基于排水系統(tǒng)的PLC控制與組態(tài)含4張CAD圖

基于排水系統(tǒng)的PLC控制與組態(tài)含4張CAD圖,基于,排水系統(tǒng),plc,控制,節(jié)制,組態(tài),cad 外文資料 INDUCTION MOTOR STARTING METHODS AND ISSUES Abstract - Many methods can be used to start large AC induction motors. Choices such as full voltage, reduced voltage either by auto-transformer or Wyes - Delta, a soft starter, or usage of an adjustable speed drive can all have potential advantages and trade offs. Reduced voltage starting can lower the starting torque and help prevent damage to the load. Additionally, power factor correction capacitors can be used to reduce the current, but care must be taken to size them properly. Usage of the wrong capacitors can lead to significant damage. Choosing the proper starting method for a motor will include an analysis of the power system as well as the starting load to ensure that the motor is designed to deliver the needed performance while minimizing its cost. This paper will examine the most common starting methods and their recommended applications. Index Terms: motor starting. Reduced voltage start auto transformer, wyes-delta, power factor correction I. INTRODUCTION There are several general methods of starting induction motors: full voltage, reduced voltage, wyes-delta, and part winding types. The reduced voltage type can include solid state starters, adjustable frequency drives, and autotransformers. These, along with the full voltage, or across the line starting, give the purchaser a large variety of automotives when it comes to specifying the motor to be used in a given application. Each method has its own benefits, as well as performance trade offs. Proper selection will involve a thorough investigation of any power system constraints, the load to be accelerated and the overall cost of the equipment. In order for the load to be accelerated, the motor must generate greater torque than the load requirement. In general there are three points of interest on the motor's speed-torque curve. The first is locked-rotor torque (LRT) which is the minimum torque which the motor will develop at rest for all angular positions of the rotor. The second is pull-up torque (PUT) which is defined as the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. The last is the breakdown torque (BDT) which is defined as the maximum torque which the motor will develop. If any of these points are below the required load curve, then the motor will not start. The time it takes for the motor to accelerate the load is dependent on the inertia of the load and the margin between the torque of the motor and the load curve, sometimes called accelerating torque. In general, the longer the time it takes for the motor to accelerate the load, the more heat that will be generated in the rotor bars, shorting ring and the stator winding. This heat leads to additional stresses in these parts and can have an impaction motor life. II. FULL VOLTAGE The full voltage starting method, also known as across the line starting, is the easiest method to employ, has the lowest equipment costs, and is the most reliable. This method utilizes a control to close a contactor and apply full line voltage to the motor terminals. This method will allow the motor to generate its highest starting torque and provide the shortest acceleration times. This method also puts the highest strain on the power system due to the high starting currents that can be typically six to seven times the normal full load current of the motor. If the motor is on a