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1、异步电动机起动的方法和问题摘要：大容量的交流异步电动机有多种启动方法。可选择的如全压启动、降压启动、自耦变压器启动、星三角转换启动、软启动、或者使用可调速驱动器，都有潜在优势和选择。降压启动可以减小启动转矩，可以防止损坏负载。此外，功率因数校正电容器可以用来减小电流，但选择的型号必须适宜，否那么将会造成电容器的严重损坏 。为电动机选择一个适宜的启动方法，需要分析电力系统和启动负载以确保电机到达所需性能且本钱最少。本文将探讨最常见的几种启动方法以及它们的应用。检索词：电机启动 降压启动 自耦变压器 星三角 功率因数校正.引言异步电动机有多种启动方法：全压启动、降压启动、星三角转换启动和局部绕组等
2、类型。降压启动包括固态启动器、变频启动和自耦变压器启动。这些连同全压或者直接启动，当电动机的应用场合被确定后可以给购置者大量类型的变化。每种方法都有它自己的好处，以及贸易业绩。合理的选择包括对电力系统透彻的研究，负载的加速以及设备的全部本钱。为了使负载能够很好的加速，电动机必须产生比负载需求更大的转矩。一般来说，在机械特性曲线上集中有三点。第一点是堵转转矩LRT，使电机由静止到旋转的最小转矩。第二点是最小启动转矩，使电机由静止加速到出现制动转矩是的最小转矩。最后一点是临街转矩，就是电机能产生的最大转矩。如果任何一段虚线在负载曲线以下，那么电机就不能启动。如图1所示。电机的加速时间是由负载的惯性
3、以及电机的机械特性曲线和负载的特性曲线之间的差额决定的。总的来说，电机的加速时间越长，那么电机转子铜条、端环、定子绕组产生的热量也就越多。这些热量会给这些部件带来额外的压力甚至还会影响到电机的使用寿命。.全电压全压文启动的方法也叫直接启动，是最早被使用的方法，且设备本钱最低工作最可靠。这种方法利用一个控制器闭合电流接触器给电动机输入全压。此方法允许电动机产生最大的启动转矩使加速时间最短。由于用此方法启动电时机使启动电流到达电机额定电流的六到七倍，因此方法也是给供电系统带来最大的压力的方法。如果供电系统比拟薄弱，大功率电机的突然启动不仅会使电动机的电压瞬间下降，而且会使给电机供电的整个母线端电压
4、下降。电压下降会使此电动机的启动转矩和工作在同一母线上的电动机的转矩下降。异步电动机的转矩大约和输入电压的平方成比例变化。因此，由于电压的大幅下降，工作在此供电系统的电动机可能停车。另外许多控制系统监控器工作在低电压下，第二个全压启动时电压问题会使正在运行的电机离线。同时，母线电压变化，直接启动的另一个问题驱动设备突然被加载。由于瞬时转矩可以超过转子制动转矩的600%，这个冲击负荷会增加设备的磨损，如果负载不能承受由电机启动产生的力矩，甚至会造成灾难性故障。A电容器和启动异步电动机功率因数通常很低,因此在开始有很大的感性无功。 如图2所示。 在启动时通过给电机增加电容器可以降低对系统的这种影响
5、。电机需要大量的无功电流滞后于输入电压90度。这个无功功率不产生任何输出，但是这是电机运行所必需的。输入电压产生无功功率和这个无功功率组成的可用无功功率功率表测量。电容器担任提供一个超前90度的电流。由电容器产生的超前电流取消了电机需要的滞后电流，降低了从供电系统输入的感性无功功率。防止过电压和电机损坏 应小心谨慎确保电容器被切除当电机到达额定转速时,否那么由于功率损失，电动机利用由电容器提供的磁化电流将不会进入发电模式. 这将在下一段和附录中详述。B功率因数校正电容器也可永久留下提高满载功率因数，当使用这种方式时被称为功率因数校正电容器。该电容器的容量不能大于电动机的励磁电流，除非当功率降低
6、时他们可以和电机别离。附加的电容器将会改变电机的开环时间常数，时间常数表示电机断电后电压衰减到原来的36.8%所需要的时间。没有电容的典型值是两到三秒。由于电容器和电动机的前端相连，在电机断开电源之后电容可以继续提供励磁电流。这说明该系统的时间常数较大。如果电机驱动高惯性负载，电动时机利用电容提供的励磁电流和负载的带动转变为发电状态。这可能导致电机端部的电压上升到额定电压的50%。如果在电压剧烈衰减之前再次通电会产生很大的开关电流和转矩，会造成电动机和驱动设备的严重损坏。这种现象的例子在附录中有概述。.降电压每一种降压方法都是通过控制电动机的输入电压来减小电动机起动电流对供电系统的冲击。在考虑
7、具体的降压启动方法时，了解负载特性是非常重要的。当电机制造商要确定他们的设计时必须知道电机的机械特性和被驱动设备的转动惯量。可以通过初始状态或脱离转矩，以及不少于四个速度上升的数据点和全速时的转矩构成启动状态的曲线。在很多情况下可以被假设成一个离心曲线或直角曲线，但是在一些应用场合这还是有问题的。例如螺杆式压缩机在低速时比离心式鼓风机类负载需要更大的转矩。通过详细了解负载的特性，厂商可以确定选择的启动方法可以产生足够大的力矩启动负载。A自耦变压器电动机连接在变压器的低压侧。80%、65%和50%是最常用的接头。电压为原来的50%电流那么为全压转子堵转时的25%。电动机降低电压启动，当到达预先设
8、定的条件时就切换成全压。这个条件可以是预先设定的时间、电流、母线电压或者电动机的转速。这种变化既可以采用闭环方式也可以采用开环方式。采用开环方式在低压和全压切换时线路电压严重，应小心使用，以确保不会由于切换产生暂态问题。这个潜在的问题可以采用闭环的方法消除。采用闭环的方式可以给电机提供连续的电压。利用自耦起动另一个好处是尽可能的降低了振动和噪音。因为电机的转矩随输入电压的平方变化而变化。应非常谨慎,以确保可以从电动机获得足够的加速转矩。应该用驱动设备的机械特性和惯性来验证电动机的设计。一个好的经验法那么是至少有10%的额定转矩，作为该曲线各点的极限。此外，对加速时间应进行计算，以确保电机有足够
