Permanent Magnet (PM) DC Gearmotors & Motors - Bodine Electric
Transcript of Permanent Magnet (PM) DC Gearmotors & Motors - Bodine Electric
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Permanent Magnet (PM) DC Gearmotors & Motors
www.bodine-electric.com • 800-7BODINE (800-726-3463)
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
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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.
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Torque
Figure 3
Spee
d
V1 V2 V3 V4 V5
CutawayviewofaPMDCBodinetype33A-GBright-anglegearmotor
Fig.3:Atypicalfamilyofspeed/torquecurvesforaPMDCmotoratdifferentvoltageinputs,withV5>V4>V3>V2>V1.
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