outerframe:Itistheouterbodyofthemotor.Its
functionistosupportthestatorcoreandtoprotect
theinnerpartsofthemachine.
•Usually,itismadeofcostiron.
•Toplacethemotoronthefoundation,feetare
providedintheouterframeasshown

4. (ii) Stator core:
•Thestatorcoreistocarrythealternatingmagneticfieldwhich
produceshysteresisandeddycurrentlosses.
•Tominimisetheselosseshighgradesiliconsteelstampingsareused
tobuildcore.
•Thestampingsareassembledunderhydraulicpressureandarekeyed
totheouterframe.
•Thestampingsareinsulatedfromeachotherbyathinvarnishlayer.
•Thethicknessofthestampingusuallyvariesfrom0.3to0.5mm.
•Slotsarepunchedontheinnerperipheryofthestampingsto
accommodatestatorwinding.

5. (iii) Stator winding:
•Thestatorcorecarriesasinglephasewindingwhich
isusuallysuppliedfromasinglephaseACsupply
system.
•Theterminalsofthewindingareconnectedinthe
terminalboxofthemachine.
•Thestatorofthemotoriswoundfordefinite
numberofpoles,aspertheneedofspeed.

6. 2. Rotor: It is the rotating part of the motor.
•Asquirrelcagerotorisusedinsinglephase
inductionmotors.

7. •Itconsistsofalaminatedcylindricalcoreofsome
highqualitymagneticmaterial.
•Semi-closedcircularslotsarepunchedattheouter
periphery.
•Aluminiumbarconductorsareplacedintheseslots
andshortcircuitedateachendbyaluminiumrings,
calledshortcircuitingrings
•Thus,therotorwindingispermanentlyshort
circuited.

8. Therotorslotsareusuallynotparalleltotheshaftbut
areskewed.Skewingofrotorhasthefollowing
advantages:
(a)Itreduceshummingthusensuringquietrunningof
amotor
(b)Itresultsinasmoothertorquecurvesfordifferent
positionsoftherotor
(c)Itreducesthemagneticlockingofthestatorand
rotor
(d)Itincreasestherotorresistanceduetothe
increasedlengthoftherotorbarconductors.

9. Nature of Field Produced in Single Phase Induction Motors

12. Torque Produced by Single-phase Induction Motor

13. •Thetworevolvingfieldswillproducetorquesinopposite
directions.LetthetworevolvingfieldsbefieldNo.1andfield
No.2revolvinginclockwiseandanticlockwisedirection.
•Theclockwisefieldproducestorqueinclockwisedirection,
whereas,theanticlockwisefieldproducestorquein
anticlockwisedirection.
•Theclockwisetorqueisplottedaspositiveandanticlockwise
asnegative.
•Atstandstill,slipforbothfieldsisone.Atsynchronousspeed,
forclockwisedirection,thefield-1willgiveconditionofzero
slipbutitwillgiveslip=2forfieldNo.2.
•Atsynchronousspeedinacounterclockwisedirection,will
giveconditionofzeroslipforfield-2butslip=2forfieldNo.
1.
•Theresultanttorquedevelopedintherotorisshownbythe
curvepassingthroughzeropositionasshowninFig.11.4.

14. •Theresultanttorqueitisobservedthatthestarting
torque(torqueatslip=1)iszero.Andexceptat
startingthereisalwayssomemagnitudeof
resultanttorque,
•Atposition1-1’,thetorquedevelopedbyfield-1is
dominating,therefore,motorwillpick-upthespeed
inclockwisedirection.
•Atposition2-2’,thetorquedevelopedbyfield-2is
dominating,therefore,motorwillpick-upthespeed
inanti-clockwisedirection).
•Thisshowsthatifthistypeofmotorisoncestarted
(rotated)ineitherdirectionitwilldeveloptorquein
thatdirectionandrotorwillpick-uptherequired
speed.

15. •Theaboveanalysisshowsthatsinglephase
inductionmotorwithsinglewindingdevelopsno
startingtorque
•butiftherotorisrotatedinanydirectionbysome
auxiliarymeansinwilldeveloptorqueinthesame
directionandwillstartrotatinginthatdirection.
•Sotheproblemistofindouttheauxiliarymeansto
givethestartingtorquetothemotor.

16. Methods to make Single-phase Induction Motor Self-starting
•Asingle-phaseinductionmotorinherentlyisnotself-starting.
•Tomakeitself-starting,somemethodisrequiredtobeevolvedtoproducea
revolvingmagneticfieldinthestatorcore.
•Thismaybeobtainedbyconvertingasingle-phasesupplyintotwo-phase
supplywhichcanbeachievedbyusinganadditionalwinding(thisadditional
windingmaybeormaynotbedisconnectedoncethemotorstartsandpicks-
upthespeed)orbycreatinganotherfield(byinduction)usingashort
circuitingbandorring.
•Accordingly,dependinguponthemethodusedtomakea1-phaseinduction
•motorself-starting,single-phaseinductionmotorscanbeclassifiedas:
1.Split-phasemotors:Thesemotorsarestartedbyemployingtwo-phase
motoractionthroughtheuseofanauxiliarywindingcalledstartingwinding.
2.Capacitormotors:Thesemotorsarestartedbyemployingtwo-phasemotor
actionthroughtheuseofanauxiliarywindingwithcapacitor.
3.Shaded-polemotors:Thesemotorsarestartedbytheinteractionofthe
fieldproducedbyashadingbandorshortcircuitingringplacedarounda
portionofthepolestructure.

