all motors are stepping motors

8/4/2019 All Motors Are Stepping Motors

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AllMotorsareSteppingMotors

In1938,GeneralElectricbeganproductionofatwophasesynchronousinductionmotorwhich,at60Hz,ranat

75RPM. Thelowspeedresultedfromtherebeingadifferentnumberofrotortostatorpolesorteethwhichmadethe

motoragoodbidirectionalcontrolmotor. ThismotorwasusedbySuperiorElectricCompanyofBristol,CTforrunning

powerdrivenautotransformersusedtodimlightsinauditoriumsandsimilarapplications. GeneralElectricceased

productiononthismotorinthemid1950sbecausesaleswerenothighenoughtosupportproduction.

SuperiorElectricengineersredesignedthemotortorunat72RPMon60Hzandbegantoconsiderusingafour

stepDCvoltagesequencetohavethemotormoveinincrementsof1.8degreesperstepor200stepsperrevolution.

Microswitches,magneticreedswitchesandrelayswereusedtodemonstratethebasicoperationatengineering

conferencesandseminars. Intheearly1960stransistorsmadeoperatingthesteppingmotorpracticalandapplications

quicklydeveloped. Thehybridsteppingmotorcamealive.

Theprincipleofthesteppingmotor

Ifyoucreatetwomagneticfieldsofoppositepolarities,theywillattracteachothercreatingmotion. Ifoneof

themagneticfieldsisfixedonashaft,youhaverotarymotionofsomeangle. Now,tocontinuethisrotarymotion,you

havetocreateanewmagneticfieldatadifferentposition.

Figure 1

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Figure1illustratesatypicalstepsequenceforatwophasemotor.InStep1phaseAofatwophasestatoris

energized.Thismagneticallylockstherotorinthepositionshown,sinceunlikepolesattract.Instep2,phaseAisturned

offandphaseBisturnedon. Therotorrotates90clockwise.InStep3,phaseBisturnedoffandphaseAisturnedon

butwiththepolarityreversedfromStep1.Thiscausesanother90rotation.InStep4,phaseAisturnedoffandphaseB

isturnedon,withpolarityreversedfromStep2.Repeatingthissequencecausestherotortorotateclockwisein90

steps. SeeFigure2foranelectricalrepresentationofthecurrentflowinthetwophases.

Thereisanelectronicdrivethatsendsthe

electricalpulsestothetwophasesatthe

appropriatetimetocreaterotation. Inthis

example,therotationis90degreesper

pulseorstep. Inpractice,themotorshavemorepolestocreatesmallerstepanglessuchas15degrees,7.5degreeso

1.8degreesperstep. Thetermsteppingmotortraditionallyreferstoamotorthatrunsfrompulsesfromanelectron

drive. Bysendingthemotoraspecificnumberofpulses,youwouldknowtherotorpositionatanytime. Themovement

createdbyeachpulseispreciseandrepeatable,whichiswhysteppermotorsaresoeffectiveforpositioningapplications.

However,allmotorsarereallysteppingmotors. Theonlydifferenceisthesizeofthestepangleandhowtheswitching

ofthepulsesiscreated. ManyACmotorscanberunassteppingmotors. Thetwophasealternatingcurrent90

electricaldegreesapart,createsafourstepsequence

justliketheelectronicdriveexceptthatitisasinewave

instead

of

a

square

wave.

r

ic

(SeeFigure3)

Thelimitationisthatyouarelimitedtoonlyonespeed,

the60hertzoftheincomingpower. Inthiscase,since

themotorrunssynchronouslywiththeincoming

frequency,itisnowcalledasynchronousmotornota

steppingmotor. However,itisthesamemotor. Nothinghaschanged. Themotorcanalsobemadetorunonasingle

phaseACcurrent. Aphaseshiftingcircuitisusedtosplitthesinglephaseintotwophases.

VariableReluctanceMotors

Avariablereluctancemotorisasteppingmotorthatdoesnotuseapermanentmagnet. Astepisachievedby

theprinciplethattherotorwillrotatetominimizethereluctancepathofamagneticcircuit. (SeeFigure4)


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Figure 55

Inthefirststep,pole1ismagnetizednorthandpole4ismagnetizedsouth. Instep2,poles2and5areenergizedand

poles1and4areturnedoff. Therotorrotates60degrees. Instep3,poles3and6areenergizedandsoon.

Inthefirststep,pole1ismagnetizednorthandpole4ismagnetizedsouth. Instep2,poles2and5areenergizedand

poles1and4areturnedoff. Therotorrotates60degrees. Instep3,poles3and6areenergizedandsoon.

