Module 2-Electric Actuators electrical engineering robotics

electrical engineering 6th sem electric actuators pdf forrobotics and automation. It covers a range of topics from
1 Actuators and Grippers : Electric Actuators
2 Hydraulic Actuators
3 Pneumatic Actuators
4 Selection of Motors
5 Grippers

1. MODULE 2 ( PPT-2/4)
ROBOTICS AND
AUTOMATION
21ECE1673

2. SYLLABUS
MODULE-2
1 Actuators and Grippers : Electric Actuators T1 : 3.1
2 Hydraulic Actuators T1 : 3.2
3 Pneumatic Actuators T1 : 3.3
4 Selection of Motors T1 : 3.4
5 Grippers T1 : 3.5

3. ELECTRIC ACTUATORS

4. ELECTRIC ACTUATORS
Electric actuators
are generally those where an
electric motor drives robot links
through some mechanical transmission, e.g., gears, etc.

5. ELECTRIC ACTUATORS
In the early years of industrial robotics,
hydraulic robots were the most common,
but recent improvements in electric motor design
have meant that most new robots are of all-electric construction.
.

6. INDUSTRIAL ROBOT
ABBpioneeredtheworld’s
first commercial all-
electric microprocessor-
controlledrobotin1974.
Since then, it has
introduced anumber of
robotics innovations from
high-speed“delta”robots,
forpickingandpackaging,
totheworld’sfirsttruly
collaborative industrial
robot,YuMi,in2015.

7. ELECTRIC ACTUATOR
COMPONENTS

8. ELECTRIC ACTUATOR -ADVANTAGES
1.Widespread availabilityof power supply.
2.Basic drive element electric motor is usually lighterthan pressurized
fluid or compressed air.
3.High power-conversionefficiency.
4.No pollution of working environment.
5.Accuracy and repeatability of electric drive robots are normally better
than fluid power
6.Being relatively quietand clean, they are very acceptable
environmentally.
7.They are easilymaintainedand repaired.
8.Structural components can be lightweight.
9.The drive system is well suited to electronic control.

9. ELECTRIC ACTUATOR -
DISADVANTAGES
1.Electricallydrivenrobotsoftenrequiretheincorporationofsomesortofmechanical
transmissionsystem.
2.Additionalpowerisrequiredtomovetheadditionalmassesofthetransmissionsystem.
3.Unwantedmovementsduetobacklashandplaysinthetransmissionelements.
4.Duetotheincreasedcomplexitywiththetransmissionsystem,weneedcomplexcontrol
requirementandadditionalcostfortheirprocurementandmaintenance.
5.Electricmotorsarenotintrinsicallysafe,mainly,inexplosiveenvironments

10. WHAT’S BEING DONE ???
1.Theabovedisadvantagesaregraduallybeingovercomewiththeintroductionofdirect-drive
motorsystem,inwhichtheelectricmotorisapartoftherelevantrobotarmjoint,thus,
eliminatingthetransmissionelements.
2.Moreover,theintroductionofnewerbrushlessmotorsallowelectricrobotstobeusedin
somefire-riskapplicationssuchasspraypainting,asthepossibilityofsparkingatthe
motorbrushesiseliminated.
3.Differenttypesofelectricmotorsaresteppermotors,andthedcandacmotors

11. STEPPER MOTORS
DC MOTORS
AC MOTORS
DIFFERENT TYPES OF ELECTRIC
MOTORS

12. STEPPER MOTOR

13. STEPPER MOTORS
Stepper motor (Bipolar, 200 Steps/Rev, 20 X30 mm, 3.9 V, 0.6 A/Phase)
[Courtesy: http://www.polulu.com]

14. STEPPER MOTOR
Asteppermotorisanelectricmotorwhosemainfeatureisthatitsshaftrotatesby
performingsteps,thatis,bymovingbyafixedamountofdegrees.
Thisfeatureisobtainedthankstotheinternalstructureofthemotor,andallowsto
knowtheexactangularpositionoftheshaftbysimplycountinghowmaysteps
havebeenperformed,withnoneedforasensor.

