异步电动机起动
中英文资料对照外文翻译文献综述
英文资料：
INDUCTION MOTOR STARTING METHODS
Abstract -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 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.
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 V ARS (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.
Ⅲ. 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 Starting
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.
E .Star delta Starting
This approach started with the induction motor, the structure of each phase of the terminal are placed in the motor terminal box. This allows the motor star connection in the initial startup, and then re-connected into a triangle run. The initial start time when the voltage is reduced to the original star connection, the starting current and starting torque by 2 / 3. Depending on the application, the motor switch to the triangle in the rotational speed of between 50% and the maximum speed. Must be noted that the same problems, including the previously mentioned switch method, if the open circuit method, the transition may be a transient problem. This method is often used in less than 600V motor, the rated voltage 2.3kV and higher are not suitable for star delta motor start method.
Ⅴ. 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.
IV .Conclusion
There are many ways asynchronous motor starting, according to the constraints of power systems, equipment costs, load the boot device to select the best method. From the device point of view, was the first full-pressure launch the cheapest way, but it may increase the cost efficiency in the use of, or the power supply system in the region can not meet their needs. Effective way to alleviate the buck starts the power supply system, but at the expense of the cost of starting torque.These methods may also lead to increased motor sizes have led to produce the required load torque. Inverter can be eliminated by the above two shortcomings, but requires an additional increase in equipment costs. Understand the limitations of the application, and drives the starting torque and speed, allowing you for your application to determine the best overall configuration.
英文资料翻译：
异步电动机起动的方法
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，但选择的型号必须合适，否则将会造成电容器的严重损坏。为电动机选择一个合适的启动方法，需要分析电力系统和启动负载以确保电机达到所需性能且成本最少。本文将探讨最常见的几种启动方法。
Ⅰ.引言
异步电动机有多种启动方法：全压启动、降压启动、星三角转换启动和部分绕组等类型。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买者大量类型的变化。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成本。
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，则电机就不能启动。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，则电机转子铜条、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。
Ⅱ.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，且设备成本最低工作最可靠。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机会使启动电流达到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。如果供电系统比较薄弱，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。
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。
避免过电压和电机损坏应小心谨慎确保电容器被切除当电机达到额定转速时,否则由于功率损失，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，除非当功率降低时他们可以和电机分离。
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。