电动机控制外文文献翻译、中英文翻译、外文翻译
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附录一：文献翻译第 3 部分电动机控制3.3 继电器和接触器3.3.1 控制继电器磁继电器作为辅助设备控制开关控制电路和大型发动机起动器、接触器线圈来控制小负载小型汽车、螺线管、电动加热器,灯光、音响信号装置和其他继电器。磁继电器是由一个电磁铁,电磁铁时打开或关闭电气接触器断开。继电器一般用于放大机或放大接触能力,或试验设备的开关函数添加更多电路。大多数继电器用于控制电路;因此,他们的低评级(0-15 安培最大到 800 伏特)显示的当前水平降低它们的运作方式。磁继电器不提供电机过载保护。通常这种类型的继电器用于两个线控制系统(任何电接触式设备有两个电线)。当它被设计为使用瞬时接触试验设备,如按钮。任何可用的常开触点可以连接作为一个持有三线制电路。初学者来说,接触器,继电器是相似的在建设和运营,但并不相同。控制继电器在各种单或双掷安排常开的组合(不)和常闭(NC)接触电路。虽然有一些 single-break 用于工业继电器的接触,大部分的继电器用于机床制 15:40 接触。这可能是特别感兴趣的电工改变联系人,常常闭,或反过来数控没有。大多数机床继电器有一些手段使这种变化。它的范围从简单的翻联系删除搬迁弹簧位置变化。同时,通过重叠接触器在这种情况下,一个接触可以安排操作不同的时间相对于另一个在同一继电器接触。例如,常开前联系关闭(使)常闭触点打开(休息)。继电器在不同电压等级、联系人、联系重排,物理尺寸,和附件提供辅助功能,如机械自锁和时机。在使用继电器为特定的应用程序中,应确定的第一个步骤控制(线圈)的电压继电器将必要的评级必须接触,联系人的数量以及其他特征。因为各种各样的风格的继电器,可以选择正确的传递几乎任何应用程序。继电器通常用来打开和关闭操作控制电路比功率电路。FIG.3-5 晶体管固态开关装置典型应用包括电动机起动器的控制和接触器线圈的开关螺线管和其他继电器的控制。继电器是一个小而重要的开关的组成部分很多复杂的控制系统。低压继电器系统广泛用于切换住宅和商业照明电路和照明灯具。而控制继电器各制造商在外观和结构不同,他们是可以互换的控制布线系统产品规格是否匹配系统的需求。在许多形状和配置控制继电器是可用的。有防尘,控制继电器的透明外壳。端子插入,就像一个电子管。另一个继电器的类型是非常小的簧片继电器,接触封闭的玻璃。它操作一个磁场。3.3.2 固态继电器相比一个电磁继电器,固态继电器没有线圈或只需要最小值电压和电流的打开和关闭它,固态继电器依赖于电子设备,如晶体管和可控硅整流器(可控硅)切换。3.3.3 晶体管作为开关图 3 - 5 显示了一个基本的固态开关装置用于逻辑组件(继电器)。晶体管是的核心元素。晶体管的基础控制发射器和之间的电流收集器。在这种类型的晶体管,一个负面电压允许发射极基极电流,固定在底座上流。这是由于材料的属性发射极和基极的结。的发射极基极电流使晶体管进行从发射极电流收集器。一个正电压固定在底座上防止发射极基极电流通过,晶体管停止开展。因此,它表现为一个封闭的接触在第一个国家和作为一个开放联系在第二。因为这个原因,行动被称为固态开关,即没有转动的联系人需的。只有一个电子信号来打开或关闭电路。结果,固态设备非常可靠和有一个非常漫长的一生。固态设备不灭弧,磨损,或恶化,磁继电器3.3.4 过载保护当固态设备使用磁开关、电压瞬态抑制器有必要阻止一些有害的电子“噪音”。更加复杂的需要保护微机控制机器人组装生产线。3.3.5 接触器磁接触器 eletromagnetically 操作开关提供一个安全、方便意味着连接和中断分支电路。校长之别接触器和电动机起动器,接触器不包含过载继电器。接触器结合试验控制装置用于开关照明和取暖负荷和控制交流电机过载保护的情况下单独提供。更大的接触器尺寸用于提供远程控制相对大电流的电路太贵的权力运行导致远程控制位置,FIG.3-6。这种灵活性是主要优势之一的电磁控制手动控制。飞行员等设备按钮, 浮子开关、压力开关、限位开关、恒温器提供接触器的操作。FIG.3-6 远程控制负载的一个优势磁防重型接触 arc-chutes 提供大多数的更大的接触器。的降落伞包含重铜线圈称为防线圈,安装以上联系人在系列负载提供更好的消弧线。这些防磁线圈帮助扑灭电弧在接触交流和直流负载下开放。电弧可能是相似的强度随着电弧焊接过程。一个 arc-quenching 设备使用保证接触寿命更长。自热电弧从接触技巧非常迅速,联系人保持冷静,所以他们持续时间更长。接触和电动机起动器接触,经常打破沉重的水流受破坏性的燃烧FIG.3-6 远程控制负载的一个优势磁防重型接触 arc-chutes 提供大多数的更大的接触器。包含重铜线圈称为防线圈,安装以上联系人在系列负载提供更好的消弧线。这些防磁线圈帮助扑灭电弧在接触交流和直流负载下开放。电弧可能是相似的强度随着电弧焊接过程。一个 arc-quenching 设备使用保证接触寿命更长。自热电弧从接触技巧非常迅速,联系人保持冷静,所以他们持续时间更长。接触和电动机起动器接触,经常打破沉重的水流受破坏性的燃烧效果如果电弧不容易熄灭。时形成的弧接触开放可以延长,扑灭由电动机动作如果在一个磁场。这个磁场是由磁防线圈提供。因为磁铁的线圈通常与线串联,磁场强度和灭火行动是弧的大小比例。图 3 - 7 是一个草图防磁铁的直导线(ab)位于与磁铁场和串联。这个数字可以表示直流极性或瞬时交流,交流电流、防线圈磁场和导体(电弧)磁场同时反向。根据弗莱明的左手定则, 电机动作会力量售票员在一个向上的方向。应用右手定则的信号指挥表明,导线周围的磁场艾滋病的主要领域和底部反对在顶部,售票员因此产生一个向上的力量。图 3 - 8 显示了年代的图 3 - 7 节线(ab)取而代之的是一组联系人。联系人已经开始开放,他们之间有一个弧。图 3 - 9 显示了什么是因为磁性的行动。部分显示了开始时的挠度弧由于影响电动机的行动。B 部分表明,接触更多的然后和弧开始爬上角,因为汽车行动的影响增加温度。图 3 - 9 所示的 C 部分的角附近的弧。在这点,电弧加长,将熄灭。 防磁的作用是将电弧向上在同一时间接触弧。因此,电弧延长速度超过通常会发生因为单独的接触。很明显,时间越短弧允许存在,伤害就越少联系。大多数弧淬火动作在这一原则。3.3.6 机械交流接触器和继电器举行的机械继电器或接触器,是由电磁铁但电磁铁在继电器自动断开了联系。因此,这些继电器机械在位置和没有电流流过开关后操作这些电磁铁的线圈。很明显,因此,在连续操作多个单位的实质性的大小会降低电能的需求。同时,磁保持继电器,相比之下,将改变接触位置的电压损失电磁铁,而机械继电器只会回应的行动控制装置。操作顺序指的是图 3 - 10,暂时按下“关”按钮时,当前的流动从 L1 到“on”按钮接触通电线圈通过现在的关闭清除接触,L2。现在的继电器关闭和锁存器机械。在同一时间关闭联系人(在图 3-11),照明灯具的银行当断路器关闭。门闩,继电器,从而关闭灯,暂时关闭按钮被按下,门闩,继电器和打开联系人,关掉灯。大多数操作线圈不为连续运行而设计的。