变压器毕业设计论文.doc

变压器 1）介绍要从远端发电厂送出电能，必须应用高压输电。因为最终的负荷，在一些点高电压必须降低。变压器能使电力系统各个部分运行在电压不同的等级。本文我们讨论的原则和电力变压器的应用。2）双绕组变压器变压器的最简单形式包括两个磁通相互耦合的固定线圈。两个线圈之所以相互耦合，是因为它们连接着共同的磁通。在电力应用中，使用层式铁芯变压器(本文中提到的)。变压器是高效率的，因为它没有旋转损失，因此在电压等级转换的过程中，能量损失比较少。典型的效率范围在92到99%，上限值适用于大功率变压器。从交流电源流入电流的一侧被称为变压器的一次侧绕组或者是原边。它在铁圈中建立了磁通，它的幅值和方向都会发生周期性的变化。磁通连接的第二个绕组被称为变压器的二次侧绕组或者是副边。磁通是变化的；因此依据楞次定律，电磁感应在二次侧产生了电压。变压器在原边接收电能的同时也在向副边所带的负荷输送电能。这就是变压器的作用。3）变压器的工作原理当二次侧电路开路是，即使原边被施以正弦电压Vp，也是没有能量转移的。外加电压在一次侧绕组中产生一个小电流I。这个空载电流有两项功能为在铁芯中产生电磁通，该磁通在零和m之间做正弦变化，m是m铁芯磁通的最大值；它的一个分量说明了铁芯中的涡流和磁滞损耗。这两种相关的损耗被称为铁芯损耗。变压器空载电流I一般大约只有满载电流的2%5%。因为在空载时，原边绕组中的铁芯相当于一个很大的电抗，空载电流的相位大约将滞后于原边电压相位90。显然可见电流分量Im=I0sin0，被称做励磁电流，它在相位上滞后于原边电压VP90。就是这个分量在铁芯中建立了磁通；因此磁通与Im同相。第二个分量Ie=I0sin0，与原边电压同相。这个电流分量向铁芯提供用于损耗的电流。两个相量的分量和代表空载电流，即应注意的是空载电流是畸变和非正弦形的。这种情况是非线性铁芯材料造成的。如果假定变压器中没有其他的电能损耗一次侧的感应电动势Ep和二次侧的感应电压Es可以表示出来。因为一次侧绕组中的磁通会通过二次绕组，依据法拉第电磁感应定律，二次侧绕组中将产生一个电动势E，即E=N/t。相同的磁通会通过原边自身，产生一个电动势Ep。正如前文中讨论到的，所产生的电压必定滞后于磁通90，因此，它于施加的电压有180的相位差。因为没有电流流过二次侧绕组，Es=Vs。一次侧空载电流很小，仅为满载电流的百分之几。因此原边电压很小，并且Vp的值近乎等于Ep。原边的电压和它产生的磁通波形是正弦形的；因此产生电动势Ep和Es的值是做正弦变化的。产生电压的平均值如下：即是法拉第定律在瞬时时间里的应用，它遵循：其中N是指线圈的匝数，从交流电原理可知，有效值是一个正弦波，其值为平均电压的1.11倍，因此因为一次侧绕组和二次侧绕组的磁通相等，所以绕组中每匝的电压也相同。因此并且其中Np和Es是一次侧绕组和二次侧绕组的匝数。一次侧和二次侧电压增长的比率称做变比。用字母a来表示这个比率，如下式假设变压器输出电能等于其输入电能这个假设适用于高效率的变压器。实际上我们是考虑一台理想状态下的变压器；这意味着它没有任何损耗。因此或者这里PF代表功率因素。在上面公式中一次侧和二次侧的功率因素是相等的；因此从上式我们可以得知它表明端电压比等于匝数比，换句话说，一次侧和二次侧电流比与匝数比成反比。匝数比可以衡量二次侧电压相对于一次恻电压是升高或者是降低。为了计算电压，我们需要更多数据。终端电压的比率变化有些根据负载和它的功率因素。实际上,变比从标识牌数据获得,列出在满载情况下原边和副边电压。当副边电压Vs相对于原边电压减小时，这个变压器就叫做降压变压器。如果这个电压是升高的，它就是一个升压变压器。在一个降压变压器中传输变比a远大于1(a1.0)，同样的，一个升压变压器的变比小于1(a1.0),whileforastep-uptransformeritissmallerthanunity(a1.0).Intheeventthata=1,thetransformersecondaryvoltageequalstheprimaryvoltage.Thisisaspecialtypeoftransformerusedininstanceswhereelectricalisolationisrequiredbetweentheprimaryandsecondarycircuitwhilemaintainingthesamevoltagelevel.Therefore,thistransformerisgenerallyknowsasanisolationtransformer.Asisapparent,itisthemagneticfluxinthecorethatformstheconnectinglinkbetweenprimaryandsecondarycircuit.Insection4itisshownhowtheprimarywindingcurrentadjustsitselftothesecondaryloadcurrentwhenthetransformersuppliesaload.Lookingintothetransformerterminalsfromthesource,animpedanceisseenwhichbydefinitionequalsVp/Ip. From,wehaveVp=aVsandIp=Is/a.IntermsofVsandIstheratioofVptoIpisButVs/IsistheloadimpedanceZLthuswecansaythatThisequationtellsusthatwhenanimpedanceisconnectedtothesecondaryside,itappearsfromthesourceasanimpedancehavingamagnitudethatisa2timesitsactualvalue.Wesaythattheloadimpedanceisreflectedorreferredtotheprimary.Itisthispropertyoftransformersthatisusedinimpedance-matchingapplications.4) TRANSFORMERSUNDERLOADTheprimaryandsecondaryvoltagesshownhavesimilarpolarities,asindicatedbythe“dot-making”convention.Thedotsneartheupperendsofthewindingshavethesamemeaningasincircuittheory;themarkedterminalshavethesamepolarity.Thuswhenaloadisconnectedtothesecondary,theinstantaneousloadcurrentisinthedirectionshown.Inotherwords,thepolaritymarkingssignifythatwhenpositivecurrententersbothwindingsatthemarkedterminals,theMMFsofthetwowindingsadd.Sincethesecondaryvoltagedependsonthecoreflux0,itmustbeclearthatthefluxshouldnotchangeappreciablyifEsistoremainessentiallyconstantundernormalloadingconditions.Withtheloadconnected,acurrentIswillflowinthesecondarycircuit,becausetheinducedEMFEswillactasavoltagesource.ThesecondarycurrentproducesanMMFNsIsthatcreatesaflux.Thisfluxhassuchadirectionthatatanyinstantintimeitopposesthemainfluxthatcreateditinthefirstplace.Ofcourse,thisisLenzslawinaction.ThustheMMFrepresentedbyNsIstendstoreducethecoreflux0.ThismeansthatthefluxlinkingtheprimarywindingreducesandconsequentlytheprimaryinducedvoltageEp,ThisreductionininducedvoltagecausesagreaterdifferencebetweentheimpressedvoltageandthecounterinducedEMF,therebyallowingmorecurrenttoflowintheprimary.ThefactthatprimarycurrentIpincreasesmeansthatthetwoconditionsstatedearlierarefulfilled:thepowerinputincreasestomatchthepoweroutput,andtheprimaryMMFincreasestooffsetthetendencyofthesecondaryMMFtoreducetheflux.Ingeneral,itwillbefoundthatthetransformerreactsalmostinstantaneouslytokeeptheresultantcorefluxessentiallyconstant.Moreover,thecoreflux0dropsveryslightlybetweennoloadandfullload(about1to3%),anecessaryconditionifEpistofallsufficientlytoallowanincreaseinIp.Ontheprimaryside,IpisthecurrentthatflowsintheprimarytobalancethedemagnetizingeffectofIs.ItsMMFNpIpsetsupafluxlinkingtheprimaryonly.Sincethecoreflux0remainsconstant.I0mustbethesamecurrentthatenergizesthetransformeratnoload.TheprimarycurrentIpisthereforethesumofthecurrentIpandI0.Becausetheno-loadcurrentisrelativelysmall,itiscorrecttoassumethattheprimaryampere-turnsequalthesecondaryampere-turns,sinceitisunderthisconditionthatthecorefluxisessentiallyconstant.ThuswewillassumethatI0isnegligible,asitisonlyasmallcomponentofthefull-loadcurrent.Whenacurrentflowsinthesecondarywinding,theresultingMMF(NsIs)createsaseparateflux,apartfromtheflux0producedbyI0,whichlinksthesecondarywindingonly.Thisfluxdoesnolinkwiththeprimarywindingandisthereforenotamutualflux.Inaddition,theloadcurrentthatflowsthroughtheprimarywindingcreatesafluxthatlinkswiththeprimarywindingonly;itiscalledtheprimaryleakageflux.Thesecondary-leakagefluxgivesrisetoaninducedvoltagethatisnotcounterbalancedbyanequivalentinducedvoltageintheprimary.Similarly,thevoltageinducedintheprimaryisnotcounterbalancedinthesecondarywinding.Consequently,thesetwoinducedvoltagesbehavelikevoltagedrops,generallycalledleakagereactancevoltagedrops.Furthermore,eachwindinghassomeresistance,whichproducesaresistivevoltagedrop.Whentakenintoaccount,theseadditionalvoltagedropswouldcompletetheequivalentcircuitdiagramofapracticaltransformer.Notethatthemagnetizing branchisshowninthiscircuit,whichforourpurposeswillbedisregarded.Thisfollowsourearlierassumptionthattheno-loadcurrentisassumednegligibleinourcalculations.Thisisfurtherjustifiedinthatitisrarelynecessarytopredicttransformerperformancetosuchaccuracies.Sincethevoltagedropsarealldirectlyproportionaltotheloadcurrent,itmeansthatatno-loadconditionstherewillbenovoltagedropsineitherwinding.