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电动机软启动控制

2007年03月31日 10:02:00 中国传动网

三相感应电动机在线直接启动需要一个很高的电动机启动力矩及启动电流,软启动装置中的半导体必需有足够的电量以抵御芯片温度变化,并且需要很好的负荷循环能力。拥有双面可控硅芯片冷却,软启动的反平行可控硅模块SEMISTART较传统模块产生一半内部热阻抗,保证了启动期间高的过电流能力。 材料提供:Norbert Schafer & Ralf Herrmann,德国纽伦堡 反平行可控硅模块SEMiSTART(图一)专业设计用于软启动装置,拥有双面可控硅芯片冷却,模块设计较传统模块产生一半内部热阻抗。这个紧凑的模块还运用成熟的压力接点技术,实践中,运用三种不同类型的电动机启动控制,详细信息如下:
图一:反平行可控硅模块SEMiSTART设计专用于软启动装置
在线直接起动器 三相感应电机(异步电机)在线直接启动需要一个很高的电动机启动力矩及启动电流。高的电动机启动力矩可以导致机械损伤;例如,将会撕破三相感应电机带动的传送带。高启动电流可以导致网格内的电压尖脉冲。电动机越大这种影响也越大,为了防止不希望的损伤,在启动阶段感应电机的电压是受到控制的,这意味着启动电流和启动力矩可以受到限制(见图二)。
图二:直接在线启动和软启动电动机电流
星形三角形起动器 星形三角形起动器(即星/三角转换起动器)是一个简单的方案,当电动机提高运转速度时电动机定子绕组以星形接法连接,一旦电动机达到了额定转速,绕组即以三角接法连接,以星形连接的方法启动的作用是:在电动机达到正常转速的过程中经过每个定子绕组的电压是正常经过定子绕组电压的三分之一。从星形接法转到三角形接法通常是通过一个机械接触器来完成的。然而,因为只有两种交换连接法(星形接法和三角形接法),控制也就不是一个很适当的方法,此外,机械接触器由于火花容易造成磨损而需要替换,这种起动器控制维修率很高。 软起动器 在启动阶段控制感应电机的电压需要一个软启动装置(软启动器)。软启动器需要一个可控硅来控制电压。(见图三)
图三:软启动原理示范
反平行可控硅在电动机绕组和网格间串联,在加速到正常运行速度期间(斜坡启动)经过电动机绕组的电压是相位控制的。触发何时延迟意味着起动力矩和启动电流可以设定到期望值。软启动控制的领一个优点在于启动时间也可以得到控制。 通过可控硅的电流在半导体中产生功率消耗,这就使得半导体受热,然后半导体需要冷却,为了阻止斜坡启动阶段后功率的进一步消耗,半导体通过一个机械开关(接触器)旁通。旁通开关可以相对较小,因为无需开关大电流负荷。系统已经达到了额定运行速度,无需通过旁落开关转换更大的电压降落,唯一的电压降落是来自软启动器的构造设计和通过可控硅的触发。这即意味着无需转换更大的电流负载,这就使得软启动器维护率非常低。 半导体要求 为了确保软启动器同时具有紧凑和高性价比的特性,而不降低其可靠性,软启动器中的半导体需要满足重要的要求。 即使当软启动器在启动阶段运用的启动电流是额定电流的几倍(3-5倍),在大规模系统中,启动电流峰值通常达到好几千安培,因此半导体必需能够带动这样的高启动电流,同时,软启动器必需经济实惠并且尽可能简洁,因此,所用的半导体必需尽可能小(包括散热片)。 出于成本的原因,在实际中运用可控硅元件,其额定电流远远低于大型系统启动电流。这就是可控硅芯片在短暂的启动阶段充分加热的原因,即:从T斜坡启动温度40°C到130°C,芯片温度相差90K。如果系统每天转换三次,每天8小时,那么十年后负载总变化将达到87,600小时。可控硅必需能够在数十年内承受启动阶段的过载电流。 直到现在,软启动生产商在为其设备寻找适宜的半导体方面还存在困难。这也是反平行可控硅模块SEMiSTART可插足的市场,因为该模块是专为软启动装置研发。 安装和连接技术 组装和连接硅片有多种不同的方法。在许多模块中,用单面模块冷却将硅片同时焊接在阴极和阳极。(见图四)
图四:焊接模块原理
积聚在模块中的热量通过底盘(单面冷却)分散到散热片。一个特殊的问题是可控硅模块单个元件具有不同的热膨胀系数。在带有焊接头的模块中,可控硅芯片,焊料和铜(主端子)有不同的膨胀系数。 随着时间推移,由于负荷循环操作,不同的膨胀系数导致连接芯片和铜端子的焊料疲劳,结果出现焊料分层,即:焊料层出现细裂纹。焊料疲劳裂纹又导致热电阻增加,反过来,热电阻的增加导致了芯片温度升高,从而最终导致芯片失效。事实上,焊接模块中芯片失效不常发生。 模块是基于压力接点技术的,与之相反的是,芯片通过接点压力连接在主端子间,在这些模块中,主端子间不焊接芯片,而需要一个非常高的接点压力以固定主端子间的芯片。事实上,在高功率负荷应用中(额定电流>200A),运用压力接点技术连接的元件由于没有运用焊接,其负荷循环能力连接相对较高。 这就是赛米控推荐在大额定电流的软启动装置中使用压力接点元件的原因。而在SEMiSTART中也使用了压力接点技术。(见图五)
图五:SEMiSTART模块中所用压力接点技术
SEMiSTART模块中在两个散热片间模压可控硅芯片 这种安装和连接类型不包括焊接层,这也是SEMiSTART模块拥有卓越的负荷循环能力的原因,也正是由此使其拥有长效的使用寿命。 散热片选择最佳尺寸以适合芯片的尺寸和软启动装置,这就使得模块非常简洁小巧,可控硅和散热片间的总热阻远低于其他传统元件总热阻。因为芯片直接模压在散热片间,同时受到两面冷却,热阻就非常低。另外一个优势就是安装SEMiSTART模块非常方便,与安装模块一样,在安装塑封可控硅,时也无需特殊夹具,另外,在安装模块时也无需热胶。 当然,SEMiSTART模块可以应用于其他方面,如保护电路。SEMiSTART模块有三种不同尺寸和五种不同的电流等级。二十秒钟最大电流时间(斜坡启动时间)电流在500A – 3000A间。可控硅断态电压最大1800V。 结论 未来几年内,由于这些软启动控制元件方案较传统元件更为突出,软启动市场将继续发展,由于使用了双面可控硅芯片冷却及其紧凑的设计,软启动器反平行可控硅模块SEMISTART较传统模块产生一半的热阻。因此会出现短期的较高过电流,此外,由于使用了压力接点技术,这些模块具有高度可靠性。 original text [COLOR=#708090][b]POWER ELECTRONICS EUROPE MOTOR CONTROL Soft-Start Control of Electric Motors Soft-Start Control of Electric Motors[/b] Figure 1:The anti-parallel thyristor module SEMiSTART is designed specifically for use in soft-start devices For a three-phase induction motor direct on-line start involves a very high motor starting torque and a very high starting current. Semiconductors in soft-start devices must be extremely robust to resist considerable chip temperature changes and must demonstrate very good load cycle capability. The antiparallel thyristor module SEMISTART for soft-starters provides half the internal thermal impedance of conventional components, thanks to double-sided thyristor chip cooling and thus, ensures high over-current capability for the starting period. Norbert Schafer and Ralf Herrmann, SEMIKRON, Nuremberg, Germany The anti-parallel thyristor module SEMiSTART (Figure 1), designed specifically for use in soft-start devices, provides half the internal thermal impedance of conventional components in modular designs, thanks to double-sided thyristor chip cooling. This compact module also uses proven pressure contact technology. In practice, three different types of motor starter control are used which are described in the following. Direct on-line starters For a three-phase induction motor (asynchronous motor), direct on-line start involves a very high motor starting torque and a very high starting current. The high motor starting torque can lead to mechanical