Table1.DCMotorDesign Overview DCMotors Design Characteristics Brushed DCMotors Brushless DCMotors Conventional DCStepper Motors HybridDCStepper Motors Specialty Brushless DCMotors Toroidal Brushless DCMotors Ironless Brushless DCMotorsBrushes control powerdistribution inthewindings (internally commutated.) Simple DCsupplyoperation. Brushlifelimited. Twoorthreephases. Nobrushes. External driveelectronics andposition sensing forcommutation arerequired. Versatile overbroadspeed-torque ranges. Motorcogging anddamping arerequired. Rotates indiscrete stepswithout external commutation sensing. Driveelectronics required. Performance analysis iscomplex. Unique lamination pattern permits asmallstepangle(typically 1.8degrees) in asmalldiameter. Requires driveelectronics butnocommutation feedback is required. Custom variations ofBDCMthatprovide special performance suchasfor extremely lowtorquerippleorlowmotorlossesathighspeeds. Highmotor constant issacrificed. Magnetic circuitironringiswrapped withwindings. Nocogging. Highmotor constant issacrificed forsmooth rotation orlargelimitedangleofrotation. Difficult towind. Non-metallic statorslots,largeairgap,andsmooth rotation. Magnetic circuit ironringliesoutside coilwindings. Highmotorconstant issacrificed for smooth torqueandlowlosses. Understanding andSpecifying DCMotors Characteristics ofDCMotors Whileitiscommonly understood thatelectric motorsconvert electrical energy intoarotating mechanical output, itislesscommonly understood howthisisaccomplished withDCmotors. NotunliketheirAC counterparts, theDCmotors requireanintricate assembly ofmagnetically permeable materials, electrical windings, andconductive media. DCmotors, however, utilizepermanent magnet fieldstosetthestage fordynamic interaction whendirectcurrent electricity isapplied tothemotorwindings. Energized windings produce opposing magnetic fieldsthatcreatearotating forceortorque. Powermustbecorrectly sequenced inthemotorwinding phases toperpetuate therotation through theprocess ofcommutation. Oneofthetwomembers, eitherthewoundmember orthepermanent magnet member, freelyrotatesand transmits torquetotheexternal load.DCmotors generally sharecommon design, specification, and operation terminology todescribe howthisoccurs. Examination ofthevarious typesofDCmotors willclarifywhichdesigns havewound stators with magnetized rotorsandwhichdonot.Generally, thebrushed DCmotoristheexception, withitswound member rotating insideofthemagnetized structure. Forthemajority ofDCmotors, thelaminated and wound member iscommonly referred toasthe"stator", anditisusually thestationary non-rotating member surrounding therotating member. Thepermanent magnets arelocated ontheinner,rotating member referred toasthe"rotor."Additional circuitry mayberequired toassistwiththecommutation or distribution ofpowerintothemotorphases intheformofacoppercommutator, Halleffectsensors, ora resolver. Stators (oranarmature ifitisabrushed motor)relyonabonded stackofmagnetically permeable metal laminations thatservetwofunctions. First,theyareslottedtopermittheinstallation andretention ofthe electric motorwindings. Second, lamination stacksaremagnetically permeable havingamaterial thatis wellsuitedforcarrying magnetic fluxproduced bythemagnets. Ifthemotorlamination isdesigned correctly, itwillproperly conduct andchannel themagnetic fluxwhilemaintaining sufficient clearance for thewindings. Thestackislaminated ratherthanmadeasasolidtorestrictthecirculation ofelectrical currents thatareinduced byrotation ofthemagnet fields.Thesespeed-dependent, "eddycurrents" decrease thenetoutputtorqueandwastefully consume electrical power. Bylaminating thestack,the insulation between thelaminations servesasbarriers torestricteddycurrent flowandassociated losses. Byadjusting thelamination thickness andmaterial, thelossescanbefurtherreduced depending onthe criticality oftheapplication. NASA/CP--2002-211506 90