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(dòng)機(jī)起動(dòng)的方法和問(wèn)題
摘要：大容量的交流異步電動(dòng)機(jī)有多種啟動(dòng)方法。可選擇的如全壓?jiǎn)?dòng)、降壓?jiǎn)?dòng)、自耦變壓器啟動(dòng)、星三角轉(zhuǎn)換啟動(dòng)、軟啟動(dòng)、或者使用可調(diào)速驅(qū)動(dòng)器，都有潛在優(yōu)勢(shì)和選擇。降壓?jiǎn)?dòng)可以減小啟動(dòng)轉(zhuǎn)矩，可以防止損壞負(fù)載。此外，功率因數(shù)校正電容器可以用來(lái)減小電流，但選擇的型號(hào)必須合適，否則將會(huì)造成電容器的嚴(yán)重?fù)p壞 。為電動(dòng)機(jī)選擇一個(gè)合適的啟動(dòng)方法，需要分析電力系統(tǒng)和啟動(dòng)負(fù)載以確保電機(jī)達(dá)到所需性能且成本最少。本文將探討最常見的幾種啟動(dòng)方法以及它們的應(yīng)用。
檢索詞：電機(jī)啟動(dòng) 降壓?jiǎn)?dòng) 自耦變壓器 星三角 功率因數(shù)校正
Ⅰ.引言
異步電動(dòng)機(jī)有多種啟動(dòng)方法：全壓?jiǎn)?dòng)、降壓?jiǎn)?dòng)、星三角轉(zhuǎn)換啟動(dòng)和部分繞組等類型。降壓?jiǎn)?dòng)包括固態(tài)啟動(dòng)器、變頻啟動(dòng)和自耦變壓器啟動(dòng)。這些連同全壓或者直接啟動(dòng)，當(dāng)電動(dòng)機(jī)的應(yīng)用場(chǎng)合被確定后可以給購(gòu)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(jī)。合理的選擇包括對(duì)電力系統(tǒng)透徹的研究，負(fù)載的加速以及設(shè)備的全部成本。
為了使負(fù)載能夠很好的加速，電動(dòng)機(jī)必須產(chǎn)生比負(fù)載需求更大的轉(zhuǎn)矩。一般來(lái)說(shuō)，在機(jī)械特性曲線上集中有三點(diǎn)。第一點(diǎn)是堵轉(zhuǎn)轉(zhuǎn)矩（LRT），使電機(jī)由靜止到旋轉(zhuǎn)的最小轉(zhuǎn)矩。第二點(diǎn)是最小啟動(dòng)轉(zhuǎn)矩，使電機(jī)由靜止加速到出現(xiàn)制動(dòng)轉(zhuǎn)矩是的最小轉(zhuǎn)矩。最后一點(diǎn)是臨街轉(zhuǎn)矩，就是電機(jī)能產(chǎn)生的最大轉(zhuǎn)矩。如果任何一段虛線在負(fù)載曲線以下，則電機(jī)就不能啟動(dòng)。電機(jī)的加速時(shí)間是由負(fù)載的慣性以及電機(jī)的機(jī)械特性曲線和負(fù)載的特性曲線之間的差額決定的?？偟膩?lái)說(shuō)，電機(jī)的加速時(shí)間越長(zhǎng)，則電機(jī)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(huì)給這些部件帶來(lái)額外的壓力甚至還會(huì)影響到電機(jī)的使用壽命。
Ⅱ.全電壓
全壓文啟動(dòng)的方法也叫直接啟動(dòng)，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(gè)控制器閉合電流接觸器給電動(dòng)機(jī)輸入全壓。此方法允許電動(dòng)機(jī)產(chǎn)生最大的啟動(dòng)轉(zhuǎn)矩使加速時(shí)間最短。
由于用此方法啟動(dòng)電機(jī)會(huì)使啟動(dòng)電流達(dá)到電機(jī)額定電流的六到七倍，因此方法也是給供電系統(tǒng)帶來(lái)最大的壓力的方法。如果供電系統(tǒng)比較薄弱，大功率電機(jī)的突然啟動(dòng)不僅會(huì)使電動(dòng)機(jī)的電壓瞬間下降，而且會(huì)使給電機(jī)供電的整個(gè)母線端電壓下降。電壓下降會(huì)使此電動(dòng)機(jī)的啟動(dòng)轉(zhuǎn)矩和工作在同一母線上的電動(dòng)機(jī)的轉(zhuǎn)矩下降。