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 versatility．By 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 operation．Because 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 output．The essential features of a DC machine are shown schematically．The 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 axis．As we know，the．AC voltage generated in each rotating armature coil is converted to DC in the external armature terminals by means ofnl，the m．m．f．F 2 rotates in the direction opposite to the rotor rotation and，when interacting with m．m．f．F l，develops the electromagnetic torque directed against emMrotor rotation．Consequently，torque is a retarding torque。and the eminduction machine operates as a generator．To construct the vector diagram of an induction generator，let US lay off the main fluxvector in the positive direction on the abscissa m?axis．The current vector I。and the e．m．f．vector El=E’2will then occupy their usual positions on the diagram，but the current vector 2'Iwill now be in the second and not in the third quadrant as would be the case for the motor operating condition．The primary current = —1m．The voltage across the stator terminals is Vl= 一 El+ ．The angle2'I 1ZI>90°，i．e． ，the electric power of the machine inegative．This means 1?that 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 circuit．The magnetic flux is produced in the induction generator by the m?14magnetising current For this purpose synchronous generators are used mIto feed the external circuit together with the induction generator．Since current Im amounts to from 25％to 45％of and is fed to the generator at nIcircuit voltage，the excitation power(in kVA)amounts to the same 25％to 45％of the rating of the generator．In 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 generators．Recall that the power of excitation of a large synchronous generator is less that 1％．This difference in excitation power being unfavourable for the induction generator is its essential drawback as compared with the synchronous generator．In addition，the current lags the voltage by practically mI90°．Consequently，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 active．The connection of an induction generator to the circuit does not present any difficulties．The rotor is run in the same direction in which the flux is rotating at a speed as close as possible to synchronism．When the generator is connected to the circuit the same phenomena arise as when transformers or induction motors are connected．A 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 shaft．The efficiency of an induction generator is not lower than that of a synchronous generator。In practice induction generators are used only in low—power stations，most frequently in automatic hydropower stations and wind—driven installations．If the induction generator is to work alone into an external circuit，the magnetising current may be obtained id the process of self—excitation of the induction generator．For this purpose it is necessary to connect to the stator terminals a battery of chosen condensers and run the rotor at required speed．The indispensable condition for self—excitation of the induction generator．For this purpose it is necessary to connect to the stator terminals a battery of chosen condensers and run 15the rotor at required speed．The indispensable condition for self—excitation of the induction generator is the presence of a residual magnetic flux in the rotor steel．With the external circuit disconnected from the stator，the residual magnetic flux creates a certain resid?e．m．f． in the stator winding，the effect of which is to cause residEcurrent to flow into the condenser battery，thus strengthening fluxcI．The process then proceeds in the same way as in the case of self—resid?excitation of D．C．shunt generators．The most costly part of induction self—excited generator installations is the condenser battery，that is why such generators have not obtained wide use．C)Electromagnetic brake conditions．If 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 stall．After this，we may run the rotor opposite the field with the help of some auxiliary motor．We have previously agreed to call these operating conditions electromagnetic brake conditions．Since the speed in these conditions should be assumed to be negative，then ln1，磁勢 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 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和電勢向量 1= 在向量圖上將仍處于原位置，但電流向量 現(xiàn)E'2 '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In25％～45％，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 25~45％，換句話說，若由相同容量的 2～4 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調(diào)節(jié)施加于發(fā)電機(jī)轉(zhuǎn)軸上的機(jī)械功率來實(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 φ resid在定子繞組中產(chǎn)生的剩磁電勢 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 φ resid。隨后的過程便與直流并勵(lì)發(fā)電機(jī)的自勵(lì)過程的情形是一樣的。自勵(lì)感應(yīng)發(fā)電機(jī)組中最費(fèi)錢的部分是電容器組，這便是該類發(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)子朝磁場相反的方向運(yùn)轉(zhuǎn)。前面我們已經(jīng)約定，稱該運(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 l
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