DCmotorwindingsvaryconsiderably dependingonthetypeofmotor.Windingsusuallyhaveasymmetry thatisbasedaroundthenumberofmotorpolesanddesirednumberofmotorphases.Inthecaseof stepperandbrushlessDCmotors,windingsmaybetwo,three,orothernumbersofphases,buttwoor threeistypical.Eachphaseconsistsofanumberofcoilsthatisbasedonthenumberofmotormagnets. Windingcoilspan,turncount,andwiresizeplaycriticalrolesinprovidingthedesiredperformance. Torquecapability,speedrange,andwindinginductance aredirectlyrelatedtothenumberofturnsper coilandcoilplacement. Figure1showstwocommonphaseandconnection diagrams,"wye"and"delta", usedforthreephasebrushlessDCmotorsandsteppermotors. A WyeConnectior. DeltaConnection Figure1.DCMotorWinding PhaseDiagrams Motorrotorsusually contain thepermanent magnets. Anevennumber ofmagnets isalways placedon thecircumference oftherotoralternating between NorthandSouthpoles.Olderdesigns useAlnico magnet materials polarized axiallyalongtherotorcircumference. Newermotordesigns usemagnets madefromrareearthmaterials suchassamarium cobalt,andtheyareusually polarized perpendicular to therotorcircumference. Theperpendicularly oriented or"radial" magnets forcemagnetic fluxdirectly into thespacethatseparates therotorouterdiameter andthestatorinnerdiameter forrotational clearance. Theclearance airgapbetween therotorandstatorisacriticalfeature thataffectspractically everyaspect ofthemotordesign. Underthebroadcategory ofDCmotors, itshouldbementioned thatsomemotors arecharacteristically DCbutinfactoperate fromalternating sinusoidal drivers. Thisapparent conflict isreconciled iftheissue ofmotorcommutation isexamined. WhilemostDCmotortypesrelyoncommutation thatbringsrotation andtorquethrough discrete phaseswitching, certain typesdonot.Sine/cosinedrivenbrushless and stepper motors aredesigned withpermanent magnets toproduce torquewhencontinuously alternating sineandcosine currents areapplied totwoorthreephases. Withtheproper continuous position feedback fromanintegral resolver, thesemotorsarecapable ofproducing verysmooth, almostripplefree torqueoutput. ThemajorissueforDCmotorsforspaceapplications istounderstand thattherearetradeoffs between typesofmotors andtheirfeatures. Certain typesofdesigns lendthemselves morereadily tomeeting certaintypesofrequirements, whichisexplained further. And,fornewmotordesigns, therearetradeoffs thatcanbemadewithintherealmofslot-pole combinations, materials, airgap,andwindings tooptimize thedesign foraspecific application. Before examining specific motortypesandtheirfeatures, itis necessary tounderstand thegeneral terminology usedtospecify theirelectrical, magnetic, and mechanical characteristics. Electromechanical DCMotorSpecifications Itisimportant tounderstand keymotordesign differences andhowtheprimary requirements shouldbe specified foreveryapplication. Asafoundation forthis,therearecommon designfactorsassociated with allDCmotors regardless oftheiruniquedesignfeatures. Thecommon factors maybederived fromthe factthatDCmotors usepermanent magnets, magnetic irontoestablish fluxpaths,andcopper windings withmultiple phases. Hence, thecommon terminology forDCmotors isshowninTable3.Whilethereis somevariation withintheindustry intermsofsymbols andunits,theconstants applyuniversally. Manufacturer's literature isreadilyavailable thatexplains motorterminology inmoredetail. NASA/CP--2002-211506 91