15. APPLICATIONS
3D Printers: XY Table Drive, Media
Drive
Video Cameras: Pan, Tilt, Zoom,
Focus
Robots: Arms, End Effectors

16. APPLICATIONS
Engraving Machines: XY Table Motion ATM Machines: Bill Movement, Tray Elevators

17. STEPPER MOTORS
Steppermotors(alsocalledsteppingmotorsorstepmotors)werefirstusedforremotecontrol
ofthedirectionindicatorsoftorpedotubesandgunsinBritishwarshipsandlater,forasimilar
purpose,intheUSNavy.
Avariablereluctance-typesteppermotorwasfirstpatentedin1919byC.L.Walker,aScottish
civilengineer.
However,itscommercialproductiondidnotstartuntil1950.

18. STEPPER MOTORS
Usedwithsmallandmediumrobotsandwithteachingandhobbyrobots.
Usedinindustrialapplications.(advantage:notrequiringanyfeedback
system)
Theyarecompatiblewithmanyfeedbackdevices.
Usedinfullservo-controlconfigurationsinmediumdutyindustrialrobots.

19. STEPPER MOTORS
Becausetheyaredigitallycontrolled,theycanbereferredtoasdigitalmotors
oractuators.
Thesemotorsdonotrequiretheexpenseofdigital-to-analogconversion
equipmentwhenconnectedtoacomputer-controlsystem

20. WORKING

21. WORKING
Normally,theshaftofasteppermotorsrotatesincrementallyinequal
stepsinresponsetoaprogrammedinputpulsetrain.
Acurrentinanyofthetwophases,i.e.,withP1andP2,willmagnetize
thepairintonorthandsouthpoles,indictedwithNandS,respectively.
Accordingly,thepermanentmagnetatthecentrewillrotateinorderto
alignitselfalongaparticularphase,whichisdemonstrated.
Switchingthecurrentsinthetwophasesinanappropriatesequencecan
produceeitherclockwise(CW)orcounterclockwise(CCW)rotations.

22. WORKING
Theswitchingsequencecorrespondstowhatisknownashalf-steppingwithastep
angleof45
o
,whereasthefull-steppingcorrespondsto90
o
inwhichonephaseis
energizedatatime

23. WORKING
Micro-stepping (non-identical steps) up to 1/125 of full-step by changing the currents in small steps instead of
switching them on and off, as in the case of half-and full-stepping.
While micro-steppingis advantageous from the point of view of accurate motion control using a stepper motor, it
has the disadvantage of reduced motor torque.
The steps are achieved through phase activation or switching sequences triggered by the pulse sequences.
The switching logic that decides the states of the phases of a given step can be generated using a look-up table.
The same sequences can also be generated using a logic circuitry which is typically an electronic device.

24. WORKING
Astherotorindexesroundaspecificamountforeachcontrolpulse,any
errorinpositioningisnoncumulative.
Toknowthefinalpositionoftherotor,allthatisrequiredistocountthe
numberofpulsesfedintothestator’sphasewindingofthemotor.
Thenumberofpulsesperunittimedeterminesthemotorspeed.
Therotorcanbemadetoindexslowly,comingtorestaftereachincrement
oritcanmoverapidlyinacontinuousmotiontermedslewing.Maximum
dynamictorqueinasteppermotoroccursatlowpulserates.

25. WORKING
Therefore,itcaneasilyaccelerateaload.Oncetherequiredpositionisachievedandthe
commandpulsescease,theshaftstopswithouttheneedforclutchesorbrakes.
Theactualrotationalmovementsorstepanglesoftheshaftareobtainabletypicallyfrom1.8°to
90°dependingontheparticularmotorchoices.
Thus,withanominalstepangleof1.8°,astreamof1000pulseswillgiveanangular
displacementof1800°orfivecompleterevolutions.
Theyhavealsoalowvelocitycapabilitywithouttheneedforgearreduction.Forinstance,ifthe
previouslymentionedmotorisdrivenby500pulsespersecond,itwillrotateat150rpm.
Otheradvantagesofthesteppermotorarethatthemotorinertiaisoftenlow,andalsoifmore
thanonesteppermotorisdrivenfromthesamesourcethentheywillmaintainperfect
synchronizations.

26. STEPPER MOTOR ADVANTAGES
•Duetotheirinternalstructure,steppermotorsdonotrequireasensortodetectthemotor
position.
•Sincethemotormovesbyperforming“steps,”bysimplycountingthesesteps,youcan
obtainthemotorpositionatagiventime.
•Inaddition,steppermotorcontrolisprettysimple.
•Themotordoesneedadriver,butdoesnotneedcomplexcalculationsortuningtowork
properly.
•Steppermotorsoffergoodtorqueatlowspeeds,aregreatforholdingposition,andalsotend
tohavealonglifespan.