weak power system, the sudden high power draw can cause a temporary voltage drop, not only at the motor terminals, but the entire power bus feeding the starting motor. This voltage drop will cause a drop in the starting torque of the motor, and a drop in the torque of any other motor running on the power bus. The torque developed by an induction motor varies roughly as the square of the applied voltage. Therefore, depending on the amount of voltage drop, motors running on this weak power bus could stall. In addition, many control systems monitor under voltage conditions, a second potential problem that could take a running motor offline during a full voltage start. Besides electrical variation of the power bus, a potential physical disadvantage of an across the line starting is the sudden loading seen by the driven equipment. This shock loading due to transient torques which can exceed 600% of the locked rotor torque can increase the wear on the equipment, or even cause a catastrophic failure if the load can not handle the torques generated by the motor during staring. A. Capacitors and Starting Induction motors typically have very low power factor during starting and as a result have very large reactive power draw. See Fig. 2. This effect on the system can be reduced by adding capacitors to the motor during starting. The large reactive currents required by the motor lag the applied voltage by 90 electrical degrees. This reactive power doesn't create any measurable output, but is rather the energy required for the motor to function. The product of the applied system voltage and this reactive power component can be measured in VARS (volt-ampere reactive). The capacitors act to supply a current that leads the applied voltage by 90 electrical degrees. The leading currents supplied by the capacitors cancel the lagging current demanded by the motor, reducing the amount of reactive power required to be drawn from the power system. To avoid over voltage and motor damage, great care should be used to make sure that the capacitors are removed as the motor reaches rated speed, or in the event of a loss of power so that the motor will not go into a generator mode with the magnetizing currents provided from the capacitors. This will be expanded on in the next section and in the appendix. B. Power Factor Correction Capacitors can also be left permanently connected to raise the full load power factor. When used in this manner they are called power factor correction capacitors. The capacitors should never be sized larger than the magnetizing current of the motor unless they can be disconnected from the motor in the event of a power loss. The addition of capacitors will change the effective open circuit time constant of the motor. The time constant indicates the time required for remaining voltage in the motor to decay to 36.8% of rated voltage after the loss of power. This is typically one to three seconds without capacitors. With capacitors connected to the leads of the motor, the capacitors can continue to supply magnetizing current after the power to the motor has been disconnected. This is indicated by a longer time constant for the system. If the motor is driving a high inertia load, the motor can change over to generator action with the magnetizing Current from the capacitors and the shaft driven by the load. This can result in the voltage at the motor terminals actually rising to nearly 50% of rated voltage in some cases. If the power is reconnected