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Permanent Magnet (PM) DC Gearmotors & Motors


Features & Benefits:• Continuousdutyoperation• DCpowersupply(batteryorspeedcontrols)• Reversibilityatrestorduringrotationwith

currentlimiting• Relativelyconstantandadjustablespeed• Startingtorque175%andupofratedtorque• Highstartingcurrent,relativetofullload

runningcurrent

Design and Operation: PermanentmagnetDC(PMDC)mo-torsprovideacomparativelysimpleandreliableDCdrivesolution in applications requiring high efficiency, highstartingtorqueandalinearspeed/torquecurve.Withthegreatstridesmadeinceramicandrareearthmagnetma-terials,combinedwithelectroniccontroltechnology,thePMDCmotorisacost-competitivesolutionforadjustablespeedapplications-deliveringsignificantperformanceinarelativelycompactsize.

The single design feature which distinguishes the per-manent magnet DC motor from other DC motors is thereplacementofthewoundfieldwithpermanentmagnets.Iteliminatestheneedforseparatefieldexcitationandat-tendantelectricallossesinthefieldwindings.

Advantages: Perhaps themost important advantageof PM field motors is theirsmaller overall size madepossible by replacing thewound field with ceramicpermanent magnets. ThePM motor’s ring and mag-net assembly is consider-ably smaller in diameterthan its wound field coun-terpart, providing substan-tialsavingsinbothsizeandweight.SeeFig.1.AndsincethePMDCmotorisnotsusceptibletoarmaturereaction,thefieldstrengthremainsconstant.

If we examine the field construction of the wound fieldDCmotorversusthePMDCfieldmotors,wecanexplainthe differences in armature reaction and correspondingdifferences in speed/torque characteristics of the twomotortypes.Thearmaturemagnetizingforceinawoundfieldconstruction“sees”averyhighpermeability(lowre-luctance)ironpathtofollow.InthePMfielddesign,thisarmaturemagnetizingforceisresistedbythelowpermea-bility(highreluctance)pathoftheceramicmagnet,whichtendstoactasaverylargeairgap.ThenetresultisthatthearmaturecannotreactwiththefieldinaPMDCmotor,therebyproducinga linearspeed / torquecharacteristicthroughoutitsentiretorquerange.

PMDC motors offer benefits in a number of ways:a)Theyproducerelativelyhightorquesatlowspeeds,en-

ablingthemtobeusedassubstitutesforgearmotorsinmanyinstances.PMDCmotorsoperatedatlowspeedsare especially useful where “backlash” and inherentmechanical“windup”ofgearingingearmotorscannotbe tolerated. It should be noted that if PMDC motorsarecontinuouslyoperatedathightorquelevels(aboverated),theycangenerateseriousoverheating,ormotordamagecanresult.

b)Thelinearspeed/torquecurveofPMDCmotors,cou-pledwiththeirabilitytobeeasilycontrolledelectroni-cally,makethemidealforadjustablespeedandservomotorapplications.

c)ThelinearoutputperformancecharacteristicsofPMDCmotors also make it easier to mathematically predicttheirdynamicperformance.SeeFig.3.

ThePMDCmotor’shighstartingtorquecapabilitycanbea valuable asset in many “motor only” (non-gearmotor)applicationsaswellasinertialloadapplicationsrequiringhighstartingtorquewithlessrunningtorque.PMDCmo-torsfunctionwellastorquemotorsforactuatordrivesandinotherintermittentdutyapplications.

PM Field

Torque

Speed/Torque curve of a PMDC motor

Spee

d

Speed/TorquecurveofaPMDCmotor

Fig.1:PMmotorringandmagnetassembly

©200807481082.A

ThesizereductioninPMDCmotorsisgenerallyaccom-plished without any significant change in the tempera-tureriseratingforagivenhorsepower.Infact,theelec-tricalefficiencyofthePMDCmotorisveryoften10%to15%higherduetotheeliminationoffieldcopperlosseswhich occur in wound field motors. PMDC motors canbe produced in TENV (totally enclosed non-ventilated)construction,eliminatingtheneedforfansandprovidingmuchgreaterapplicationflexibility.

Withtheirhigherinherentefficiency,PMDCmotorsandgearmotorsoffer lowercurrentdrain formoreefficientbattery operation in portable applications. The perma-nentmagnetsalsoprovidesomeself-braking(lessshaftcoast) when the power supply is removed. PMDC mo-torsrequireonlytwoleads(shunt-woundmotorsrequirefour). The leads can be reversed by simply changingthe polarity of the line connection. Dynamic braking isachievedbymerelyshuntingthetwoleadsafterdiscon-nectingthemfromthepowersource.PermanentmagnetDCmotorsalsoprovidesimilarperformancecharacteris-ticstoshunt-woundDCmotorswhenusedwithallcom-moncontrolmethods(exceptfieldweakening).

Design Considerations: While today’s ceramic mag-nets have properties which make them very reliable,

certaincharacteristicsofthesematerialsmustbethor-oughlyunderstoodifproperoperationofceramicmag-net PMDC motors or gearmotors is to be obtained. Atlowertemperatures(0°Candbelow),ceramicmagnetsbecomeincreasinglysusceptibletopermanentdemag-netizingforces.

Strongarmaturefieldscapableofproducingpermanentdemagnetizationof themagnets takeongreater impor-tance at lower temperatures. Therefore, special atten-tionmustbegiventooverloadcurrentconditionsinclud-ing“starting,”“lockedrotor”and“plugreversing”whenapplying PMDC motors to low temperature use. Plugreversingrequirescurrentlimiting,evenatnormaltem-peratures.

Thedesignofthemotor’spowersupplyisalsoimportant.PWMandSCRcontrolsaredesignedtoprovidecurrentregulatingand/orlimitingfeaturestoprotectthemotororgearmotor.TheactualapplicationparametersinvolvedvarywitheachparticularPMDCmotordesign,sincetheprotectionagainstdemagnetizationispartofthemotor’sdesignandmustbeconsideredaccordingly.Itisbesttoconsultthemanufactureriflowtemperatureuseorplugreversingiscontemplated.

As operating temperature increases, the residual orworkingfluxofPMDCmotorsdecreasesatamoderaterate.Thisfluxdecreaseismuchlikethedecreaseoffieldfluxstrengthinwoundfieldmotorsascopperresistanceincreaseswithtemperature.

Application information: Becauseof theirhighstartingtorquecharacteristic,caremustbeexercised inapply-ing PMDC gearmotors. A PMDC gearmotor applicationshouldbecarefullyreviewedforanyhighinertial loadsorhighstartingtorqueloads.Thesetypesofloadscouldcause the motor to transmit excessive torque to thegearheadandproduceoutputtorquewhichexceedsitsdesign(rated) limits.PWMorSCRspeedcontrolswithbuilt-incurrentlimitingcircuits,oroverloadslipclutchesare sometimes employed to protect gearing used withPMDCmotors.


Torque

Figure 3

Spee

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V1 V2 V3 V4 V5

CutawayviewofaPMDCBodinetype33A-GBright-anglegearmotor

Fig.3:Atypicalfamilyofspeed/torquecurvesforaPMDCmotoratdifferentvoltageinputs,withV5>V4>V3>V2>V1.

©200807481082.A