27. STEPPER MOTOR DISADVANTAGES
1.They can miss a step if the load torque is too high.
2.These motors always drain maximum current even when still, which makes efficiency worse and
can cause overheating.
3.Stepper motors have low torque and become pretty noisy at high speeds.
4.Finally, stepper motors have low power density and a low torque-to-inertia ratio.
5.Low efficiency
6.Drive inputs and circuitry have to be carefully designed in relation to the torque and speed
required.

28. TORQUE –SPEED
CHARACTERISTICS

29. TYPES OF STEPPER MOTORS

30. TYPES OF STEPPER MOTORS
Types based on motor’s rotor:
1.VariableReluctance
2.PermanentMagnet
3.Hybrid

31. VARIABLE-RELUCTANCE STEPPER MOTOR
Rotor rotates due to reluctance that is offers to magnetics flux lines. When
motor moves Reduction in reluctance in air,moreenergy in magnetic field.

32. WORKING
Magneticreluctance,orreluctance,istheanalogofelectricalresistance.Justascurrentoccursonlyina
closedloop,somagneticfluxoccursonlyaroundaclosedpath,althoughthispathmaybemorevaried
thanthatofthecurrent.
Therotorismadeofsoftsteelandithasfourpoles,whereasthestatorhassixpoles.Whenoneofthe
phases,sayAA’,isexcitedduetoadccurrentpassingthroughthecoilsaroundthepoles,therotor
positionsitselftocompletethefluxpath.
Notethatthereisamainfluxpaththroughthealignedrotorandstatorteeth,withsecondaryfluxpaths
occurringasindicated.
Whenrotorandstatorteetharealignedinthismanner,thereluctanceisminimizedandtherotorisatrest
inthisposition.Thisfluxpathcanbeconsideredratherlikeanelasticthreadandalwaystryingtoshorten
itself.

33. WORKING
Therotorwillmoveuntiltherotorandstatorpolesarelinedup.Thisistermedasthepositionof
minimumreluctance.
Torotatethemotorcounterclockwise,thephaseAA’isturnedoffandphaseBB’isexcited.
Thisformofasteppermotorgenerallygivesstepanglesof7.5°or15°,whicharereferredashalf-
steppingandfull-stepping,respectively.
Notethatadisadvantageofvariable-reluctancesteppermotorsisthatithaszeroholdingtorque
whenthestatorwindingsarenotenergized(poweroff)becausetherotorisnotmagnetized.
Hence,ithasnocapacitytoholdaloadinpower-offmodeunlessmechanicalbrakesare
employed.

34. PERMANENT-MAGNET STEPPER MOTOR

35. WORKING
Thebasicmethodofoperationofapermanent-magnettypeis
similartothevariable-reluctancetype.
TherearetwocoilsAandB,eachofthemhavingfourpoles
butdisplacedfromeachotherbyhalfapolepitch.
Therotorisofpermanent-magnetconstructionandhasfour
poles.Eachpoleiswoundwithfieldwinding,thecoilson
oppositepairsofpolesbeinginseries.Currentissupplied
fromadcsourcetothewindingthroughswitches.

36. WORKING
Itcanbeseenthatthemotorisatrestwiththepoles
ofthepermanentmagnetrotorheldbetweentheresidual
polesofthestator.Inthisposition,therotoris
lockedunlessaturningforceisapplied.
Ifthecoilsareenergizedand,inthefirstpulse,the
magneticpolarityofthepolesofthecoilAisreversed,
therotorwillexperience atorqueandwillrotate
counterclockwise.ThereversepolesareshownasA’.

37. WORKING
IfthecoilBpolesarenowreversedtoB’therotorwillagain
experienceatorque,andoncemorethepolesoftherotorare
positionedmidwaybetweenthestatorpoles.Thus,byswitchingthe
currentsthroughthecoils,therotorrotatesby45°.
Ifinthefirstpulse,thepolesofthecoilBhadbeenreversedthenthe
motorwouldhaverotatedclockwise.Withthistypeofmotor,
commonlyproducedstepanglesare1.8°,7.5°,15°,30°,34°,90°.