異步電動(dòng)機(jī)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(dòng)機(jī)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(gè)全壓?jiǎn)?dòng)時(shí)電壓?jiǎn)栴}會(huì)使正在運(yùn)行的電機(jī)離線。同時(shí)，母線電壓變化，直接啟動(dòng)的另一個(gè)問(wèn)題驅(qū)動(dòng)設(shè)備突然被加載。由于瞬時(shí)轉(zhuǎn)矩可以超過(guò)轉(zhuǎn)子制動(dòng)轉(zhuǎn)矩的600%，這個(gè)沖擊負(fù)荷會(huì)增加設(shè)備的磨損，如果負(fù)載不能承受由電機(jī)啟動(dòng)產(chǎn)生的力矩，甚至?xí)斐蔀?zāi)難性故障。
A．電容器和啟動(dòng)
異步電動(dòng)機(jī)功率因數(shù)通常很低,因此在開始有很大的感性無(wú)功。 如圖2所示。 在啟動(dòng)時(shí)通過(guò)給電機(jī)增加電容器可以降低對(duì)系統(tǒng)的這種影響。
電機(jī)需要大量的無(wú)功電流滯后于輸入電壓90度。這個(gè)無(wú)功功率不產(chǎn)生任何輸出，但是這是電機(jī)運(yùn)行所必需的。輸入電壓產(chǎn)生無(wú)功功率和這個(gè)無(wú)功功率組成的可用無(wú)功功率功率表測(cè)量。電容器擔(dān)任提供一個(gè)超前90度的電流。由電容器產(chǎn)生的超前電流取消了電機(jī)需要的滯后電流，降低了從供電系統(tǒng)輸入的感性無(wú)功功率。
避免過(guò)電壓和電機(jī)損壞 應(yīng)小心謹(jǐn)慎確保電容器被切除當(dāng)電機(jī)達(dá)到額定轉(zhuǎn)速時(shí),否則由于功率損失，電動(dòng)機(jī)利用由電容器提供的磁化電流將不會(huì)進(jìn)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(dāng)使用這種方式時(shí)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(dòng)機(jī)的勵(lì)磁電流，除非當(dāng)功率降低時(shí)他們可以和電機(jī)分離。
附加的電容器將會(huì)改變電機(jī)的開環(huán)時(shí)間常數(shù)，時(shí)間常數(shù)表示電機(jī)斷電后電壓衰減到原來(lái)的36.8%所需要的時(shí)間。沒(méi)有電容的典型值是兩到三秒。
由于電容器和電動(dòng)機(jī)的前端相連，在電機(jī)斷開電源之后電容可以繼續(xù)提供勵(lì)磁電流。這表明該系統(tǒng)的時(shí)間常數(shù)較大。如果電機(jī)驅(qū)動(dòng)高慣性負(fù)載，電動(dòng)機(jī)會(huì)利用電容提供的勵(lì)磁電流和負(fù)載的帶動(dòng)轉(zhuǎn)變?yōu)榘l(fā)電狀態(tài)。這可能導(dǎo)致電機(jī)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(huì)產(chǎn)生很大的開關(guān)電流和轉(zhuǎn)矩，會(huì)造成電動(dòng)機(jī)和驅(qū)動(dòng)設(shè)備的嚴(yán)重?fù)p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(guò)控制電動(dòng)機(jī)的輸入電壓來(lái)減小電動(dòng)機(jī)起動(dòng)電流對(duì)供電系統(tǒng)的沖擊。在考慮具體的降壓?jiǎn)?dòng)方法時(shí)，了解負(fù)載特性是非常重要的。當(dāng)電機(jī)制造商要確定他們的設(shè)計(jì)時(shí)必須知道電機(jī)的機(jī)械特性和被驅(qū)動(dòng)設(shè)備的轉(zhuǎn)動(dòng)慣量?？梢酝ㄟ^(guò)初始狀態(tài)或脫離轉(zhuǎn)矩，以及不少于四個(gè)速度上升的數(shù)據(jù)點(diǎn)和全速時(shí)的轉(zhuǎn)矩構(gòu)成啟動(dòng)狀態(tài)的曲線。在很多情況下可以被假設(shè)成一個(gè)離心曲線或直角曲線，但是在一些應(yīng)用場(chǎng)合這還是有問(wèn)題的。例如螺桿式壓縮機(jī)在低速時(shí)比離心式鼓風(fēng)機(jī)類負(fù)載需要更大的轉(zhuǎn)矩。通過(guò)詳細(xì)了解負(fù)載的特性，廠商可以確定選擇的啟動(dòng)方法可以產(chǎn)生足夠大的力矩啟動(dòng)負(fù)載。
