Permanent Magnet (PM) DC Gearmotors & Motors - Bodine Electric
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Handbook Bodine ® www.bodine-electric.com • 800-7BODINE (800-726-3463) Permanent Magnet (PM) DC Gearmotors & Motors www.bodine-electric.com • 800-7BODINE (800-726-3463) Features & Benefits: • Continuous duty operation • DC power supply (battery or speed controls) • Reversibility at rest or during rotation with current limiting • Relatively constant and adjustable speed • Starting torque 175% and up of rated torque • High starting current, relative to full load running current Design and Operation: Permanent magnet DC (PMDC) mo- tors provide a comparatively simple and reliable DC drive solution in applications requiring high efficiency, high starting torque and a linear speed/torque curve. With the great strides made in ceramic and rare earth magnet ma- terials, combined with electronic control technology, the PMDC motor is a cost-competitive solution for adjustable speed applications - delivering significant performance in a relatively compact size. The single design feature which distinguishes the per- manent magnet DC motor from other DC motors is the replacement of the wound field with permanent magnets. It eliminates the need for separate field excitation and at- tendant electrical losses in the field windings. Advantages: Perhaps the most important advantage of PM field motors is their smaller overall size made possible by replacing the wound field with ceramic permanent magnets. The PM motor’s ring and mag- net assembly is consider- ably smaller in diameter than its wound field coun- terpart, providing substan- tial savings in both size and weight. See Fig. 1. And since the PMDC motor is not susceptible to armature reaction, the field strength remains constant. If we examine the field construction of the wound field DC motor versus the PMDC field motors, we can explain the differences in armature reaction and corresponding differences in speed/torque characteristics of the two motor types. The armature magnetizing force in a wound field construction “sees” a very high permeability (low re- luctance) iron path to follow. In the PM field design, this armature magnetizing force is resisted by the low permea- bility (high reluctance) path of the ceramic magnet, which tends to act as a very large air gap. The net result is that the armature cannot react with the field in a PMDC motor, thereby producing a linear speed / torque characteristic throughout its entire torque range. PMDC motors offer benefits in a number of ways: a) They produce relatively high torques at low speeds, en- abling them to be used as substitutes for gearmotors in many instances. PMDC motors operated at low speeds are especially useful where “backlash” and inherent mechanical “windup” of gearing in gearmotors can not be tolerated. It should be noted that if PMDC motors are continuously operated at high torque levels (above rated), they can generate serious overheating, or motor damage can result. b) The linear speed / torque curve of PMDC motors, cou- pled with their ability to be easily controlled electroni- cally, make them ideal for adjustable speed and servo motor applications. c) The linear output performance characteristics of PMDC motors also make it easier to mathematically predict their dynamic performance. See Fig. 3. The PMDC motor’s high starting torque capability can be a valuable asset in many “motor only” (non-gearmotor) applications as well as inertial load applications requiring high starting torque with less running torque. PMDC mo- tors function well as torque motors for actuator drives and in other intermittent duty applications. PM Field Torque Speed Speed/Torque curve of a PMDC motor Fig. 1: PM motor ring and magnet assembly © 2008 074 81082.A
Transcript of Permanent Magnet (PM) DC Gearmotors & Motors - Bodine Electric
HandbookB
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www.bodine-electric.com • 800-7BODINE (800-726-3463)
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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