38. HYBRID STEPPER MOTOR

39. WORKING
Hybridsteppermotorsarethemostcommonvarietyofsteppermotorsin
engineeringapplications.
Theycombinethefeaturesofboththevariablereluctanceandpermanent-
magnetmotors,havingapermanentmagnetencagedinironcapswhicharecut
tohaveteeth.
Ahybridsteppermotorhastwostacksofrotorteethonitsshaft
(WRStepper,2013).Thetworotorstacksaremagnetizedtohaveopposite
polarities,whiletwostatorsegmentssurroundthetworotorstacks.
Therotorsetsitselfintheminimumreluctancepositioninresponsetoa
pairofstatorcoilsbeingenergized.Typicalstepanglesare0.9°and1.8°.

40. STEPPER MOTORS SUMMARY
Fromthedescriptionsofsteppermotors,itis,therefore,apparentthattherateatwhich
thepulsesareapplieddeterminesthemotorspeed,thetotalnumberofpulsesdetermines
theangulardisplacement,andtheorderofenergizingthecoilsinthefirstinstance
determinesthedirectionofrotation.
Itisbecauseofthiseaseofdrivingusingdirectdigitalcontrolthatsteppermotorsarewell
suitedforuseinacomputercontrolledrobot,althoughthemotordoesrequireinterfacing
withahigh-currentpulsesource.

41. EXAMPLE

42. EXAMPLE

43. EXAMPLE

44. DC MOTOR

45. DC MOTORS
Traditionally, roboticists haveemployedelectrically drivendc(direct-
current)motorsforrobotsbecause,notonlyarepowerfulversionsavailable,
buttheyarealsoeasilycontrollablewithrelativelysimpleelectronics.
Althoughdirectcurrentisneeded,batteriesarerarelyused(fornonmobile
robots)butinsteadtheacsupplyisrectifiedintoasuitabledcequivalent.

46. DC MOTORS
Theoperationofanyelectricmotorisbasedupontheprinciplewhichstatesthataconductorwill
experienceaforceifanelectriccurrentinthatconductorflowsatrightanglestoamagneticfield.
Therefore,toconstructamotor,twobasiccomponentsarerequired.
Onetoproducethemagneticfieldusuallytermedthestator,
andanothertoactastheconductorusuallytermedthearmatureortherotor.

47. DC MOTORS
Theprincipleisforoneelementofadcmotor,whereasatwo-
poledcmotorisshown
Themagneticfieldmaybecreatedeitherbyfieldcoilswoundon
thestatororbypermanentmagnets.
Thefieldcoils,ifused,wouldbeprovidedwithanelectric
currenttocreatemagneticpolesonthestator.
Thecurrentissuppliedtoaconductorviathebrushesand
commutators.

48. DC MOTORS

49. Thecurrentpassingthroughthefieldproducesatorque,or
moreaccuratelystatictorqueontheconductors.

50. TYPES OF WINDINGS OF DC
MOTOR

51. SHUNTWOUND
Inashunt-woundmotor,thearmaturewindingsandfieldwindingsare
connectedinparallel.Atsteadystate,thebackelectromotiveforce(e.m.f.)
dependsdirectlyonthesupplyvoltage.Sincethebacke.m.f.isproportionalto
thespeed,itfollowsthatthespeedcontrollabilityisgoodwiththeshunt-wound
configuration.

52. SERIES WOUND
Inseries-woundmotorstheyareconnectedinseries.Therelationbetweenthebacke.m.f.andsupply
voltageiscoupledthroughboththearmaturewindingsandthefieldwindings.Hence,itsspeed
controllabilityisrelativelypoor.Butinthiscase,arelativelylargecurrentflowsthroughbothwindings
atlowspeedsofthemotor,givinghigherstartingtorque.Also,theoperationisapproximatelyat
constantpower.

53. COMPOUND WOUND
Inthecompoundwoundmotor,apartofthefieldwindingsrelatestothearmaturewindingsinseriesand
theotherpartisconnectedinparallel,thiskindofmotorsprovidesacompromiseperformancebetweenthe
extremesofspeedcontrollabilityandhigherstartingtorquecharacteristics,asprovidedbytheshuntwound
andseries-woundmotors,respectively.

54. DC MOTORS
Foranindustrialrobot,ingeneral,itissaidthatthecurrentexcitedfieldcontrolmethods
involvetooslowaresponsetimeandincurlossesthatmakepermanent-magnetfieldsand
armaturecontrolmoreattractive,whichareexplainednext.

55. TYPES OF DC MOTOR

56. TYPES OF DC MOTORS
Permanent magnet dc Motors
Brushless Permanent-Magnet dc Motors
DC Servo motors

57. PERMANENT-MAGNET (PM) DC MOTORS
The permanent-magnet dc motor, which is also referred as torque motor, can provide high torque.
Here, no field coils are used and the field is produced by the permanent magnets themselves.
These magnets should have high-flux density per unit yielding a high torque/mass ratio.
Typical materials with desired characteristic of such dc motors are rare-earth materials such as samarium
cobalt, etc.