Oneofthedisadvantagesofasteppingmotor

istheringingoftherotorwhenyoutakea

singlestep. (SeeFigure5) Therotordoesno

stopinstantlybutoscillatesaboutitsfinal

positionforseveralmilliseconds. Ifasecon

stepisintroducedwhiletherotorismovin

anegativedirection,youhaveaconflictand

lossoftorque. Inextremecases,youcan

causearesonanceandtherotorwillvibrateor

Oneofthedisadvantagesofasteppingmotor

istheringingoftherotorwhenyoutakea

singlestep. (SeeFigure5) Therotordoesno

stopinstantlybutoscillatesaboutitsfinal

positionforseveralmilliseconds. Ifasecon

stepisintroducedwhiletherotorismovin

anegativedirection,youhaveaconflictand

lossoftorque. Inextremecases,youcan

causearesonanceandtherotorwillvibrateor

t

d

gin

a

price. Inabrushtypeorbrushlessmotor,theswitchingis

doneatexactlytherighttimeandthisproblemisminimized.

t

d

gin

a

price. Inabrushtypeorbrushlessmotor,theswitchingis

doneatexactlytherighttimeandthisproblemisminimized.

stop.stop.

TherearewaystodampenthisoscillationbutthisincreasestheTherearewaystodampenthisoscillationbutthisincreasesthe

Figure 4


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BrushtypeDCmotorshaveseveralcoilswoundonarotororarmature. Thesecoilsareconnectedtoa

commutator. Thecommutatorconsistsofcoppersegments. (SeeFigure6) Inthisexampleshown,therearetwelvecoils

soonestepis or30degrees. Theswitchingisdonemechanicallyinsteadofelectronicallybymeansofa

commutatorwith24coppersegments. Thecoilsareconnectedtothecoppersegments. Twobrushes,180degreesapart

connecttotheappropriatecoilwhichrotatestherotor30degrees. (SeeFigure7)

Thisrotationcausesthebrushesto

connecttothenextcoilwhichcauses

another30degreerotation. Thisin

effect,causescontinuousrotation.

ThismotoriscalledabrushtypeDC

motor. Thefieldcanbeeither

permanentmagnetsorawoundfield.

Ifitisawoundfield,itiscalleda

universalmotorbecauseitcanrunon

alternatingcurrentordirectcurrent.

Another

version

of

this

motor

replaces

the

commutator

and

brushes

with

hall

cells

and

a

magnetic

ring

magnetized

with

manypoles. (SeeFigure8) Thehallcellisamagnetic

switchwhichswitchesoninthepresenceofasouth

magneticfield. Thisenergizesthepropercoiltotakethe

nextstep.

Figure 6

Figure 7

Figure 8

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ThisiscalledabrushlessDCmotor. AphotoofabrushlessDCmotorsisshowninFigure9.

Figure 9

InductionandSquirrelCageMotors

Inductionandsquirrelcagemotorshavebutasingleturnelectricalconductorintherotor. Theygenerallyuse

copperbarsorcastaluminum. Theelectriccurrentisinducedintotherotorfromthestatorfield. TheseareACmotors

andsteppingisderivedfromthelinefrequency. Therearemanyotherspecializedmotors. However,theendresultis

stillthesame;toobtainrotarymotion,youhavetocreatearotatingmagneticfield.

Everyonceinawhileyouhearthatthismotorisbetterthananothermotor.

Thereisnosuchthingasthebestmotor. Ifthisweretrue,therewouldonlybeonemotoronthemarket. Whenyou

haveanapplication,youhavetodeterminewhichisthebestmotorforthatapplication. Youwouldhavetoconsider

suchthingsascost,dependability,lifeandmanyotherfactors.

Insummary,therearefourwaystoobtainstepsformotors;electroniccommutation,mechanical,magneticand

electrical. Hybrid,permanentmagnetandvariablereluctancesteppingmotorsreceivetheirstepsfromanelectronic

drive. ThebrushtypeDCmotorreceivesitsstepsbyamechanicalcommutatorandbrushes. Thisassemblyconverts

directcurrentintosteps. ThebrushlessDCmotorusesmagnetichallcellsandmagnetstoconvertdirectcurrentinto

steps.

AC

motors

such

as

the

induction

motor,

squirrel

cage

motor,

hysteresis

synchronous

motor

and

many

other

motorsusethelinefrequencyof60hertzfortheirsteps. Eachalternationofthesinewaveisastep.

Inthe1930sand1940s,HaydonmanufacturingCompanywasmakingclockmotorsandtimingmotors. During

WorldWarII,theyweremakingDCmotorsusedinaircraft,tanks,submarinesandavarietyofportablemilitary

equipment. Inthe1960sHaydonbegantomakesteppingmotors. Today,HaydonKerkMotionSolutions,Inc.isa

recognizedworldwideleaderinpermanentmagnetandhybridsteppingmotorsandlinearactuators. Theyhaveseveral

patentsrelatedtosteppingmotors.


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all motors are stepping motors

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