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1、LoadInertiaandMotorSelectionByJohnMazurkiewiczBaldorElectricConsidermatchingloadandmotorinertia.Mismatchcanextendthetimetospeed(accelerationtime),andexcessivemismatchesproducelessthanoptimalresponseandcanwastepower.InductionmotorsBecauseamotor’sspeedcanbevar
2、ieddoesnotmeanit’saviablecandidateforamotioncontrolpositioningsystem.Forexample,aninductionmotorandinvertercontrollercanrunfromabout300RPMtoabasespeedof1,800.Butpositioningsystemsrequirespeedsdowntozeroandoftenhigherthan1,800RPM.However,inductionmotorsareide
3、alforcentrifugalfans,conveyors,pumps,andmixersbecausetheyarerelativelyinexpensive,simpleandeasytouse,highlyreliable,andvibrationfree.Additionallyapeakoverloadcapacityof150%anddirectionreversalarepossible.ThecontrolshavepresetspeedsandprogrammableI/Os.Figure1
4、-InductionmotortorquevsspeedFigure2-MotortimetospeedPower(hp)Speed(RPM)1.5BrushlessServo(ferritemagnets)1800PeakDCServoVectorTorque1.0ConstantTorqueConstanthp0Speed(RPM)0Time(sec)BaseSpeed3.5xbaseSpeed0.050.100.200.300.401800RPM6300RPMThetorque/speedcurvefor
5、a1.0hpinductionmotorshowsThetimetoreachbasespeedvariesconsiderablyforvarious1constanttorqueuptobasespeed,butconstanthpupto3.5timeshpmotors.Eachisdrivenintheconstantcurrentmodeandthethebasespeed.Thepeaktorqueiswithinthenormalratingsforaload=motorinertia.Vecto
6、rmotorstaketimetoreachspeedtypicalmotor.(400msec)whilebrushlessservomotorsarethefasterbyasmuchasafactorofeight(50msec).VectormotorsHowever,don’tdismissinductionmotorsformotioncontrolaltogether.Addinganencoderfeedbackdevicetransformsitintoavectormotor.Thiscan
7、beacandidateforpositioningsystems.Closingthelooparoundtheencoder,controller,andmotorletsthemotorrundownto0RPMwithfulltorque.Vectormotorswithclosedloopcontrolsprovidetightspeedregulationandconstanttorquefrom0RPMtobasespeed,asshowninFigure1.Operatingintheconst
8、anthorsepowermodeletsthemotorrunuptothreetimesthebasespeed.Controlsincludeprogrammablefeaturessuchascontrolledacceleration,deceleration,etc.Atypical1hpvectormotorwithaninertiaof0.052lb–in–sec2ca