因此,他们自动断开联系防止意外线圈倦怠。这些线圈清算联系人交替变化的位置接触器闭锁位置的变化。图 3 - 12 显示了一个三相电力负荷的应用程序使用一个主接触器断开配电盘。选择性单、三相分支电路可以切换由其他独立机械接触器或继电器举行。这些机械接触器和继电器机电设备,图 3。他们提供了一个安全、方便的切换电路,安静的操作,能量电路连接效率和连续性的要求安装。例如,电路连续性在电源故障自动处理设备通常是重要的,在一系列的操作必须继续从权力后中断吗恢复,而然后返回序列的开始。接触器和安静的操作继电器是必需的在许多控制系统用于医院、学校、办公楼。机械接触器和继电器通常用于地方轻微的嗡嗡声,交流磁设备的特点,是令人反感的。此外, 机械继电器机床控制电路中经常使用。这些继电器可以锁住和拉开通过限位开关的操作, 时间继电器,起动器联锁、timeclocks 光电电池,其他控制继电器、按钮。一般来说, 机械继电器可在 10 - 15-ampere 大小;机械接触器也可以在尺寸范围从 30 安培 1200 安培。3.4.2 流体减震器时间继电器磁,油缓冲器延时继电器可能用于电压 800 伏交流电直流。联系人是由铁芯的运动。的磁场电磁线圈拳的铁芯对制动力活塞移动充油的减震器。这种类型的继电器不是很准确。活塞必须允许定居下来之间的缓冲器的底部连续时间周期。如果活塞不允许全部返回,时间是不稳定的。减震器的延时继电器延时后磁铁是精力充沛。接触不同可能常开或常闭应用程序。与磁过载继电器、减震器延时继电器运行电压线圈连接整个线通过接触或开关。过载继电器操作的电流线圈所受影响电动机电流负载。流体减震器时间继电器用于许多应用程序: 控制加速电机接触器开始 时间关闭或打开阀门的制冷设备 为任何应用程序的操作序列延迟。有必要的时间延迟是非常准确的。这些继电器使用硅胶减震器流体不是一个石油。流体有助于消除不同粘度的影响时间由于环境温度的变化。硅胶流体运作成功的环境温度范围+ 48.9到-34.4。的可以轻松调整时间范围从 2 秒到 30 秒。多次接触缓冲器时间继电器用于直流电机起动。当线圈这种类型的延时继电器精力充沛,纷纷联系密切,中间有一个时间差每一个结束。3.4.3 气动计时器的结构和性能特点气动(空气)定时器使它适合多数工业应用。气动计时器有以下特点: 不受正常环境温度和大气压力的变化 可调在一个广泛的时间周期 良好的重复精度 可用各种各样的联系和时间安排。这种类型的继电器有一个气动延时单元,由一个机械操作磁铁的结构。延时功能取决于空气通过一个限制的转移孔板钢筋的使用合成橡胶波纹管或隔膜。时间范围是调整定位针阀不同数量的孔或排气限制。激励的过程或断电路可以控制的气动时间继电器试验设备(如按钮、限位开关或恒温继电器。自从权力了由时间继电器线圈是小, 敏感的控制设备可用于控制操作序列。气动时间继电器用于汽车加速和自动控制电路。自动控制是必要的在应用程序重复精度是必需的,例如控制序列操作的机床和控制,工业过程操作,输送线。3.4.4 磁限时继电器如果线圈中的电流增加,那么由于自感电动势方向反对行为增加线圈的电流。如果线圈中的电流减少,emf 由于自感线圈的行为方向反对降低线圈的电流。一种磁期限继电器有一个线圈绕在一个空心铜柱含铁核心。其他时间限制继电器线圈铜夹套。对任何一种继电器、时间延迟继电器下降时提供。线圈的电路断开时,电流迅速下降到零。因此,作为线圈的磁通量减少,削减短路铜缸和在汽缸产生一个电压。这种感应电压发送电流通过铜汽缸。由于当前,通量是磁铁电枢的产生一段时间后,线圈电路坏了。延迟时间是有限的在电路中线圈和铁。Figure3-15 素描的磁时间限制继电器。继电器是精力充沛时,电枢(M)画的核心(N)。同时,在吗弹簧的张力会画出电枢远离核心。维护的通量当前在铜套死,时间电枢释放所依赖一些吗春天的紧张程度。释放时间也可以通过插入不同的铜垫片在 M 和 N 之间的差距,更厚的垫片,通量越低, 越早电枢释放。磁期限继电器用于短路电阻启动的步骤马达。这种类型的继电器衔铁, 皮卡是瞬时的。辍学的时间延迟通过使用非磁性电枢调整垫片和弹簧。3.4.5 电容器限时继电器假设为电容器充电连接直流线,然后它暂时电容器直流通过继电器线圈放电。当前感应线圈会慢慢衰减,根据电容的相对价值,电感和电阻的放电电路。如果一个继电器线圈和电容器并联直流线路,电容器充电线电压的值和当前出现在线圈。如果线圈和电容器组合是现在从这条线,当前的线圈将开始减少曲线如图 3-16 所示。如果调整继电器,使衔铁释放在当前 i1,t1 的时间延迟获得的。时间延迟可以增加到一个值的t2 通过调整继电器,以便电枢不会被释放,直到i2 的电流降低到一个值。电位器用作一个可调电阻来改变时间。这阻容(RC)理论是用于工业电子和固态控制也。这个计时器是高度精确和用于汽车加速度控制,在许多工业过程中。3.4.6 电子计时器电子计时器使用固态组件来提供所需的时间延迟。图 3 - 17 所示图这个计时器。计时器有一个发光二极管(LED)当时间继电器断开,而时机一一闪过,而精力充沛。的单位符合标准的工业控制继电器安装。3.4.7 选择延时继电器在选择一个特定的时间继电器应用程序,应该是以下因素仔细考虑。 长度所需的时间延迟 时间范围要求 容许误差 周期或操作和重置时间 成本 额外的需求长度所需的时间延迟所需的时间延迟是由机器或过程的类型计时器将控制。时间延迟将范围从一小部分第二个几分钟。所需的时间范围这句话时间范围意味着的各种时间间隔计时器调整。计时器是可用的,可以设置为 1 秒的时间延迟,100 秒,之间的延迟或任何值 1 和 100 秒。在选择使用一台机器或一个计时器过程中,应该足够宽范围可能需要处理的各种延时时间机器或过程。具体时间值必须找到时间范围内任何位置的审判和错误。规模提供了一个计时器主要目的是允许快速重置计时器到时间位置之前确定是正确的对于一个给定的操作。容许误差，计时器都受到一些错误,也就是说,可能会有正负时间变化连续时间操作之间相同的设置。误差随的数量类型的定时器和操作条件。这个错误通常的百分比表示时间设定。任何计时器的误差百分比取决于类型的计时器,环境温度尤其是低温), 线圈温度、线电压和之间的时间长度操作。所需的操作周期和重置时间为一种计时器,计时器成为手术当电路打开或关闭。时间延迟,那么发生在应用程序进程开始之前。一旦特定过程操作完成时,计时器电路重置本身。必须精力充沛或电路每次时机行动需要断开。第二种类型的计时器被称为一个过程计时器。当连接到一个电路,提供了一系列事件,控制在另一个地方。不断循环重复,直到电路断开。一个计时器的选择时要考虑的一个重要因素是速度定时器重置。重置时间继电器机制所需的时间回到原来的位置。一些工业过程要求瞬间继电器复位。其他进程需要一个缓慢重置时间。重置时间随延时继电器的类型和时间延迟的长度。成本当有几种电磁计时器,满足给定的需求应用程序。建议选择计时器与最小数量的操作部分。在句话说,选择最简单的定时器。在大多数情况下,这个计时器可能是最低的成本。3.5 二线控制双线式控制可能是拨动开关,压力开关,浮子开关,限位开关,温控器,或任何其他类型的开关有明确的立场。表示,设备类型通常被设计成处理小电流。