A．自耦變壓器
電動(dòng)機(jī)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(lái)的50%電流則為全壓轉(zhuǎn)子堵轉(zhuǎn)時(shí)的25%。電動(dòng)機(jī)降低電壓?jiǎn)?dòng)，當(dāng)達(dá)到預(yù)先設(shè)定的條件時(shí)就切換成全壓。這個(gè)條件可以是預(yù)先設(shè)定的時(shí)間、電流、母線電壓或者電動(dòng)機(jī)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(shí)線路電壓嚴(yán)重，應(yīng)小心使用，以確保不會(huì)由于切換產(chǎn)生暫態(tài)問(wèn)題。這個(gè)潛在的問(wèn)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(jī)提供連續(xù)的電壓。利用自耦起動(dòng)另一個(gè)好處是盡可能的降低了振動(dòng)和噪音。
因?yàn)殡姍C(jī)的轉(zhuǎn)矩隨輸入電壓的平方變化而變化。應(yīng)非常謹(jǐn)慎,以確?？梢詮碾妱?dòng)機(jī)獲得足夠的加速轉(zhuǎn)矩。應(yīng)該用驅(qū)動(dòng)設(shè)備的機(jī)械特性和慣性來(lái)驗(yàn)證電動(dòng)機(jī)的設(shè)計(jì)。一個(gè)好的經(jīng)驗(yàn)法則是至少有10%的額定轉(zhuǎn)矩，作為該曲線各點(diǎn)的極限。
此外，對(duì)加速時(shí)間應(yīng)進(jìn)行計(jì)算，以確保電機(jī)有足夠的熱容量將電機(jī)在長(zhǎng)時(shí)間加速時(shí)產(chǎn)生的熱量散發(fā)掉。
B．固態(tài)軟啟動(dòng)器
這些裝置采用可控硅整流器或晶閘管，通過(guò)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(dòng)電流，在一個(gè)正弦周期中僅讓部分通過(guò)，來(lái)啟動(dòng)電動(dòng)機(jī)。最常用的軟起動(dòng)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(jī)的電壓斜坡上升一直到限制值，并使保持電流不變使電機(jī)加速。利用轉(zhuǎn)速表和軟啟動(dòng)器可以控制加速時(shí)間，軟啟動(dòng)器調(diào)整輸出電壓使電機(jī)維持一個(gè)恒定的加速度。關(guān)于啟動(dòng)力矩和其他降壓?jiǎn)?dòng)方法有同樣的問(wèn)題，另一個(gè)問(wèn)題是由于晶閘管的觸發(fā)角會(huì)使電機(jī)產(chǎn)生振蕩轉(zhuǎn)矩，依靠驅(qū)動(dòng)設(shè)備，可能導(dǎo)致系統(tǒng)共振。
C．星三角啟動(dòng)
用這種方法啟動(dòng)的異步電機(jī)，結(jié)構(gòu)上每一相的引出端都被放置在電機(jī)的接線盒內(nèi)。這允許電機(jī)在初始啟動(dòng)時(shí)星形連接，然后重新接成三角形運(yùn)行。初始啟動(dòng)時(shí)用星形連接時(shí)電壓降為原來(lái)的，啟動(dòng)電流和啟動(dòng)力矩下降2/3。根據(jù)應(yīng)用，電機(jī)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(wèn)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(guò)渡可能是一個(gè)問(wèn)題。這種方法經(jīng)常用在低于600V的電機(jī)上，額定電壓在2.3kV及更高的電機(jī)則不適合用星三角啟動(dòng)的方法。
Ⅳ.結(jié)論