damage; for instance, the conveyor belt driven by the three-phase induction motor may tear. The high starting current can also result in voltage spikes in the grid. The larger the motor, the more serious the effects. To combat such undesired effects, the voltage applied to the induction motor during the start-up phase is controlled. This means that the starting current and, consequently, the starting torque can be limited (see Figure 2). Figure 2: Motor current at direct on-line start and with soft start Star-delta starters A simple solution is the star-delta starte (also known as the wye-delta starter). Here, the motor stator windings are connected in star (or wye) connection as the motor accelerates up to its running speed; once the motor reaches near rated speed, the windings are connected in delta. The effect of starting in star connection is that the voltage across each stator winding during the build-up to normal running speed is 1/3 of the normal. The changeover from star to delta connection is normally done using a mechanical contactor. As, however, there are only 2 switch connections (star and delta), controlling‘ is not a particularly appropriate term in this case. Moreover, this type of starter ‘control‘ is not low- maintenance, as the mechanical contactors are prone to wear caused by sparking and need to be replaced. Soft-starters To control the voltage applied to the induction motor during the start-up phase, a soft-start device (soft starter), is needed. In soft starters, thyristors are used for voltage control (see Figure 3). Figure 3:Principle schematic of a soft starter Two anti-parallel thyristors are connected in series between the motor windings and the grid. During the run-up to normal running speed (ramp-up), the voltage across the motor windings is the controlled by way of phase control. Depending on when the thyristors are fired (trigger delay angle a), this means that the starting torque and the starting current can be set to desired values. A further advantage of soft-start control is that the starting time can also be controlled. The current flowing through the thyristors produces power dissipation in the semiconductors. This power dissipation heats up the semiconductors, which then have to be cooled. To prevent further power dissipation in the semiconductors after the ramp-up phase the semiconductors are bypassed by a mechanical switch (mechanical contactor). This bypass switch can be relatively small since it does not have to switch large loads. The system has already reached normal running speed, no large voltage drop that has to be switched by the contacts of the bypass switch occurs. The only voltage drop is that resulting from the mechanical design and that across the fired thyristors. This means that no large loads are being switched, which is why soft-starters are low- maintenance devices. Semiconductor requirements To ensure that a soft-starter is both compact and cost-efficient without compromising reliability, the semiconductors used in a soft-starter must meet a number of important requirements. Even when a soft-starter is used the starting current during the starting phase of a drive system is still several times larger than the rated current (3-5 times higher). In large-scale systems, the peak starting current is often several thousand amperes. The semiconductors used therefore have to be able to carry this high starting current during the start phase. At the same time, however, the soft-starter must be cost-optimised and as compact as possible. For this reason, the semiconductors used (including heatsink) must be as small as possible. Thus, for reasons of cost, thyristor components whose rated current is far lower than the large system starting current are used in practice. This is why the thyristor chips heat up substantially during the brief start phase, e.g. from T Ramp-up = 40°C to T = 130°C, resulting in a chip temperature difference of 90K. If a system is switched on 3 times per hour, 8 hours a day on 365 days a year, the total number of load changes after 10 years is 87,600. These thyristors must be able to carry the overload current that occurs during the