5/14/2015 Zone4info.comMotorProtectionandContactorSizing

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Zone4info.comMotorProtectionandContactorSizing

MotorProtectionandContactorSizing

PostedbyAshokKumarApril29

Theoverloadrelayissituatedinthemotorline.Therefore,thecurrenttobeadjustedislowerthanthemotorsratedcurrentbyafactorof1_3=0.58.Duetothethirdharmonicscurrentscirculatinginthemotorwindings,ahighersettingoftheoverloadrelaymayberequired.Thismayonlybecarriedoutonthebasisofutilisingameasuringdevicewhichrecordsthecorrectr.m.s.value.Conductorcrosssectionalareasmustbeofasuitablesizeinorderthattheywillbeprotectedagainsttemperaturerisesresultingfromoverloadconditions.Therefore,theconductorsizeselectedmustbeinaccordancewiththeprotectivedevice(s)rating.

5/14/2015 Zone4info.comMotorProtectionandContactorSizing

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Formotorprotectionbymeansofpowercircuitbreakerswithmotorprotectioncharacteristics,thepowercircuitbreakerisswitchedintothenetworksupplylines,asitalsocarriesoutshortcircuitprotectionofstarterandlines.Inthiscase,thecurrentissettotheratedmotorcurrent.Acorrectionofthesetvaluebecauseofthethirdharmonicsisirrelevantunderthesecircumstances.Thelinesaretobethermallyproportioneddependingonthepowercircuitbreakerssetting.

Fornormalstardeltastarting,thecontrolgearmustbesizedinaccordancewiththefollowingcurrents:

MaincontactorK1M0.58IeDeltacontactorK2M0.58IeStarcontactorK3M0.34Ie

Forstartingtimesexceedingapproximately15seconds,abiggerstarcontactorhastobeselected.Ifthestarcontactorisequaltothemaincontactor,starttimesofuptoapproximatelyoneminutearepermissible.

Reference:

www.rockwellautomation.com