異步電動(dòng)機(jī)有許多啟動(dòng)方法，依據(jù)電力系統(tǒng)的制約、設(shè)備成本、負(fù)載設(shè)備來(lái)選擇最好的啟動(dòng)方法。從設(shè)備的角度來(lái)看，全壓?jiǎn)?dòng)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?jiǎn)?dòng)方式有效的緩解了供電系統(tǒng)，卻以犧牲啟動(dòng)力矩為代價(jià)的。這些方法也可能導(dǎo)致不得不加大電機(jī)尺寸以產(chǎn)生轉(zhuǎn)矩帶動(dòng)所需的負(fù)荷。采用變頻器可以消除以上的兩個(gè)缺點(diǎn)，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(dòng)設(shè)備的啟動(dòng)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(shí)也是成本最高的。這種裝置不僅電壓變化，而且頻率也同時(shí)變化，使電機(jī)的工作在一個(gè)恒壓頻比條件下。這可以使電動(dòng)機(jī)在整個(gè)調(diào)速度范圍內(nèi)產(chǎn)生滿載轉(zhuǎn)矩，最高可達(dá)10:1。在啟動(dòng)過(guò)程中，典型的電流值為額定電流的150%。這使電動(dòng)機(jī)帶載啟動(dòng)時(shí)對(duì)電源的要求有很大的降低并減小了電機(jī)產(chǎn)生的熱量，所有這些加起來(lái)，很大的提高了工作效率。利用變頻器也可以使小電機(jī)在低速范圍內(nèi)產(chǎn)生很大的轉(zhuǎn)矩。但電機(jī)的型號(hào)必須大于要帶動(dòng)的負(fù)載。在電機(jī)加速時(shí)變頻器可以允許一個(gè)很大程度的控制，而其他降壓?jiǎn)?dòng)方式去不容易做到。變頻器相對(duì)于其他方式最大的缺點(diǎn)就是成本高！使用成本高，只能在特殊電機(jī)的設(shè)計(jì)上使用?；隍?qū)動(dòng)器的輸出信號(hào)，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(lái)自共模電壓的潛在鐵芯電流。沒(méi)有這些結(jié)構(gòu)特征，鐵芯電流通過(guò)鐵芯、軸承、機(jī)殼形成回路，造成軸承彎曲使軸承過(guò)早的損壞。當(dāng)使用電機(jī)和驅(qū)動(dòng)裝置在危險(xiǎn)環(huán)境和區(qū)域中時(shí)應(yīng)該考慮這種潛在的危險(xiǎn)。電機(jī)用在變頻器上的額外結(jié)構(gòu)特征需要電機(jī)繞組有一個(gè)更好的絕緣系統(tǒng)。沒(méi)有濾波直接從驅(qū)動(dòng)器出來(lái)的信號(hào)會(huì)在電機(jī)中產(chǎn)生諧波電壓毛刺，威脅電機(jī)繞組的絕緣。值得注意的是，在變頻器上電機(jī)的這些特征可以用來(lái)啟動(dòng)和運(yùn)行。如果這些驅(qū)動(dòng)僅用于啟動(dòng)電機(jī)，這些特征可能沒(méi)有必要，可以和電機(jī)制造商協(xié)商申請(qǐng)具體的要求。
E.串電阻器或電抗器起動(dòng)
這種方法是利用一個(gè)串聯(lián)電阻或是電抗器放置在電機(jī)回路中。串電阻啟動(dòng)方式在小電機(jī)上應(yīng)用更頻繁。
在電機(jī)啟動(dòng)時(shí)，電阻器限制涌流并使電機(jī)輸入端的電壓下降。電阻可以選定使輸入電壓降為原來(lái)的50%。隨著電動(dòng)機(jī)轉(zhuǎn)速的上升，產(chǎn)生一個(gè)反電動(dòng)勢(shì)來(lái)對(duì)抗輸入電壓，進(jìn)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(dòng)機(jī)的轉(zhuǎn)矩增加。在預(yù)定的時(shí)間，電阻器將被短接，快速打開啟動(dòng)開關(guān)，將電阻器和電路分離。這種方法提供了一個(gè)封閉式過(guò)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(dòng)時(shí)起限制電流的作用。在啟動(dòng)結(jié)束后他們依然被短接掉并封閉的過(guò)渡到全壓。
Ⅳ增量型
第
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