start phase for decades. Up till now, manufacturers of soft-starters have had difficulties in finding the optimum semiconductors for their devices on the market. This is where the anti-parallel thyristor module SEMiSTART steps in, as this module was developed specifically for use in soft-start devices. Mounting and connecting technology There are a number of different ways of assembling and connecting a silicon chip. In many modules, the silicon chip is soldered on both sides (anode and cathode side) with single-sided module cooling (see Figure 4). Figure 4:Principle behind the solder contact module The heat that builds up in the module is dissipated to the heatsink via the baseplate (single-sided cooling). A particular problem here is the different thermal expansion coefficients of the individual components used in a thyristor module. In modules with soldered connections, the thyristor chip, solder and copper (main terminals) have different expansion coefficients. Over time, these different coefficients lead to fatigue in the solder that connects the chip and the copper terminal due to load cycle operation. As a result, delamination of the solder layer occurs, i.e. fine hairline cracks appear in the solder layer. The solder fatigue cracking then results in an increase in thermal impedance which, in turn, leads to an increase in chip temperature and, ultimately chip failure. In fact, it is not unusual for chip failure to occur in soldered modules. In modules based on pressure contact technology, by way of contrast, the chip is connected between the main terminals by contact pressure. In these modules, the chip is not soldered between the main terminals. Instead, very high contact pressure (several kN) is applied to ‘retain‘ the chip between the main terminals. In practice it has been shown that, especially in applications with large power loads (rated currents >200A), the load cycle capability of the components connected using pressure contact technology is far superior owing to the non-use of soldered connections. This is why Semikron recommends using pressure-contacted components in soft-start devices with larger rated currents. And it is this very pressure contact technology that is used in SEMiSTART (see Figure 5). Figure 5:Pressure contact technology used in the SEMiSTART module In SEMiSTART modules the two thyristor chips are ‘pressed‘ between two heatsinks. This type of mounting and connection does not contain solder layers, which is why the SEMiSTART modules boast very good load cycle capability and, consequently, a long service life. The heatsinks are optimally dimensioned for the chip dimensions and for use in soft-start devices. The result is very compact modules. The total thermal resistance between the thyristor chips and heatsink is far lower than that of other conventional components. As the chips are pressed directly between two heatsinks and are cooled on both sides, the thermal resistances are very low. Another advantage is that very little mounting effort is necessary for SEMiSTART modules: no special clamps are needed as is the case when assembling capsule thyristors. Plus, no thermal paste is needed as in the case of module assembly. SEMiSTART modules can, of course, also be used in other applications, e.g. protective circuits. SEMiSTART modules come in three different sizes and a total of 5 different current classes. The current range is 500A – 3000A for a maximum current flow time (ramp-up time) of 20 seconds. The thyristors have a maximum off-state voltage of 1800V. Conclusion The market for soft-starters will continue to grow over the coming years as the advantages that these components boast over conventional solutions become more apparent. The antiparallel thyristor module SEMISTART for soft-starters provides half the internal thermal impedance of conventional components thanks to double-sided thyristor chip cooling and its compact design. Extremely high overcurrents are therefore possible for a short period. What‘s more, thanks to the use of pressure contact technology, these modules offer a high degree of reliability. [/COLOR]
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