18. Split-phase induction motor

20. Thedirectionofrotationofa1-phase(splitphase)inductionmotorcan
bereversedbyreversing(interchanging)theconnectionsofeither
startingwindingorrunningwinding.

21. Capacitor Motors

22. Capacitor start motors

24. Performance and characteristics:
Speedisalmostconstantwithin5%slip.Thistypeof
motordevelopshighstartingtorqueabout4to5
timesthefullloadtorque.Itdrawlowstarting
current.
Thedirectionofrotationcanbechangedby
interchangingtheconnectionofeitherstartingor
runningwinding.

26. Capacitor run motors (fan motors):
•Apapercapacitorispermanentlyconnectedinthe
startingwinding.Inthiscase,electrolyticcapacitor
cannotbeusedsincethistypeofcapacitoris
designedonlyforshorttimeratingandhence
cannotbepermanentlyconnectedinthewinding.
•Bothmainaswellasstartingwindingisofequal
rating.
Performance and characteristics.
•Startingtorqueislowerabout50to100%offull
loadtorque.
•Powerfactorisimprovedmaybeaboutunity.
Efficiencyisimprovedtoabout75%.

28. Capacitor start and capacitor run motors
•Inthiscase,twocapacitorsareusedoneforstarting
purposeandotherforrunningpurpose.
•Forstartingpurposeanelectrolytictypecapacitor(C
s)
isusedwhichisdisconnectedfromthesupplywhen
themotorattains75%ofsynchronousspeedwiththe
helpofcentrifugalswitchS.
•Whereas,apapercapacitorC
Risusedforrunning
purposewhichremainsinthecircuitofstarting
windingduringrunningconditions.
•Thistypeofmotorgivesbestrunningandstarting
operation.
•StartingcapacitorC
Swhichisofhighervaluethanthe
valueofrunningcapacitorC
R.

29. Performance and characteristics:
•Suchmotorsoperateastwophasemotorsgiving
bestperformanceandnoiselessoperation.
•Startingtorqueishigh,startingcurrentislowand
givebetterefficiencyandhigherp.f.
•Theonlydisadvantageishighcost.

30. Shaded Pole Motor
Construction
•Shadedpolemotorisconstructedwithsalientpoles
instator.
•Eachpolehasitsownexcitingwinding
•A1/3rdportionofeachpolecoreissurroundedby
acopperstripformingaclosedloopcalledthe
shadingband
•Rotorisusuallysquirrelcagetype.

32. •When a single phase supply is given to the stator
(exciting) winding, it produces alternating flux.
•When the flux is increasing in the pole, a portion of
the flux attempts to pass through the shaded
portion of the pole.
•This flux induces an emf and hence current in the
shading band or copper ring.
•As per Lenz’s law the direction of this current is such
that it opposes the cause which produces it i.e.,
increase of flux in shaded portion. Hence in the
beginning, the greater portion of flux passes
through unshaded side of each pole and resultant
lies on unshaded side of the pole.

33. •When the flux reaches its maximum value, its rate of
change is zero, thereby the emfand hence current in
the shading coil becomes zero.
•Flux is uniformly distributed over the pole face and the
resultant field lies at the centreof the pole.
•After this the main flux tends to decrease, the emfand
hence the current induced in the shading coil now
tends to increase the flux on the shaded portion of the
pole and resultant lies on the shaded portion of the
pole
•Hence, a revolving field is set up which rotates from
unshadedportion of the pole to the shaded portion of
the pole as marked by the arrow head
•Thus, by electromagnetic induction, a starting torque
develops in the rotor and the rotor starts rotating.
•After that its rotor picks up the speed.

36. Split PhaseCapacitor
Start
Capacitor runCapacitor
start and run
Shaded pole
Characteristi
cs
Low and
medium
starting
torque
High Starting
torque
Medium
torque
High starting
torque
Low starting
torqu
Starting
torque
100% to250%
of rated value
250% to400%
of rated value
100% to200%
of rated value
200% to300%
of rated value
40% to60%
of rated value
Braking
torque
Upto300% Upto350% Upto250% Upto250% Upto10%
Power
factor
0.5 to 0.650.5 to 0.650.75 to 0.90.75 to 0.90.25 to 0.4
Efficiency55% to 65%55% to 65%60% to 70%60% to 70%25% to 40%
Rating 0.5 to 1 HP0.125to 1 HP0.125to 1 HP0.125to 1 HPUpto40W
ApplicationsFans,
blowers,
Centrifugal
pumps,
washing
machines
Compressors,
Pumps,
Conveyors,
Refrigerators,
AC and
Washing
Machines
Fans,
blowers,
Centrifugal
pumps,
Compressors,
Pumps,
Conveyors,
Refrigerators
Fans, turn
Tables, Hair
driers,
Motion
picture
projectors,
Blowers,