重新分配任务的英语作业第一课Word下载.docx

重新分配任务的英语作业第一课Word下载.docx

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1.ElectricalMeasuringInstruments

Electricalpersonnelusemanydifferenttypesofmeasuringinstruments.Somejobsrequireveryaccuratemeasurementswhileotherjobsneedonlyroughestimates.Someinstrumentsareusedsolelytodeterminewhetherornotacircuitiscomplete.Themostcommonmeasuringandtestinginstrumentsarevoltagetesters,voltmeters,ammeters,ohmmeters,continuitytesters（连续性测试仪）,megohmmeters,wattmeters（功率计）,andwatt-hourmeters（电度表，电表）.

Allmetersusedformeasuringelectricalvaluesarebasicallycurrentmeters.Theymeasureorcomparethevaluesofcurrentflowingthroughthem.Themetersarecalibratedandthescaleisdesignedtoreadthevalueofthedesiredunit.【1】

1.1SAFETYPRECAUTION

Correctmeterconnectionsareveryimportantforthesafetyprecautionoftheuserandforpropermaintenanceofthemeters.Abasicknowledgeoftheconstructionandoperationofmeterswillaidtheuserinmakingproperconnectionsandmaintainingtheminsafeworkingorder.【2】

ManyinstrumentsaredesignedtobeusedonDCorAConly,whileotherscanbeusedinterchangeably.Note:

Itisveryimportanttouseeachmeteronlywiththetypeofcurrentforwhichthemeterisdesigned.Usingameterwithanincorrecttypeofcurrentcanresultindamagetothemeterandmaycauseinjurytotheuser.

Somemetersareconstructedtomeasureverylowvalues.Othermeterscanmeasureextremelyhighvalues.

CAUTION:

Neverallowametertoexceeditsratedmaximumlimit.Theimportanceofneverallowingtheactualvaluetoexceedthemaximumvalueindicatedonthemetercannotbeoveremphasized.Exceedingmaximumvaluescandamagetheindicatingneedle,interferewithpropercalibration,andinsomeinstancesmaycausethemetertoexplode,resultingininjurytotheuser.Somemetersareequippedwithovercorrectprotection.However,acurrentmanytimesgreaterthantheinstrument’sdesignlimitmaystillbehazardous.【3】

1.2BASICMETERCONSTRUCTIONANDOPERATION

Manymetersoperateontheprincipleofelectromagneticinteraction.Thisinteractioniscausedbyanelectriccurrentflowingthroughaconductorwhichisplacedbetweenthepolesofapermanentmagnet.Thistypeofmeterisespeciallysuitablefordirectcurrent.

Wheneveranelectriccurrentflowsthroughaconductoramagneticforceisdevelopedaroundtheconductor.Themagneticforcecausedbytheelectriccurrentreactswiththeforceofthepermanentmagnet.Thiscausestheindicatingneedletomove.Thelargertheamountofcurrent,thefarthertheneedlewillmove.【4】

以上716完成

Theconductorisformedintocoil,whichisplacedonapivotbetweenthepolesofthepermanentmagnet.Thecoilisconnectedtotheterminalsoftheinstrumentthroughtwospiralsprings.Thesespringssupplyareactingforceproportionaltothedeflection.Whennocurrentisflowing,thespringscausetheneedletoreturntozero.【5】

Themeterscaleisdesignedtoindicatetheamountofcurrentbeingmeasured.Themovementofthecoil（andthusthemovementoftheindicatingneedle）isproportionaltotheamountofcurrentflowinginthecoil.Ifitisnecessarytomeasurelargercurrentsthanthecoilcansafelycarry,abypasscircuit,orshunt,isincluded.Theshuntmanybecontainedwithinthemeterhousingorconnectedexternally.【6】

Example1

Ameterisconstructedtomeasure10Aonamaximumscale.Thecoilcansafelycarry0.001A.Theshuntmustbedesignedtocarry9.999A.Themeterisdesignedtoindicate10Awhen0.001Aflowsinthecoil.

Fig.1.1（a）illustratesapermanent-magnetmeter.Fig.1.1（b）showanexternalshuntconnectedacrossthemeterterminals.Thepermanent-magnetmetercanbeusedasanammeteroravoltmeter.Whenthescaleisdesignedtoindicatecurrentandtheinternalresistanceiskepttoaminimum,themeterfunctionsasanammeter.Whenthescaleisdesignedtoindicatevoltage,theinternalresistancewillberelativelyhigh,dependinguponthevalueofvoltageforwhichthemeterisdesigned.Note:

Regardlessofthedesign,thedistancetheneedlemovesisdeterminedbytheamountofcurrentflowinginthecoil.【7】

以上717完成

AslightchangemustbemadeinthedesigninordertousethistypeofmeteronAC.ArectifierisadevicewhichchangesACtoDC.ItmustbeincorporatedintothemeterandthescalemustbedrawntoindicatethecorrectvalueofACvoltage.Rectifier-typeACmeterscannotbeusedonDCandaregenerallydesignedasvoltagemeters.【8】

Theelectrodynamometer,Fig.1.2isanotherdesignforbothammetersandvoltmetersthatcanbeusedonalternatingcurrent.Thisinstrumentconsistsoftwostationarycoilsandonemovablecoil.Thethreecoilsareconnectedinserieswitheachotherthroughtwospiralsprings.Thespringalsosupportthemovablecoil.Whencurrentflowsthroughthecoilsthemovablecoilmovesinaclockwisedirectio