双线式控制装置不会携带足够的电流操作大型汽车。此外,230 伏电机和三相电机通常需要不止一个联系人联系提供二线设备。双线式控制可以连接到操作线圈的磁开关,如图3-18 所示。开关关闭时,通过线圈控制电路完成 M),当线圈激励它关闭联系人在 M 和电机运行。3.6.1 三线控制三线控制电路采用瞬时接触,起停站和电路联锁连接在开始按钮的同时,保持电路。一般来说,三线设备连接,如图 3-19 所示。虽然安排的各个部分可能会有所不同从一个制造商的切换到另一个,基本电路是相同的。这个电路的操作顺序是:当开始按钮被推,通过线圈电路完成(显示为 M)和联系人在 M 关闭。的权力电路接触电动机也密切(没有显示)。开始按钮被释放时,保持联系在保持这个辅助触点。当起动器中使用这种方式,它据说是“维护”或“封口”。保持联系关闭,电路仍然是通过线圈完成。如果停止按钮被推,线圈的电路坏了,失去了能源和联系人在开放。停止按钮被释放时,电路仍然开放因为保持联系和开始按钮必须再次被推到完成电路。过载保护的操作打开控制电路,导致相同的的效果。如果电源电压失败,电路断开。当电源电压恢复,电路仍然开放,直到重新开始按钮推。这样的安排被称为无电压保护和保护运营商和设备。按钮站接线图图 3-19(B)代表物理站。它显示单元的相对位置,内部线路,连接起动器。电线终端贴上 1、2 和 3(引起名称“三线控制”)。常闭辅助触点用于开关指示灯。电动机没有运行,这些联系是开放的,当电机停止,关闭,指示灯。一个指示灯可以安装时显示电机运行。对于这个情况下,指示灯控制终端 3 和 2 号线之间的连接。除了这个修改电路是一个基本的三线,按键控制电路。3.6.2 顺序来推动测试指示灯有必要重新启动电机后,停止了三线控制电路低电压保护。一个指示灯通常是当电动机停止,这样它的信号可以导致罢工的问题是清除后重新启动。因为飞行员灯是一个重要组成部分在这种情况下,他们经常以确保操作进行测试。来推动测试飞行员灯光炫耀立即如果电路或者如果灯烧坏了。图 3 的一部分显示了这种电路的线路。三线电动机起动器控制电路像往常一样连接。注意,信号灯是能量从三号航站楼到 C,通过常闭按钮 L2。测试,灯被打开在 C 和电路在 L1 关闭它。按钮的安排,测试灯直接跨线 1 和 23.6.3 报警沉默电路警报喇叭,大声蜂群还用于生产系统调用故障。问题是承认企图压制这种“噪音污染”。一个典型电路如图 3 - 21 所示。假设一个工业系统的高压危险的继续。这样的条件将关闭一个压力开关。这个开关关闭时,闹钟响起通过常闭触点 s .此外,红色的指示灯光。提醒时,维修人员可以通过压低沉默闹钟“关闭”按钮。红灯继续默默地宣布这个问题,直到它被清除。后压力开关打开时,报警系统可以重新激活通过按按钮。有几乎是无限的控制电路采用三线制。3.6.4 单独控制它有时需要操作按钮或其他控制设备在一些低电压电动机的电压。控制系统的这种情况下,一个单独的来获得的隔离变压器或一个独立的电压供应提供控制的权力电路。这个独立的电压是电动机的主电源分开。一种单独的控制图 3-22 所示。这是一个冷却的原理图电路对商业空调安装。恒温器要求冷却时,压缩机电动机起动器线圈(显示为 M)是通过降压隔离精力充沛变压器。当线圈 M 是亢奋时,在 240 伏电源接触开关启动制冷压缩机电机。由于控制电路是电源电路的分开隔离控制变压器,没有两个电路之间的电气连接。对于这个原因,起动器上的跳线连接到 L2 为不同电压应该删除。然而,过载继电器控制必须包括在单独的控制线路接触。维护技术人员还必须确保控制变压器电压匹配电压,使用适当的连接。附录二：英文原文PART 3 ELECTRIC MOTOR CONTROL3.3 RELAYS AND CONTACTORS3.3.1 Control RelaysControl magnetic relays are used as auxiliary devices to switch control circuits and large motor starter and contactor coils, and to control small loads such as small motors, solenoids, electric heaters, pilot lights, audible signal devices and other relays.A magnetically held relay is operated by an electromagnet which opens or closes electrical contactors when the electromagnet is deenergized. FIG.3-3 Elementary diagram using (A)FIG.3-4 Heavy duty, double selectorA single-break, three-position selectorswitch for (A)Two position switch and (B) Two-position, single-and (B) Three positionswitchesbreak switchRelays are generally used to enlarger or amplify the contact capability, or multiply the switching function of a pilot device by adding more contacts to circuit.Most relays are used in control circuits; therefore, their lower ratings (0-15 amperes maximum to 800 volts) show the reduced current levels at which they operate. Magnetic relays do not provide motor overload protection. This type of relay ordinarily is used in a two wire control system (any electrical contact-making device with two wires).Whenever it is designed to use momentary contact pilot devices, such as push buttons.Any available normally open contact can be wired as a holding circuit in a three-wire system. Starters, contactors, and relays are similar in construction and operation but are not identical.Control relays are available in single- or