R2=R1•(234.5+T2)/(234.5 +T1) Eq.(2) where T1istheambienttemperature ofthemotorwindingsindegreesC,typicallytakenas25degreesC T2isthemodifiedtemperature ofthemotorwindingsindegreesCtakingintoaccountexternal temperature andinternalmotorheating. R1isthemotorresistanceatambienttemperature indegreesC R2isthemotorresistanceatthemodifiedtemperature indegreesCtakingintoaccountexternal temperature andinternalmotorheating. Eq.(1)forBrushed DCMotors Thebasicperformance Equation (1)applies, butthereisaspecial setofconcerns duetomechanical brushaction. Bothminimum andmaximum supplyvoltages mustbeobserved. Itisimportant toassure sufficient voltage ispresent foradequate contact androtation. Thebrushes require asmallvoltage associated withtheirelectrical contact, called"Brush Drop"thatsubtracts fromavailable motor performance voltage. Intheothercase,themaximum voltage canbedetrimental duringcommutation depending onthecommutator bardesign, condition ofthecopper commutator, andtheambient air pressure. Theresultofexcessive voltage canbedamage tothemotorortotalmotorfailure. Because the brushes mechanically contact themotorcopper commutator surface, iftheactionisnotsmooth, the brushes can"bounce". Brushbounce, whichisobservable onanoscilloscope, willreduce theeffective voltage applied tothemotorcausing areduction inspeed. Eq.(1)forBrushless DCMotors Equation (1)mustbemodified toinclude running lossesduetohigh-speed effects. Usingeitherempirical dataoraviscous lossconstant, thetorque-cwouldbeincreased bytheamount oftheviscous torqueat thehighspeed. Usually, thisisinresultofhysteresis andeddycurrent lossesduetochanging magnetic fieldsinthemotorstatorlamination iron.Tominimize theselosses, itbecomes adesign issueof lamination material selection andthemagnetic fluxdensity permitted withinthelaminations. Rotor windage losstorquemayneedtobeaddedatveryhighspeeds. Insomecasesofhigh-speed operation, themotorinductance mayrequireadditional voltage. Eq.(1)forStepper Motors Equation (1)mustbemodified similarly tothebrushless DCmotor,butthereareadditional concerns. The stepper motorandspecific application system dynamics mustbeincluded inevaluation ofastepper motor.Astepper motor's response isnotasindependently predictable asabrushless DCmotorassessment ofthesystem mechanical anddynamic performance mustbemade.Several techniques may beused,sometimes together, toassess stepper motorperformance inthesystem application. Essentially, sincethestepper motorrotorposition ischanged byshortpowerinputsthatleavetherotorto settleintoitsnewposition onitsown,thecombined motorandsystem dynamic effectsareanimportant issue.Thetorqueoutputrequired tomovetheloadcannot simplybepredicted usingKt•I.Additional torquemargin isrequired toaddress thespecific application concerned. DCMotorClassifications andDesign Features Brushed DCMotors Brushed DCmotors, asthenameimplies, contain graphite basedelectrically conductive brushes to conduct theDCsupplypowertothemotorwindings ontheinner,rotating member. Notonlydothe brushes provide themeans ofcontact fromthestationary portion ofthemotortotheinsiderotorwith windings, butthemanner inwhichcontact ismadeservestodistribute orcommutate themotorpower. Thisisaccomplished because themotorbrushes "ride"onasystem ofelectrically isolated copper bars thatformthemotorcommutator. Thebarslaysidebysideandaremachined toprovide asmooth, round surface forthebrushes. Eachbarisjoinedtoadifferent portion ofthemotorwinding, sothatasthe brushes change barsduringmotorrotating, theDCsupplyiscarefully switched acrossdifferent bars.This means ofcommutation required noexternal switching ofpowertomaintain rotation. Onlyasimple application ofaDCsourceisrequired acrosstwomotorleads. NASA/CP--2002-211506 93