before this voltage decays severe transients can be created which can cause significant switching currents and torques that can severely damage the motor and the driven equipment. An example of this phenomenon is outlined in the appendix. Ill. REDUCED VOLTAGE Each of the reduced voltage methods are intended to reduce the impact of motor starting current on the power system by controlling the voltage that the motor sees at the terminals. It is very important to know the characteristics of the load to be started when considering any form of reduced voltage starting. The motor manufacturer will need to have the speed torque curve and the inertia of the driven equipment when they validate their design. The curve can be built from an initial, or break away torque, as few as four other data points through the speed range, and the full speed torque for the starting condition. A centrifugal or square curve can be assumed in many cases, but there are some applications where this would be problematic. An example would be screw compressors which have a much higher torque requirement at lower speeds than the more common centrifugal or fan load. See Fig. 3. By understanding the details of the load to be started the manufacturer can make sure that the motor will be able to generate sufficient torque to start the load, with the starting method that is chosen. A. Autotransformer The motor leads are connected to the lower voltage side of the transformer. The most common taps that are used are 80%, 65%, and 50%. At 50% voltage the current on the primary is 25% of the full voltage locked rotor amps. The motor is started with this reduced voltage, and then after a pre-set condition is reached the connection is switched to line voltage. This condition could be a preset time, current level, bus volts, or motor speed. The change over can be done in either a closed circuit transition, or an open circuit transition method. In the open circuit method the connection to the voltage is severed as it is changed from the reduced voltage to the line level. Care should be used to make sure that there will not be problems from transients due to the switching. This potential problem can be eliminated by using the closed circuit transition. With the closed circuit method there is a continuous Voltage applied to the motor. Another benefit with the autotransformer starting is in possible lower vibration and noise levels during starting. Since the torque generated by the motor will vary as the square of the applied voltage, great care should be taken to make sure that there will be sufficient accelerating torque available from the motor. A speed torque curve for the driven equipment along with the inertia should be used to verify the design of the motor. A good rule of thumb is to have a minimum of 10% of the rated full load torque of the motor as a margin at all points of the curve. Additionally, the acceleration time should be evaluated to make sure that the motor has sufficient thermal capacity to handle the heat generated due to the longer acceleration time. B. Solid State or Soft Starters These devices utilize silicon controlled rectifiers or Scars. By controlling the firing angle of the SCR the voltage that the device produces can be controlled during the starting of the motor by limiting the flow of power for only part of the duration of the sine wave. The most widely used type of soft starter is the current limiting type. A current limit of 175% to 500% of full load current