double-throw arrangements with various combinations of normally open (NO) and normally closed (NC) contact circuits. While there are some single-break contacts used in industrial relays, most of the relays used in machine toolcontrol have double-break contacts. It may be of particular interest to an electrician to know about changing contacts that are normally open to normally closed, or the other way around NC to NO. Most machine tool relays have some means to make this change. It ranges from simple flip-over contact to removing the contacts and relocating with spring location changes.Also, by overlapping contactors in this case, one contact can be arranged to operate at a ifferent time relative to another contact on the same relay. For example, the normally open contact closes (makes) before the normally closed contact opens (breaks). Relays differ in voltage ratings, number of contacts, contact rearrangement, physical size and in attachments to provide accessory functions such as mechanical latching and timing.In using a relay for a particular application, one of the first steps should be determine the control (coil)voltage at which the relay will operate The necessary contact rating must be made, as well as the number of contacts and other characteristics needed. Because of the variety of styles of relays available, it is possible to select the correct relay for almost any application.Relays are used more often to open and close control circuits than to operate power circuits. Typical applications include the control of motor starter and contactor coils, the switching of solenoids, and the control of other relays. A relay is a small but vital switching component of many complex control systems. Low-voltage relay systems are used extensively in switching residential and commercial lighting circuits and individual lighting fixtures.While control relays from various manufacturers differ in appearance and construction, they are interchangeable in control wiring system if their specifications arematched to the requirements of the system.Control relays are available in many shapes and configurations. There is dustproof, transparent enclosure of a control relay. The terminals plug in, like an electron tube.Another type of relay is the very small reed relay, with the contacts enclosed in glass. It is operated with a magnetic field.3.3.2 Solid-state RelayIn comparison to an electromagnetic relay, the solid-state relay has no coil or contacts and requires only minimum values of voltage and current to turn it on and off. Thesolid-state relay depends on electronic devices, such as transistors and silicon controlled rectifiers (SCR) for switching.3.3.3 The Transistor as A SwitchFigure 3-5 shows a basic solid-state switching deviceused in a logic component (relay). The transistor isthe heart of the element. The base of the transistorcontrols the current flow between the emitter and thecollector. In this type of the transistor, a negativevoltage on the base allows emitter- base current to