经验总结-制动电阻计算公式

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1、CalculatingbrakeresistorsizesDynamicbrakingresistors(DBR’s)forinvertersandDCdrivesystemsAdrivemotorcanalsoactasagenerator.Ifthedrivesystemisbuiltsoastoallowreversepowertoflowthenthispowercanbefedintoaresistor,thustakingenergyoutofthesystemandcausingwhateverisdri

2、vingthemotortoslowdown.TherateofbrakingisdeterminedbyhowfasttheenergyisputintotheDBR.TheDClinkcapacitanceofanyinverterdrivecanitselfabsorb3-5%oftheregeneratedpower.Fornon-criticalapplicationstheselosses,togetherwiththemechanicallossesinthedrivesystem,mayprovidee

3、noughbraking.Higherpowers,upto100%ormoreofthemotor’sfullloadtorquerating,canbeabsorbedandthendissipatedbyaDBRconnectedacrosstheDCbus.Wherethebrakingpowerisonlyafewtensorhundredsofwattsaresistormountedinternallytothedriveitselfmaybesuitable,butabovetheselevelsthe

4、amountofheatgeneratedmeansthataseparatelymountedDBRwithappropriatecoolingprovisionisneeded.TheDBRisswitchedonbyaseparatecontrolunit,activatedbyasensorwhichismonitoringthevoltageleveloftheDCbusandswitchingontheDBRwhenthisvoltagerisesabovesomepresettriggerlevelasa

5、resultofthereversepowerflowingintothedrive.TheremaybetemperaturesensingintheDBRtopreventoverloadingofthedrive.Alltheenergyisusedinheatingtheresistor;someisdissipatedatonce,therestafterthestopwhiletheresistorcools.Thisiswhywemustknowthecharacteristicsofthedutycyc

6、lebeforewecanspecifytherightsizefortheDBR.Whatisthestoppingenergy?TheDBRturnsthestopenergyintoheat.BothtypesofenergyaremeasuredinJoules(J);oneJouleisaverysmallquantity,soweusuallytalkaboutkJorMJ.Inordertodesignabrakingsystemwehavetoconsiderboththeamountofheat(in

7、Joules)andtherateatwhichitisgenerated.ThisisJoules/second,usuallyknownaswatts,andforthesamereasonusuallymeasuredinkWorMW.Wethereforeneedtoknowthequantityofenergyperstop,andthestopfrequency.Energyperstop:determinestheDBRpeakpowerEnergyperstop+frequency:determines

8、theDBRaveragepowerWeallhaveagoodideaofwhatanygivenlength,weightortimeintervalrepresents;thisisusuallynotsoforenergy.Bywayofillustrationherearesomeeverydayexamples:Man