58. PERMANENT-MAGNET (PM) DC MOTORS
Some PM motors do have coils wound on the magnet poles but these are simply to recharge the magnets if
their strength fails.
Due to the field flux being a constant, the torque of these motors is directly proportional to the armature
current.
Some other advantages are: excitation power supplies for the field coils are not required, reliability is improved
as there are no field coils to fail, and no power loss from field coils means efficiency and cooling are improved.
However, these types of motors are more expensive. They are cylindrical and disk types.

59. TWO TYPES OF PM CONFIGURATIONS

60. PMDC MOTOR

61. BRUSHLESS PERMANENT-MAGNET
DC MOTORS
Theproblemwithdcmotorsisthattheyrequireacommutatorandbrushesinorderto
periodicallyreversethecurrentthrougheacharmaturecoil.
Thebrushesmakeslidingcontactswiththecommutatorsandasaconsequencesparksjump
betweenthetwoandtheysufferwear.
Brusheshavetobeperiodicallychangedandthecommutatorresurfaced.
Toavoidsuchproblems,brushlessmotorshavebeendesigned.

62. BRUSHLESS PERMANENT-MAGNET
DC MOTORS
Essentially,theyconsistofasequenceofstatorcoiledandapermanentmagnetrotor.
Acurrent-carryingconductorinamagneticfieldexperiencesaforce;likewise,asaconsequenceof
Newton’sthirdlawofmotion,themagnetwillalsoexperienceanoppositeandequalforce.
Withtheconventionaldcmotor,themagnetisfixedandthecurrent-carryingconductorsmadeto
move.
Withthebrushlesspermanent-magnetdcmotorthereverseisthecase,thecurrent-carrying
conductorsarefixedandthemagneticfieldmoves.

63. BRUSHLESS PERMANENT-MAGNET
DC MOTORS
Therotorisaferriteorceramicpermanentmagnet.
Inconcept,brushlessdcmotorsaresomewhatsimilartopermanent-magnetsteppermotors
explained
Thecurrenttothestatorcoilsiselectronicallyswitchedbytransistorsinsequenceroundthecoils,
theswitchingbeingcontrolledbythepositionoftherotorsothattherearealwaysforcesactingonthe
magnetcausingittorotateinthesamedirection.

64. BRUSHLESS PERMANENT-MAGNET
DC MOTORS

65. BLDC MOTOR

66. ADVANTAGES
Thebrushlessmotorshavemanyadvantagesoverconventionaldcmotors.Forexample,
1.Theyhavebetterheatdissipation;heatbeingmoreeasilylostfromthestatorthantherotor.
2.Thereisreducedrotorinertia.Hence,theyweighlessandlowmassforaspecifiedtorquerating.
3.Themotorsinthemselvesarelessexpensive.
4.Theyaremoredurableandhavelongerlife.
5.Lowmaintenance.
6.Lowermechanicalloading.
7.Improvedsafety.
8.Quieteroperation.
9.Theyareofsmallerdimensionsofcomparablepower.
Theabsenceofbrushesreducesmaintenancecostsduetobrushandcommutatorwear,andalsoallowselectric
robotstobeusedinhazardousareaswithflammableatmospheressuchasarefoundinspray-paintingapplications.

67. DC SERVO MOTORS AND
THEIR DRIVERS
Servomotorsaremotorswithmotionfeedbackcontrol,whichareabletofollowaspecified
motiontrajectory.
Inadcservomotor,bothangularpositionandspeedmightbemeasuredusing,say,shaft
encoders,tachometers, resolvers,potentiometers, etc.,andcomparedwiththedesired
positionandspeed.
Theerrorsignalwhichisthedifferencebetweenthedesiredminusactualresponsesis
conditionedandcompensatedusinganalogcircuitryorisprocessedbyadigitalhardware
processororacomputer,andsuppliedtodrivetheservomotortowardthedesiredresponse.

