commutation scheme,theBDCmotorwillbehaveirregularly.Twoapproaches forBDCcommutation are common: (1)Six-stepperelectricalcyclemethodusingthreeHalleffectdevicesforthreephasemotors(Hall effectsaresolidstatemagneticfieldsensorsusedtoindicatetherotorposition.WhenthreeHall effectsareusedtogetherandproperlylocatedtosensetherotormagnets,theycanindicaterotor positionbasedontheircombinedstateinformation. Halleffectsusuallyarelatchingdevicesthat changebetweenhighorlowstatesdependingonthemagneticfieldpresent.Absoluterotorpositionis notrequiredforcommutation.) (2)Continuous commutation methodusingaresolverfortwoandthreephasedevices.(Thecontinuous sine/cosineoutputfromaresolver isusually converted byanalogtodigitalconverters intoadata stream theygivesconstant rotorposition datanecessary forcommutation. Withthedigitaldata,the number ofmotorphases allowable isnotfixedtotwo.BeforemorerecentAtoDconverters, the resolver electrical "speed" hadtobematched tothemotorpolecount.Anabsolute position single speedresolver isnotrequired.) IfaBDCmotorisback-driven, usinganexternal motor,thenitbecomes agenerator. Examination ofthe back-driven motorvoltage reveals several keypieces ofinformation abouthowwellthemotorwill produce torque, achieve speed, andcommutate. Thevoltage produced iscalledthebackEMF (electromotive force),andifviewed onanoscilloscope, generally approximates oneoftwostylesof waveform: trapezoidal orsinusoidal. ThebackEMFvoltage waveform canbemeasured onan oscilloscope toindicate whatthebackEMFconstant "Kb"is(voltsrequired toattainrotational speed) and whatthemotortorquesensitivity "Kt"is(torque perunitcurrent.) Byexamining thewaveform, theshape maybedescribed asflat-topped (trapezoidal) orsinusoidal. Thisshapeisimportant, because the trapezoidal backEMFlendsitselftocommutate withasix-step, three-phase brushless dcmotordriver. Themoresinusoidal motorbackEMFlendsitselftofunction withabrushless DCsine/cosinedriver.The motormagnet andwinding designs playimportant rolesinshaping thebackEMF. Thebrushless motorgenerally hasmagnets mounted ontherotorassembly thatlieswithinthewound statorassembly. SinceBDCmotors comeinmanysizesanddesignschemes, itmustbeunderstood that quiteabitofvariation canexistinrotorconstructions. Themagnets typically usedinnewerdesigns are radially oriented rareearthcobaltanddonotrequire any"keepering" likealnicomotordesigns. Therare earthcobaltmagnet material isgenerally resistant totemperature changes andprovides astable magnetic fieldforwidelyvarying spacetemperatures. BDCrotorassemblies mayhavebandsoverthe magnets duetocontainment concerns ormagnet chipping concerns associated withinstallation. ABDCstatoristheouter,stationary member thatcontains copper windings inserted intolamination stacks. Thelamination material, thickness, andnumber ofslotsarefundamental tothestatordesign. Certain lamination materials, suchasthesiliconsteels,arebetteradapted tohandling highermagnetic fluxdensities andsoaresuitable forhighmotorconstant, smallairgapdesigns toobtainthemosttorque withminimal envelope andpower. Laminations forlow-loss applications generally donothandle high magnetic fluxdensities aswell,butarenecessary forhighspeed, low-loss motors. Thinner laminations alsowillreduce motorlosses. Thenumber oflamination slotsisafunction ofthenumber ofpoles,butit factors intothemagnetic designsignificantly. Thelargerthelamination slotareaistoaccommodate wire conductors, thelesstheavailable metaltocarryfluxatareasonable magnetic fluxdensity. Hence, the amount ofcoppervolume mustbeconstrained aswell. ABDCmotorforhightorqueoutputwithahighmotorconstant willgenerally becommutated withthesixstepmethod andHalleffectdevices. Itisexpected thathigh-energy product rareearthcobaltmagnets andahighpolecountwillbeused.Further, thelamination material willlikelynotbealowlossgradebuta gradesuitable forhandling higherlevelsofmagnetic fluxdensity produced bythehigh-energy product magnets. Thepolecountwilldrivethenumber oflamination slotstothepractical limitation thatthemotor diameter willallowandstillleavesignificant volume forwinding copper. Windings willusecoilsthatspan thedistance ofadjacent polecenterlines (fullpitched coils.)Theairgapwillalsotendtobeassmallas NASA/CP--2002-211506 95