flow. This is due to the properties of the material at the junction of the emitter and the base. The emitter-base current causes the transistor to conduct a current flow from the emitter to the collector. A positive voltage on the base prevents emitter-base current from flowing, and the transistor stopsconducting. Therefore, it behaves as a closed contact in the first state and as an open contact in the second For this reason, the action is called solid-state switching, that is, no moving contacts are required. There is only an electrical signal to open or close the circuit. As a result, the solid-state device is very reliable and has an exceptionally long life.Solid-state devices are not subject to arcing, wear, or deterioration, as are magnetic relays.3.3.4 Surge ProtectionWhen solid-state devices are used with magnetic switches, a voltage transient suppressor may be necessary to prevent some of the more harmful electrical “noise”. Much more sophisticated protection is required for microcomputers that control robots on assembly production lines.3.3.5 ContactorsMagnetic contactors are eletromagnetically operated switches that provide a safe and convenientmeans for connecting and interrupting branch circuits. The principal diference between a contactor and a motor starter is that the contactor does not contain overload relays. Contactors are used in combination with pilot control devices to switch lighting and heating loads and to control ac motors in those cases where overload protection is provided separately. The larger contactor sizes are used to provide remote control of relativelyhigh-current circuits where it is too expensive to run the power leads to the remote controlling location, FIG.3-6.This flexibility is one of main advantages of electromagnetic control over manual control. Pilot devices such as push buttons, float switches, pressure switches, limit switches, and thermostats are provided to operate the contactors.FIG.3-6 An advantage of a remote control loadMagnetic BlowoutHeavy-duty contact arc-chutes are provided on most of larger contactors. The chutes contain heavy copper coils called blowout coils, mounted above the contacts in serieswith the load to provide better arc suppression. These magnetic blowout coils help to extinguish an electric arc at contacts opening under alternating current and direct-current loads. The arc may be similar in intensity as the electric arc welding process.Anarc-quenching device is used to assure longer contact life. Since the hot arc is transferred from the contact tips very rapidly, the contacts remain cool and so they last longer.Contact and motor starter contacts that frequently break heavy currents are subject to a destructive burning effect if the arc is not quickly extinguished. The arc that is formed when the contacts open can be lengthened, and extinguished by motor action if it is in a magnetic field.This magnetic field is provided by the magnetic blowout coil. Since the coil of the magnet is usually in series with the