68. DC SERVO MOTORS AND THEIR DRIVERS
Motioncontrolimpliesindirecttorquecontrolofthemotorthat
causesthemotion.Insomeapplicationslikegrinding,etc.,where
torqueitselfisaprimaryoutput,directcontrolofmotortorque
wouldbedesirable.
Thiscanbeaccomplishedusingfeedbackofthearmaturecurrentor
thefieldcurrentbecausethosecurrentsdeterminethemotortorque
isdesirable.
Thiscanbeaccomplishedusingfeedbackofthearmaturecurrentor
thefieldcurrentbecausethosecurrentsdeterminethemotortorque

69. A TYPICAL LAYOUT OF A DC
SERVOMOTOR

70. WORKING
Note that the control of a dc motor is achieved by controlling either the stator field flux or the armature flux.
If the armature and fi eld windings are connected through the same circuit, i.e., one of the winding types,
both techniques are implemented simultaneously.
Two methods of control are armature control and field control.
In armature control, the fi eld current in the stator circuit is kept constant and the input voltage to the rotor is
varied in order to achieve a desired performance.
Inthefieldcontrol,ontheotherhand,thearmaturevoltageiskeptconstantandinputvoltagetothefield
circuitisvaried.
Thesewindingcurrentsaregeneratedusingamotordriver.

71. Itisahardwareunitthatgeneratesnecessarycurrenttoenergizethewindingsofthemotor.
Bycontrollingthecurrentgeneratedbythedriver,themotortorquecanbecontrolled.
Byreceivingfeedbackfromamotionsensor(encodertachometer,etc.),theangularpositionand
thespeedofthemotorcanbecontrolled.
Thedriveunitofadcservomotorprimarilyconsistsofadriveramplifier(commerciallyavailable
amplifiersarelinearamplifierorpulse-widthmodulation,i.e.,PWM,amplifier),withadditional
circuitryandadcpowersupply.
Thedriveriscommandedbyacontrolinputprovidedbyahostcomputerthroughaninterface
(input/output)card.
WORKING

72. CONTROLLER OF A DC
SERVOMOTOR

73. Also,thedriverparameterslikeamplifiergainsaresoftwareprogrammableandcanbesetbythehost
computer.
Thecontrolcomputerreceivesafeedbacksignalofthemotormotion,throughtheinterfaceboard,and
generatesacontrolsignal,usingasuitablecontrollaw
Thesignalisthenprovidedtothedriveamplifier,againthroughtheinterfaceboard.
AninterfaceboardorDataAcquisition(DAQ)cardisahardwaremodulewithDigital-to-Analog(DAC)
andAnalog-to-Digital(ADC)capabilitiesbuilt-in.Thesearegenerallypartsofarobot’scontrolsystem.
WORKING

74. Thefinalcontrolofadcmotorisaccomplishedbycontrollingthesupplyvoltagetoeitherthearmature
circuitorthefieldcircuit.
Adissipativemethodofachievingthisinvolvesusingavariableresistorinserieswiththesupply
sourcetothecircuit.
Thismethodhasdisadvantagesofhighheatgeneration,etc.Instead,thevoltagetoadcmotoris
controlledbyusingasolid-stateswitchknownasathyristortovarytheofftimeoffixedvoltagelevel,
whilekeepingtheperiodofpulsesignalconstant.
WORKING

75. PWM
Specifically, the duty cycle of a pulse signal is varied,. This is called pulse-width modulation or
PWM.

76. DC SERVOMOTOR AND ITS DRIVER
CONTROLLER

77. STABLE AND UNSTABLE OPERATING
POINTS OF A DC MOTOR

79. AC MOTORS

80. AC MOTORS
Untilrecently,ac(alternatingcurrent)motorshavenotbeenconsideredsuitableforrobots
becauseoftheproblemsinvolvedincontrollingtheirspeeds.
Initssimplestform,anacmotorconsistsofexternalelectromagnetsaroundacentralrotor,
butwithoutanyformofmechanicalswitchingmechanismfortheelectromagnets.

81. AC MOTORS
However,becausealternatingcurrent(suchasthemainselectricitysupply)isconstantlychanging
polarity(firstflowingoneway,thentheoppositeway,severaltimesasecond,e.g.,50inIndia,and
60intheUSA),itispossibletoconnecttheacsupplydirectlytotheelectromagnets.
Thealternatingreversalofthedirectionofcurrentthroughthecoilswillperformthesametaskof
polaritychanginginacmotors.
Themagneticfieldofthecoilswillappeartorotate(almostasifthecoilsthemselveswerebeing
mechanicallyrotated).