9、的热容量将电机在长时间加速时产生的热量散发掉。B固态软启动器这些装置采用可控硅整流器或晶闸管，通过控制晶闸管的触发角，可以控制该装置产生的电压，并限制启动电流，在一个正弦周期中仅让局部通过，来启动电动机。最常用的软起动器的类型是限流型，电流被限制在满载电流的175%到500%并编入装置中。然后让输入电机的电压斜坡上升一直到限制值，并使保持电流不变使电机加速。利用转速表和软启动器可以控制加速时间，软启动器调整输出电压使电机维持一个恒定的加速度。关于启动力矩和其他降压启动方法有同样的问题，另一个问题是由于晶闸管的触发角会使电机产生振荡转矩，依靠驱动设备，可能导致系统共振。B星三角启动用这种方法启动
10、的异步电机，结构上每一相的引出端都被放置在电机的接线盒内。这允许电机在初始启动时星形连接，然后重新接成三角形运行。初始启动时用星形连接时电压降为原来的，启动电流和启动力矩下降2/3。根据应用，电机切换到三角形可在转速为50% 到最大速度之间。必须指出,同样的问题存在,包括先前谈到的切换方法，如果采用开路法，暂态过渡可能是一个问题。这种方法经常用在低于600V的电机上，额定电压在2.3kV及更高的电机那么不适合用星三角启动的方法。.结论异步电动机有许多启动方法，依据电力系统的制约、设备本钱、负载设备来选择最好的启动方法。从设备的角度来看，全压启动是最早的最廉价的方法，但它可能在使用效率上增加本钱
11、，或者该区域的供电系统不能满足其需要。降压启动方式有效的缓解了供电系统，却以牺牲启动力矩为代价的。这些方法也可能导致不得不加大电机尺寸以产生转矩带动所需的负荷。采用变频器可以消除以上的两个缺点，但需要额外的增加设备本钱。理解应用的局限性，以及驱动设备的启动力矩和速度要求，可以让你为你的应用确定最正确的整体配置。C变频器这种器件控制全面灵活，在一定量的电流可以产生最大的转矩，但同时也是本钱最高的。这种装置不仅电压变化，而且频率也同时变化，使电机的工作在一个恒压频比条件下。这可以使电动机在整个调速度范围内产生满载转矩，最高可达10:1。在启动过程中，典型的电流值为额定电流的150%。这使电动机带载
12、启动时对电源的要求有很大的降低并减小了电机产生的热量，所有这些加起来，很大的提高了工作效率。利用变频器也可以使小电机在低速范围内产生很大的转矩。但电机的型号必须大于要带动的负载。在电机加速时变频器可以允许一个很大程度的控制，而其他降压启动方式去不容易做到。变频器相对于其他方式最大的缺点就是本钱高！使用本钱高，只能在特殊电机的设计上使用。基于驱动器的输出信号，滤波还是不滤波，都需要额外的装置。这些结构特征包括绝缘轴承，铁芯接地刷和绝缘连轴中来自共模电压的潜在铁芯电流。没有这些结构特征，铁芯电流通过铁芯、轴承、机壳形成回路，造成轴承弯曲使轴承过早的损坏。当使用电机和驱动装置在危险环境和区域中时应该
13、考虑这种潜在的危险。电机用在变频器上的额外结构特征需要电机绕组有一个更好的绝缘系统。没有滤波直接从驱动器出来的信号会在电机中产生谐波电压毛刺，威胁电机绕组的绝缘。值得注意的是，在变频器上电机的这些特征可以用来启动和运行。如果这些驱动仅用于启动电机，这些特征可能没有必要，可以和电机制造商协商申请具体的要求。D. 串电阻器或电抗器起动这种方法是利用一个串联电阻或是电抗器放置在电机回路中。串电阻启动方式在小电机上应用更频繁。在电机启动时，电阻器限制涌流并使电机输入端的电压下降。电阻可以选定使输入电压降为原来的50%。随着电动机转速的上升，产生一个反电动势来对抗输入电压，进一步限制涌流。随着涌流的下降
14、，电阻器上的电压下降使电动机的转矩增加。在预定的时间，电阻器将被短接，快速翻开启动开关，将电阻器和电路别离。这种方法提供了一个封闭式过渡并消除了暂态的开关效应。电抗器有助于阻扰电流的突然变化，因此在启动时起限制电流的作用。在启动结束后他们依然被短接掉并封闭的过渡到全压。增量型第一类启动方式，我们已经讨论了适用于电机能量的处理方式。下一类型的处理方式不同，通过改变电机自身结构来处理启动问题。局部绕组法，用这种方法，电机的定子是这样设计的，有两个独立的线圈组成。最普遍的方法是半绕组法，顾名思义，其定子是有两个相同的平衡绕组组成。因此被设定的特殊启动器全压加在一半绕组上，然后经过短暂的延时到另一半上
15、。这种方式可使启动电流和启动力矩降低50%到60%。这种方法的一个缺点就是在电机启动的第一步发热比直接启动严重。因此完成第一步的间隔时间应尽量减小。这种方法在第一步时也INDUCTION MOTOR STARTING METHODS AND ISSUESAbstract - Many methods can be used to start large AC induction motors. Choices such as full voltage, reduced voltage either by autotransformer or Wyes - Delta, a soft start
16、er, 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 t
17、hem 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
18、 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 correctionI. INTRODUCTIONThere are several general methods of starting induction motors: full voltage, reduced voltage
19、, 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
20、 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, t
21、he motor must generate greater torque than the load requirement. In general there are three points of interest on the motors 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
22、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
23、 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
24、 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 VOLTAGEThe full voltage starting method, also known
25、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
26、 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 t
27、emporary 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 varie
28、s 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
29、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
30、 equipment, or even cause a catastrophic failure if the load can not handle the torques generated by the motor during staring.A. Capacitors and StartingInduction motors typically have very low power factor during starting and as a result have very large reactive power draw. See Fig. 2. This effect o
31、n 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 doesnt create any measurable output, but is rather the energy required for the motor to function. The
32、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
33、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 no
34、t 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 CorrectionCapacitors can also be left permanently connected to raise the full load power factor. When used in this manner they are c
35、alled 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. Th
36、e 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
37、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 magnetizingCurrent from the capacitors and the shaft driven by the load. This ca
38、n 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 d
39、riven equipment. An example of this phenomenon is outlined in the appendix.Ill. REDUCED VOLTAGEEach 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
40、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 tor
41、que, 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 hi
42、gher 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
43、.A. AutotransformerThe 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 af
44、ter 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 conn
45、ection 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 circui
46、t method there is a continuousVoltage 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 su
47、re 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 margi
48、n 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 StartersThese devices utilize silicon controlled rectifiers or Scar
49、s. 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 va