line, the field strength and extinguishing action are in proportion to the size of the arc.Figure 3-7 is a sketch of a blowout magnet with a straight conductor (ab) located in the field and in series with the magnet. This Figure can represent either dc polarity or instantaneous ac. With ac current, the blowout coil magnetic field and conductor (arc) magnetic field will reverse simultaneously. According to Flemings left-hand rule, motor action will tend to force the conductor in an upward direction. The application of theright-hand rule for signal conductor shows that the magnetic field around the conductor aids the main field on the bottom and opposes it on the top, thus producing an upward force on the conductor.Figure 3-8 shows s section of Figure 3-7 with the wire (ab) replaced by a set of contacts. The contacts have started to open and there is an arc between them. Figure 3-9 shows what happens because of the magnetic action. Part A shows the beginning deflection of the arc because of the effect the motor action. Part B shows that the contacts are separated more then in A and the arc is beginning to climb up the horns because of the motor action and the effect of increased temperature. Part C of Figure 3-9 shows the arc near the tips of the horns. At this point, the arc is so lengthened that it will be extinguished. FIG.3-7 Illustration of the magnetic blowoutFIG.3-8 Section of blowout magnet withprinciple. Straight conductor simulates arc.straight conductor replaced by a setconducting betweencontacts. An arc isthe contacts.The function of the blowout magnet is to move the arc upward at the same time that the contacts arc opening. As a result, the arc is lengthened at a faster rate than will normally occur because of the opening of the contacts alone. It is evident that the shorter the time the arc is allowed to exist, the less damage it will do to the contacts. Most arc quenching action is based upon this principle.3.3.6 AC Mechanically Held Contactors and RelaysA mechanically held relay, or contactor, is operated by electromagnets but the electromagnets are automatically disconnected by contacts within the relay. Accordingly, these relays are mechanically held in position and no current flows through the operating coils of these electromagnets after switching. It is apparent, therefore, that near continuous operation of multiple units of substantial size will lower the electrical energy requirements. Also, the magnetically held relay, in comparison, will change contact position upon loss of voltage to the electromagnet, whereas the mechanically held relay will respond only to the action of the control device.Sequence of OperationReferring to Figure 3-10, when the“off”push button is pressed momentarily, current flows from L1 through the “on” push button contact energizing the M coil through the now closed clearing contact, to L2. The relay now closes and latches mechanically. At the same time it closes M contacts (in Figure 3-11), lighting a bank of lamps when the circuit breaker