82. TYPICAL ADVANTAGES OF AN
AC MOTOR OVER ITS DC
COUNTERPART
Cheaper.
Convenient power supply.
No commutator and brush mechanism. Hence, virtually no electric spark
generation or arcing (safe in hazardous environment like spray painting and others)
Low power dissipation, and low rotor inertia.
High reliability, robustness, easy maintenance, and long life.
Some of the disadvantages are the following:
Low starting torque.
Need auxiliary devices to start the motor.
Difficulty of variable-speed control or servo control unless modern solid-state and variable-frequency drives
with field feedback compensation are used.

83. Duetothecomplexityinspeedcontrolofanacmotor,aspeed-controlleddcdrivegenerally
worksoutcheaperthanaspeed-controlledacdrive,thoughthepricedifferenceissteadily
droppingasaresultoftechnologicaldevelopmentsandthereductioninpriceofsolid-state
device

84. Alternating current (ac) motors can be classified into two groups, single phase and poly-
phase, with each group being further subdivided into induction or asynchronous and
synchronous motors.
Single-phase motors tend to be used for low power requirements while poly-phase motors are
used for higher powers.
Induction motors tend to be cheaper than synchronous motors and are thus very widely used.
TYPES OF AC MOTOR

85. TYPES OF AC MOTORS
Single-phase Squirrel-cage Induction Motor
Three-phase Induction Motor
Synchronous Motor
ACServomotor

86. SINGLE-PHASE INDUCTION MOTOR

87. SINGLE-PHASE SQUIRREL-CAGE INDUCTION MOTOR
Itconsistsofasquirrel-cagerotor,thisbeingcopperoraluminumbarsthatfitintoslotsinendringsto
formcompleteelectricalcircuits,asshowninFig.3.20.Therearenoexternalelectricalconnectionstothe
rotor.
Thebasicmotorconsistsofthisrotorwithastatorhavingasetofwindings.
Whenanalternatingcurrentpassesthroughthestatorwindings,analternativemagneticfieldis
produced,whichappearslikearotatingmagneticfield.
Therotatingfieldinthestatorinterceptstherotatingwindings,therebygeneratinganinducedcurrent
duetomutualinductionortransformeraction(hence,thenameinductionmotor).
Theresultingsecondarymagneticfluxinteractswiththeprimary,rotatingmagneticflux,thereby
producingatorqueinthedirectionofrotationofthestatorfield.
Thistorquedrivestherotor.
Astherotorspeedincreases,initiallythemotortorquealsoincreasesbecauseofsecondaryinteractions
betweenthestatorcircuitandtherotorcircuiteventhoughtherelativespeedoftherotatingfieldwith
respecttotherotordecreases,whichreducestherateofchangeoffluxlinkageand,hence,thedirect
transformeraction.

88. SINGLE-PHASE INDUCTION MOTOR

89. Forasingle-phasesupply,whentherotorisstationaryinitially,theforcesonthecurrent-carrying
conductorsortherotorinthemagneticfieldofthestatoraresuchastoresultinnonettorque.
Hence,themotorisnotself-starting.
Anumberofmethodsareusedtomakethemotorselfstartingandgivethisinitialimpetustostartit.
Forexample,toprovidethestartingtorque,mostsingle-phasemotorsaveamainandauxiliarywinding.
Theauxiliarywindingcurrentfromthemainwindingisphase-shifted.
Connectingacapacitorinserieswiththeauxiliarywindingcausesthemotortostartrotating.
SINGLE-PHASE INDUCTION MOTOR

90. SINGLE-PHASE INDUCTION MOTOR
Therotorrotatesataspeeddeterminedbythefrequencyofthealternatingcurrentappliedtothestator.
Foraconstantfrequencysupplytoatwo-polesingle-phasemotor,themagneticfieldwillalternateatthis
frequency.
Thisspeedofrotationofthemagneticfieldistermedsynchronousspeed.
Therotorwillneverquitematchthisfrequencyofrotation,typicallydifferingfromitbyabout1to3%.
Fora50Hzsupply,thespeedofrotationoftherotor,i.e.,wm,willbealmost50revolutionspersecond.

91. THREE-PHASE INDUCTION MOTOR

92. THREE-PHASE INDUCTION MOTOR
AsshowninFig.3.21(a),itissimilartothesingle-phaseinductionmotorbuthasastatorwiththree
windingslocated120°apart,eachwindingbeingconnectedtooneofthethreelinesofthesupply.
Becausethethreephasesreachtheirmaximumcurrentsatdifferenttimes,themagneticfieldcan
beconsideredtorotateroundthestatorpoles,completingonerotationinonefullcycleofthe
current.
Therotationofthefieldismuchsmootherthanwiththesinglephasemotor.
Thethree-phasemotorhasagreatadvantageoverthesingle-phasemotorinbeingself-starting.
Thedirectionofrotationisreversedbyinterchanginganytwoofthelineconnections,thuschanging
thedirectionofrotationofthemagneticfield.