is programmed in to the device. It then will ramp up the voltage applied to the motor until it reaches the limit value, and will then hold that current as the motor accelerates. Tachometers can be used with solid state starters to control acceleration time. Voltage output is adjusted as required by the starter controller to provide a constant rate of acceleration. The same precautions in regards to starting torque should be followed for the soft starters as with the other reduced voltage starting methods. Another problem due to the firing angle of the SCR is that the motor could experience harmonic oscillating torques. Depending on the driven equipment, this could lead to exciting the natural frequency of the system. C. Adjustable Frequency Drives This type of device gives the greatest overall control and flexibility in starting induction motors giving the most torque for an amount of current. It is also the most costly. The drive varies not only the voltage level, but also the frequency, to allow the motor to operate on a constant volt per hertz level. This allows the motor to generate full load torque throughout a large speed range, up to 10:1. During starting, 150% of rated current is typical. This allows a significant reduction in the power required to start a load and reduces the heat generated in the motor, all of which add up to greater efficiency. Usage of the AFD also can allow a smaller motor to be applied due to the significant increase of torque available lower in the speed range. The motor should still be sized larger than the required horsepower of the load to be driven. The AFD allows a great degree of control in the acceleration of the load that is not as readily available with the other types of reduced voltage starting methods. The greatest drawback of the AFD is in the cost relative to the other methods. Drives are the most costly to employ and may also require specific motor designs to be used. Based on the output signal of the drive, filtered or unfiltered, the motor could require additional construction features. These construction features include insulated bearings, shaft grounding brushes, and insulated couplings due to potential shaft current from common mode voltage. Without these features, shaft currents, which circulate through the shaft to the bearing, through the motor frame and back, create arcing in the bearings that lead to premature bearing failure, this potential for arcing needs to be considered when applying a motor/drive package in a hazardous environment, Division2/Zone2. An additional construction feature of a motor used on an AFD may require is an upgraded insulation system on the motor windings. An unfiltered output signal from a drive can create harmonic voltage spikes in the motor, stressing the insulation of the motor windings. It is important to note that the features described pertain to motors which will be started and run on an AFD. If the drive is only used for starting the motor, these features may not be necessary. Consult with the motor manufacturer for application specific requirements. D. Primary Resistor or Reactor Starting This method uses either a series resistor or reactor bank to be placed in the circuit with the motor. Resistor starting is more frequently used for smaller motors. When the motor is started, the resistor bank limits the flow of inrush current and provides for a voltage drop at the motor terminals. The resistors can be selected to provide voltage reductions up to 