DZ254廢水處理微機(jī)控制系統(tǒng)
DZ254廢水處理微機(jī)控制系統(tǒng),dz254,廢水處理,微機(jī),控制系統(tǒng)
1Introduction to DC MachinesDC machines are characterized by their versatilityBy means of various combinations of shunt,series,and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steady- state operationBecause of the ease with which they can be controlled,systems of DC machines are often used in applications requiring a wide range of motor speeds or precise contro1 of motor outputThe essential features of a DC machine are shown schematicallyThe stator has Salient poles and is excited by one or more field coils. The air-gap flux distribution created by the field winding is symmetrical about the centerline of the field poles This axis is called the field axis or direct axisAs we know,theAC voltage generated in each rotating armature coil is converted to DC in the external armature terminals by means ofnl,the mmfF 2 rotates in the direction opposite to the rotor rotation and,when interacting with mmfF l,develops the electromagnetic torque directed against emMrotor rotationConsequently,torque is a retarding torque。and the eminduction machine operates as a generatorTo construct the vector diagram of an induction generator,let US lay off the main fluxvector in the positive direction on the abscissa maxisThe current vector I。and the emfvector El=E2will then occupy their usual positions on the diagram,but the current vector 2Iwill now be in the second and not in the third quadrant as would be the case for the motor operating conditionThe primary current = 1mThe voltage across the stator terminals is Vl= 一 El+ The angle2I 1ZI90,ie ,the electric power of the machine inegativeThis means 1that at the conditions considered above the mechanical power delivered to the induction machine from the primary mover is converted into electric power and is supplied to the circuitThe magnetic flux is produced in the induction generator by the m14magnetising current For this purpose synchronous generators are used mIto feed the external circuit together with the induction generatorSince current Im amounts to from 25to 45of and is fed to the generator at nIcircuit voltage,the excitation power(in kVA)amounts to the same 25to 45of the rating of the generatorIn other words,if 2 to 4 induction generators of equal output are installed in a power station,their excitation will take the full output of one synchronous generator of the same capacity as each of the induction generatorsRecall that the power of excitation of a large synchronous generator is less that 1This difference in excitation power being unfavourable for the induction generator is its essential drawback as compared with the synchronous generatorIn addition,the current lags the voltage by practically mI90Consequently,parallel operation of induction and synchronous generators results in a considerable loss in power factor(cos )of the latter even when the external load is purely activeThe connection of an induction generator to the circuit does not present any difficultiesThe rotor is run in the same direction in which the flux is rotating at a speed as close as possible to synchronismWhen the generator is connected to the circuit the same phenomena arise as when transformers or induction motors are connectedA change in active power supplied by the generator to the circuit,as in synchronous generators,is achieved by varying the mechanical power applied to the generator shaftThe efficiency of an induction generator is not lower than that of a synchronous generator。In practice induction generators are used only in lowpower stations,most frequently in automatic hydropower stations and winddriven installationsIf the induction generator is to work alone into an external circuit,the magnetising current may be obtained id the process of selfexcitation of the induction generatorFor this purpose it is necessary to connect to the stator terminals a battery of chosen condensers and run the rotor at required speedThe indispensable condition for selfexcitation of the induction generatorFor this purpose it is necessary to connect to the stator terminals a battery of chosen condensers and run 15the rotor at required speedThe indispensable