93. SYNCHRONOUS MOTOR

94. SYNCHRONOUS MOTOR
Asynchronousmotorhasastatorsimilartothosedescribedaboveforinductionmotors,buta
rotorisapermanentmagnetasshowninFig.3.21(b).
Themagneticfieldproducedbythestatorrotatesandsothemagnetrotateswithit.
Withonepairofpolesperphaseofthesupply,themagneticfieldrotatesthrough360°inone
cycleofthesupplyandsothefrequencyofrotationwiththisarrangementisthesameasthe
frequencyofthesupply.
Synchronousmotorsareusedwhenaprecisespeedisrequired.Theyarenotself-starting
andsomesystemhastobeemployedtostartthem.

95. AC SERVOMOTOR

96. AC SERVOMOTOR
Generally,amodernservomotorreferstoanacpermanentmagnetsynchronousservomotor.
Itconsistsofastatorwindingplusrotorwithfeedbackunitslikeencoders,resolvers,etc.
Thesemotorshavetypicaladvantagesofacmotors.
Speedcontrolofacmotorsisbasedontheprovisionofavariablefrequencysupply,sincethe
speedofsuchmotorsisdeterminedbythefrequencyofthesupply.

97. AC SERVOMOTOR
Inprinciple,thehigherthefrequencyofthealternatingcurrentappliedtothemotor,thefaster
itwillrotate.
Providingvaryingfrequencysuppliestoanumberofaxisdrivessimultaneouslyhasbeen,until
recently,largelyimpractical.
Insomespecialcases,e.g.,wound-rotoracinductionmotors,speedcanbecontrolledby
accessingtherotorcircuitwheredifferentvaluesofresistancecanbeinsertedintherotor
circuit.
Electromagneticisusedtoprovideregenerativebrakingtocutdownthedecelerationtimes,
andminimizeaxisoverrun.
Manyindustrialrobots,e.g.,KUKAKR-5,useacservomotorstoday.

98. Alternating current (ac) motors can be classified into two groups, single phase and poly-
phase, with each group being further subdivided into induction or asynchronous and
synchronous motors.
Single-phase motors tend to be used for low power requirements while poly-phase motors are
used for higher powers.
Induction motors tend to be cheaper than synchronous motors and are thus very widely used.
LINE ACTUATORS

99. LINEAR ACTUATORS

100. LINEAR ACTUATORS
Linearactuatorslikesolenoidsareusedwidelyinroboticandotherautomationapplicationsforon-
offofthegripperandotherdevices.
Electricallypoweredstepperanddc/aclinearactuatorscanalsobeusedinmotiongenerationof
Cartesianrobots,etc.
Asolenoidshownhasacoilandasoft-ironcore.
Whenthecoilisactivatedbyadcsignal,thesoftcorebecomesmagnetized.
Thiselectromagnetcanserveason-off(push-pull)actuator.Solenoidsareruggedandinexpensive
devices.
Commonapplicationsofsolenoidsarevalveactuatorsmechanicalswitches,relays,etc.

101. LINEAR ACTUATORS
Mostcommonlygeneratedlinearmotionsarewiththehelpofanelectricallypoweredrotarymotor,ascoupled
withtransmissionmechanismslikenutandball-screw,cam-follower,rack-and-pinion,etc.
Thesedevicesinherentlyhaveproblemsoffrictionandbacklash.
Additionally,theyaddinertiaandflexibilitytothedrivenload,therebygeneratingundesirableresonancesand
motionerrors.Properinertiamatchingisalsoessential.
Inordertoavoidtheabovedifficultiesofusingatransmissionsystem,directrectilinearelectromechanical
actuatorsaredesirable.
Theycanbebasedonanyoftheprinciplesmentionedfortherotaryactuators,i.e.,stepperordcoracmotors.In
theseactuators,flatstatorsandrectilinearlymovingelements(inplaceofrotors)areemployed.
Thesetypesofactuatorsarealsoreferredtoaselectriccylindersinlinewithhydraulicorpneumaticcylindersthat
areexplainednext