50%. As the motor comes up to speed, it develops a counter EMF (electro-magnetic field) that opposes the voltage applied to the motor. This further limits the inrush currents. As the inrush current diminishes, so does t>e voltage drop across the resistor bank allowing the torque generated by the motor to increase. At a predetermined time a device will short across the resistors and open the starting contactor effectively removing the resistor bank from the circuit. This provides for a closed transition and eliminates the concerns due to switching transients. Reactors will tend to oppose any sudden changes in current and therefore act to limit the current during starting. They will remain shorted after starting and provide a closed transition to line voltage. IV. INCREMENT TYPE The first starting types that we have discussed have deal with the way the energy is applied to the motor. The next type deals with different ways the motor can be physically changed to deal with starting issues. Part Winding With this method the stator of the motor is designed in such a way that it is made up of two separate windings. The most common method is known as the half winding method. As the name suggests, the stator is made up of two identical balanced windings. A special starter is configured so that full voltage can be applied to one half of the winding, and then after a short delay, to the second half. This method can reduce the starting current by 50 to 60%, but also the starting torque. One drawback to this method is that the motor heating on the first step of the operation is greater than that normally encountered on across-the-line start. Therefore the elapsed time on the first step of the part winding start should be minimized. This method also increases the magnetic noise of the motor during the first step. 中文譯文 異步電動機起動的方法和問題 摘要：大容量的交流異步電動機有多種啟動方法?？蛇x擇的如全壓啟動、降壓啟動、自耦變壓器啟動、星三角轉(zhuǎn)換啟動、軟啟動、或者使用可調(diào)速驅(qū)動器，都有潛在優(yōu)勢和選擇。降壓啟動可以減小啟動轉(zhuǎn)矩，可以防止損壞負載。此外，功率因數(shù)校正電容器可以用來減小電流，但選擇的型號必須合適，否則將會造成電容器的嚴重損壞 。為電動機選擇一個合適的啟動方法，需要分析電力系統(tǒng)和啟動負載以確保電機達到所需性能且成本最少。本文將探討最常見的幾種啟動方法以及它們的應(yīng)用。 檢索詞：電機啟動 降壓啟動 自耦變壓器 星三角 功率因數(shù)校正 Ⅰ.引言 異步電動機有多種啟動方法：全壓啟動、降壓啟動、星三角轉(zhuǎn)換啟動和部分繞組等類型。降壓啟動包括固態(tài)啟動器、變頻啟動和自耦變壓器啟動。這些連同全壓或者直接啟動，當電動機的應(yīng)用場合被確定后可以給購買者大量類型的變化。每種方法都有它自己的好處，以及貿(mào)易業(yè)績。合理的選擇包括對電力系統(tǒng)透徹的研究，負載的加速以及設(shè)備的全部成本。 為了使負載能夠很好的加速，電動機必須產(chǎn)生比負載需求更大的轉(zhuǎn)矩。一般來說，在機械特性曲線上集中有三點。第一點是堵轉(zhuǎn)轉(zhuǎn)矩（LRT），使電機由靜止到旋轉(zhuǎn)的最小轉(zhuǎn)矩。第二點是最小啟動轉(zhuǎn)矩，使電機由靜止加速到出現(xiàn)制動轉(zhuǎn)矩是的最小轉(zhuǎn)矩。最后一點是臨街轉(zhuǎn)矩，就是電機能產(chǎn)生的最大轉(zhuǎn)矩。如果任何一段虛線在負載曲線以下，則電機就不能啟動。電機的加速時間是由負載的慣性以及電機的機械特性曲線和負載的特性曲線之間的差額決定的?？偟膩碚f，電機的加速時間越長，則電機轉(zhuǎn)子銅條、端環(huán)、定子繞組產(chǎn)生的熱量也就越多。這些熱量會給這些部件帶來額外的壓力甚至還會影響到電機的使用壽命。 Ⅱ.全電壓 全壓文啟動的方法也叫直接啟動，是最早被使用的方法，且設(shè)備成本最低工作最可靠。這種方法利用一個控制器閉合電流接觸器給電動機輸入全壓。此方法允許電動機產(chǎn)生最大的啟動轉(zhuǎn)矩使加速時間最短。 由于用此方法啟動電機會使啟動電流達到電機額定電流的六到七倍，因此方法也是給供電系統(tǒng)帶來最大的壓力的方法。如果供電系統(tǒng)比較薄弱，大功率電機的突然啟動不僅會使電動機的電壓瞬間下降，而且會使給電機供電的整個母線端電壓下降。電壓下降會使此電動機的啟動轉(zhuǎn)矩和工作在同一母線上的電動機的轉(zhuǎn)矩下降。異步電動機的轉(zhuǎn)矩大約和輸入電壓的平方成比例變化。因此，由于電壓的大幅下降，工作在此供電系統(tǒng)的電動機可能停車。另外許多控制系統(tǒng)監(jiān)控器工作在低電壓下，第二個全壓啟動時電壓問題會使正在運行的電機離線。同時，母線電壓變化，直接啟動的另一個問題驅(qū)動設(shè)備突然被加載。由于瞬時轉(zhuǎn)矩可以超過轉(zhuǎn)子制動轉(zhuǎn)矩的600%，這個沖擊負荷會增加設(shè)備的磨損，如果負載不能承受由電機啟動產(chǎn)生的力矩，甚至會造成災難性故障。 A．電容器和啟動 異步電動機功率因數(shù)通常很低,因此在開始有很大的感性無功。 如圖2所示。 在啟動時通過給電機增加電容器可以降低對系統(tǒng)的這種影響。 電機需要大量的無功電流滯后于輸入電壓90度。這個無功功率不產(chǎn)生任何輸出，但是這是電機運行所必需的。輸入電壓產(chǎn)生無功功率和這個無功功率組成的可用無功功率功率表測量。電容器擔任提供一個超前90度的電流。由電容器產(chǎn)生的超前電流取消了電機需要的滯后電流，降低了從供電系統(tǒng)輸入的感性無功功率。 