condition for selfexcitation of the induction generator is the presence of a residual magnetic flux in the rotor steelWith the external circuit disconnected from the stator,the residual magnetic flux creates a certain residemf in the stator winding,the effect of which is to cause residEcurrent to flow into the condenser battery,thus strengthening fluxcIThe process then proceeds in the same way as in the case of selfresidexcitation of DCshunt generatorsThe most costly part of induction selfexcited generator installations is the condenser battery,that is why such generators have not obtained wide useC)Electromagnetic brake conditionsIf we continue to load a motor more and more,its speed will decrease and then,when the load torque exceeds the maximum torque,the motor will stallAfter this,we may run the rotor opposite the field with the help of some auxiliary motorWe have previously agreed to call these operating conditions electromagnetic brake conditionsSince the speed in these conditions should be assumed to be negative,then ln1,磁勢(shì) F2以與18轉(zhuǎn)子相反方向的 轉(zhuǎn)速旋轉(zhuǎn),當(dāng)其與定子磁勢(shì) F1相互作用時(shí),便產(chǎn)生與轉(zhuǎn)子旋轉(zhuǎn)方向相反的電磁轉(zhuǎn)矩 Mem。因此,該轉(zhuǎn)矩 Mem為阻轉(zhuǎn)矩,感應(yīng)電機(jī)作發(fā)電機(jī)運(yùn)行。為了繪出感應(yīng)發(fā)電機(jī)的向量圖,我們將主磁通向量 置于橫軸的正方向上,m那么電流 向量 Im和電勢(shì)向量 1= 在向量圖上將仍處于原位置,但電流向量 現(xiàn)E2 2I在是處于第二象限而不是處于作電動(dòng)機(jī)運(yùn)行的第三象限。原邊電流 I1=Im 。定子兩端電壓為 Vl=E l+I1Z1。相位角 90,即電機(jī)的電功率為負(fù)。這就是說(shuō)在上述1討論的運(yùn)行狀態(tài)下,由原動(dòng)機(jī)提供給感應(yīng)電機(jī)的機(jī)械功率已轉(zhuǎn)換成電功率且送入了電網(wǎng)。在感應(yīng)發(fā)電機(jī)中,磁通 。是由激磁電流 Im產(chǎn)生的。為此目的,需由同步發(fā)m電機(jī)與感應(yīng)發(fā)電機(jī)一道向電網(wǎng)供電。由于激磁電流 Im的數(shù)量為額定電流的In2545,且是在網(wǎng)上電壓下供電,因此激磁功率的大小也為發(fā)電機(jī)額定功率的 2545,換句話說(shuō),若由相同容量的 24 臺(tái)感應(yīng)發(fā)電機(jī)建立一電站,那么激磁容量便占據(jù)了與感應(yīng)電機(jī)單機(jī)相同的一臺(tái)同步發(fā)電機(jī)的滿(mǎn)載輸出。而大型同步發(fā)電機(jī)的激磁功率不到額定功率的 1。感應(yīng)電機(jī)的這種所需激磁功率較大方面的差值是與同步發(fā)電機(jī)相比的主要缺點(diǎn)。除此之外,激磁電流 Im。滯后電壓接近 90,因此,盡管當(dāng)外電路負(fù)載為完全的純電阻時(shí),仍然會(huì)在與感應(yīng)發(fā)電機(jī)并聯(lián)運(yùn)行的同步發(fā)電機(jī)中產(chǎn)生由于滯后的功率因數(shù)而引起的可觀的損耗。感應(yīng)發(fā)電機(jī)并人電網(wǎng)不存在任何困難。其轉(zhuǎn)子與磁通同方向旋轉(zhuǎn),且盡可能接近同步。當(dāng)感應(yīng)發(fā)電機(jī)接入電網(wǎng),其物理現(xiàn)象與當(dāng)變壓器或感應(yīng)電動(dòng)機(jī)接到電網(wǎng)上時(shí)是一樣的。與同步發(fā)電機(jī)一樣,由發(fā)電機(jī)向電網(wǎng)提供的有功功率的變化是通過(guò)調(diào)節(jié)施加于發(fā)電機(jī)轉(zhuǎn)軸上的機(jī)械功率來(lái)實(shí)現(xiàn)的。感應(yīng)發(fā)電機(jī)的功率并不比同步發(fā)電機(jī)的效率低。在實(shí)際運(yùn)用中,感應(yīng)發(fā)電機(jī)只是用于小功率電站,其中尤其是無(wú)人值守的水電站和風(fēng)力發(fā)電裝置更為常用。如果感應(yīng)發(fā)電機(jī)要接負(fù)載單獨(dú)運(yùn)行,勵(lì)磁電流可在感應(yīng)發(fā)電機(jī)的自勵(lì)過(guò)程中得到。為此目的,必須在定子端聯(lián)接一組經(jīng)選定的電容器,并且將轉(zhuǎn)子拖至所需的轉(zhuǎn)速。感應(yīng)發(fā)電機(jī)自勵(lì)的必要條件是在轉(zhuǎn)子鐵芯中必須有剩磁。當(dāng)負(fù)載電路還未與定子端相聯(lián)時(shí),剩磁磁通 resid在定子繞組中產(chǎn)生的剩磁電勢(shì) Eresid便產(chǎn)生一流向電容器組的電流 Ic,這樣便加強(qiáng)了磁通 resid。隨后的過(guò)程便與直流并勵(lì)發(fā)電機(jī)的自勵(lì)過(guò)程的情形是一樣的。自勵(lì)感應(yīng)發(fā)電機(jī)組中最費(fèi)錢(qián)的部分是電容器組,這便是該類(lèi)發(fā)電機(jī)不能得到廣泛運(yùn)用的原因所在。c)電磁制動(dòng)狀態(tài)。如果繼續(xù)向電動(dòng)機(jī)施加越來(lái)越多的負(fù)載,其轉(zhuǎn)速將下降,而后當(dāng)負(fù)載轉(zhuǎn)矩超過(guò)其最大轉(zhuǎn)矩時(shí),電動(dòng)機(jī)將停轉(zhuǎn)。從這時(shí)后便可借助某一輔助電動(dòng)機(jī)令主電動(dòng)機(jī)的轉(zhuǎn)子朝磁場(chǎng)相反的方向運(yùn)轉(zhuǎn)。前面我們已經(jīng)約定,稱(chēng)該運(yùn)行狀態(tài)19為電磁制動(dòng)狀態(tài)。由于該狀態(tài)下的轉(zhuǎn)速 n應(yīng)該假定為負(fù),因此 s的范圍便為 ls 。由于轉(zhuǎn)差 s較大,因此轉(zhuǎn)子電流,=2I2jEsr也便較大,且滯后 角,其極限值為 90(出現(xiàn)在 s=時(shí))。此時(shí),原邊電流2EI1,的大小和其相位相對(duì)于電壓 V1的相位角 也相應(yīng)較大。電磁制動(dòng)時(shí)的向量圖與1電動(dòng)機(jī)運(yùn)行狀態(tài)時(shí)的相似,具有如前所述的所有特點(diǎn)。由于 90,因此此時(shí)感應(yīng)電機(jī)從電網(wǎng)吸收電功率。除此之外,它還從原動(dòng)1機(jī)獲取機(jī)械功率。所有這些由電機(jī)吸收的功率都消耗在電機(jī)的損耗中,此時(shí)的損耗是相當(dāng)大的。因此,從發(fā)熱的觀點(diǎn)來(lái)看,這種運(yùn)行狀態(tài)是嚴(yán)峻的,因而在額定電壓下電磁制動(dòng)狀態(tài)只允許短時(shí)運(yùn)行。
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編號(hào):146781
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上傳時(shí)間:2017-10-26
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- 關(guān) 鍵 詞:
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dz254
廢水處理
微機(jī)
控制系統(tǒng)
- 資源描述:
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DZ254廢水處理微機(jī)控制系統(tǒng),dz254,廢水處理,微機(jī),控制系統(tǒng)
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