避免過電壓和電機損壞 應(yīng)小心謹慎確保電容器被切除當電機達到額定轉(zhuǎn)速時,否則由于功率損失，電動機利用由電容器提供的磁化電流將不會進入發(fā)電模式. 這將在下一段和附錄中詳述。 B．功率因數(shù)校正 電容器也可永久留下提高滿載功率因數(shù)，當使用這種方式時被稱為功率因數(shù)校正電容器。該電容器的容量不能大于電動機的勵磁電流，除非當功率降低時他們可以和電機分離。 附加的電容器將會改變電機的開環(huán)時間常數(shù)，時間常數(shù)表示電機斷電后電壓衰減到原來的36.8%所需要的時間。沒有電容的典型值是兩到三秒。 由于電容器和電動機的前端相連，在電機斷開電源之后電容可以繼續(xù)提供勵磁電流。這表明該系統(tǒng)的時間常數(shù)較大。如果電機驅(qū)動高慣性負載，電動機會利用電容提供的勵磁電流和負載的帶動轉(zhuǎn)變?yōu)榘l(fā)電狀態(tài)。這可能導致電機端部的電壓上升到額定電壓的50%。如果在電壓劇烈衰減之前再次通電會產(chǎn)生很大的開關(guān)電流和轉(zhuǎn)矩，會造成電動機和驅(qū)動設(shè)備的嚴重損壞。這種現(xiàn)象的例子在附錄中有概述。 Ⅲ.降電壓 每一種降壓方法都是通過控制電動機的輸入電壓來減小電動機起動電流對供電系統(tǒng)的沖擊。在考慮具體的降壓啟動方法時，了解負載特性是非常重要的。當電機制造商要確定他們的設(shè)計時必須知道電機的機械特性和被驅(qū)動設(shè)備的轉(zhuǎn)動慣量。可以通過初始狀態(tài)或脫離轉(zhuǎn)矩，以及不少于四個速度上升的數(shù)據(jù)點和全速時的轉(zhuǎn)矩構(gòu)成啟動狀態(tài)的曲線。在很多情況下可以被假設(shè)成一個離心曲線或直角曲線，但是在一些應(yīng)用場合這還是有問題的。例如螺桿式壓縮機在低速時比離心式鼓風機類負載需要更大的轉(zhuǎn)矩。通過詳細了解負載的特性，廠商可以確定選擇的啟動方法可以產(chǎn)生足夠大的力矩啟動負載。 A．自耦變壓器 電動機連接在變壓器的低壓側(cè)。80%、65%和50%是最常用的接頭。電壓為原來的50%電流則為全壓轉(zhuǎn)子堵轉(zhuǎn)時的25%。電動機降低電壓啟動，當達到預先設(shè)定的條件時就切換成全壓。這個條件可以是預先設(shè)定的時間、電流、母線電壓或者電動機的轉(zhuǎn)速。這種變化既可以采用閉環(huán)方式也可以采用開環(huán)方式。采用開環(huán)方式在低壓和全壓切換時線路電壓嚴重，應(yīng)小心使用，以確保不會由于切換產(chǎn)生暫態(tài)問題。這個潛在的問題可以采用閉環(huán)的方法消除。采用閉環(huán)的方式可以給電機提供連續(xù)的電壓。利用自耦起動另一個好處是盡可能的降低了振動和噪音。 因為電機的轉(zhuǎn)矩隨輸入電壓的平方變化而變化。應(yīng)非常謹慎,以確?？梢詮碾妱訖C獲得足夠的加速轉(zhuǎn)矩。應(yīng)該用驅(qū)動設(shè)備的機械特性和慣性來驗證電動機的設(shè)計。一個好的經(jīng)驗法則是至少有10%的額定轉(zhuǎn)矩，作為該曲線各點的極限。 此外，對加速時間應(yīng)進行計算，以確保電機有足夠的熱容量將電機在長時間加速時產(chǎn)生的熱量散發(fā)掉。 B．固態(tài)軟啟動器 這些裝置采用可控硅整流器或晶閘管，通過控制晶閘管的觸發(fā)角，可以控制該裝置產(chǎn)生的電壓，并限制啟動電流，在一個正弦周期中僅讓部分通過，來啟動電動機。最常用的軟起動器的類型是限流型，電流被限制在滿載電流的175%到500%并編入裝置中。然后讓輸入電機的電壓斜坡上升一直到限制值，并使保持電流不變使電機加速。利用轉(zhuǎn)速表和軟啟動器可以控制加速時間，軟啟動器調(diào)整輸出電壓使電機維持一個恒定的加速度。關(guān)于啟動力矩和其他降壓啟動方法有同樣的問題，另一個問題是由于晶閘管的觸發(fā)角會使電機產(chǎn)生振蕩轉(zhuǎn)矩，依靠驅(qū)動設(shè)備，可能導致系統(tǒng)共振。 C．星三角啟動 用這種方法啟動的異步電機，結(jié)構(gòu)上每一相的引出端都被放置在電機的接線盒內(nèi)。這允許電機在初始啟動時星形連接，然后重新接成三角形運行。初始啟動時用星形連接時電壓降為原來的，啟動電流和啟動力矩下降2/3。根據(jù)應(yīng)用，電機切換到三角形可在轉(zhuǎn)速為50% 到最大速度之間。必須指出,同樣的問題存在,包括先前談到的切換方法，如果采用開路法，暫態(tài)過渡可能是一個問題。這種方法經(jīng)常用在低于600V的電機上，額定電壓在2.3kV及更高的電機則不適合用星三角啟動的方法。 Ⅳ.結(jié)論 異步電動機有許多啟動方法，依據(jù)電力系統(tǒng)的制約、設(shè)備成本、負載設(shè)備來選擇最好的啟動方法。從設(shè)備的角度來看，全壓啟動是最早的最便宜的方法，但它可能在使用效率上增加成本，或者該區(qū)域的供電系統(tǒng)不能滿足其需要。降壓啟動方式有效的緩解了供電系統(tǒng)，卻以犧牲啟動力矩為代價的。這些方法也可能導致不得不加大電機尺寸以產(chǎn)生轉(zhuǎn)矩帶動所需的負荷。采用變頻器可以消除以上的兩個缺點，但需要額外的增加設(shè)備成本。理解應(yīng)用的局限性，以及驅(qū)動設(shè)備的啟動力矩和速度要求，可以讓你為你的應(yīng)用確定最佳的整體配置。 D．變頻器 這種器件控制全面靈活，在一定量的電流可以產(chǎn)生最大的轉(zhuǎn)矩，但同時也是成本最高的。這種裝置不僅電壓變化，而且頻率也同時變化，使電機的工作在一個恒壓頻比條件下。這可以使電動機在整個調(diào)速度范圍內(nèi)產(chǎn)生滿載轉(zhuǎn)矩，最高可達10:1。在啟動過程中，典型的電流值為額定電流的150%。這使電動機帶載啟動時對電源的要求有很大的降低并減小了電機產(chǎn)生的熱量，所有這些加起來，很大的提高了工作效率。利用變頻器也可以使小電機在低速范圍內(nèi)產(chǎn)生很大的轉(zhuǎn)矩。但電機的型號必須大于要帶動的負載。在電機加速時變頻器可以允許一個很大程度的控制，而其他降壓啟動方式去不容易做到。變頻器相對于其他方式最大的缺點就是成本高！使用成本高，只能在特殊電機的設(shè)計上使用?；隍?qū)動器的輸出信號，濾波還是不濾波，都需要額外的裝置。 這些結(jié)構(gòu)特征包括絕緣軸承，鐵芯接地刷和絕緣連軸中來自共模電壓的潛在鐵芯電流。沒有這些結(jié)構(gòu)特征，鐵芯電流通過鐵芯、軸承、機殼形成回路，造成軸承彎曲使軸承過早的損壞。當使用電機和驅(qū)動裝置在危險環(huán)境和區(qū)域中時應(yīng)該考慮這種潛在的危險。電機用在變頻器上的額外結(jié)構(gòu)特征需要電機繞組有一個更好的絕緣系統(tǒng)。沒有濾波直接從驅(qū)動器出來的信號會在電機中產(chǎn)生諧波電壓毛刺，威脅電機繞組的絕緣。值得注意的是，在變頻器上電機的這些特征可以用來啟動和運行。如果這些驅(qū)動僅用于啟動電機，這些特征可能沒有必要，可以和電機制造商協(xié)商申請具體的要求。 E.串電阻器或電抗器起動 這種方法是利用一個串聯(lián)電阻或是電抗器放置在電機回路中。串電阻啟動方式在小電機上應(yīng)用更頻繁。 在電機啟動時，電阻器限制涌流并使電機輸入端的電壓下降。電阻可以選定使輸入電壓降為原來的50%。隨著電動機轉(zhuǎn)速的上升，產(chǎn)生一個反電動勢來對抗輸入電壓，進一步限制涌流。隨著涌流的下降，電阻器上的電壓下降使電動機的轉(zhuǎn)矩增加。在預定的時間，電阻器將被短接，快速打開啟動開關(guān)，將電阻器和電路分離。這種方法提供了一個封閉式過渡并消除了暫態(tài)的開關(guān)效應(yīng)。電抗器有助于阻擾電流的突然變化，因此在啟動時起限制電流的作用。在啟動結(jié)束后他們依然被短接掉并封閉的過渡到全壓。 Ⅳ增量型 第