is closed.To unlatch the relay, thereby turning the lamps off, the off button is pressed momentarily, unlatching the relay and opening the contacts M, turning off the lamps. Most operating coils are not designed for continuous duty. Therefore, they are disconnected automatically by contacts to prevent an accidental coil burnout. These coil clearing contacts change position alternately with a change in contactor latching position.FIG.3-9 Arc deflection between contactsFIG.3-10 Mechanically held relay control circuitFigure 3-12 shows a three-phase power load application using one main contactor to disconnect distribution panel. Selective single, or three-phase, branch circuits may beswitched independently by other mechanically held contactors or relays.These mechanically held contactors and relays are electromechanical devices, Figure 3-13. They provide a safe and convenient means of switching circuits where quiet operation, energy efficiency, and continuity of circuit connection are requirements of the installation. For example,circuit continuity during power failures is often important in automatic processing equipment,where a sequence of operations must continue from the point of interruption after power is resumedrather then return to beginning of the sequence. Quiet operation of contactors and relays is required in many control systems used in hospitals, schools, and office buildings.Mechanically held contactors and relays are generally used in locations where theslight hum,characteristic of alternating-current magnetic devices, is objectionable.In addition, mechanically held relays are often used in machine tool control circuits. These relays can be latched and unlatched through the operation of the limit switches, timing relays,starter interlocks, timeclocks, photoelectric cells, other control relays, or push buttons.Generally, mechanically held relays are available in 10- and 15-ampere sizes; mechanically held contactors are also available in sizes ranging from 30 amperes up to 1200 amperes.FIG.3-11 Load connections for a 115/FIG.3-12 Mechanically held contactor230-volt, three-phaseload loads for three-phase powerFIG.3-13 Two types latched-in or mechanically held relays in service. The upper coil is energizedmomentarily to close contacts, and the lower coil is energized momentarily toopen the contact circuit.The momentary energizing of the coil is an energy-saving feature. (Courtesy Square D Co.)3.3.7 Thermostat RelayThermostat-type are used with three-wire, gauge-type thermostat controls or other pilot controls having a slowly moving element which makes a contact for both the closed and open positions of the relay. The contacts of the thermostat control devices usually cannot handle the current to a starter coil; therefore, a thermostat relay must be used between the thermostat control and the starter, Figure 3-14.When the moving element of the thermostat control touches the closed contact, the relay closes and is held in this position by a containing contact. When the moving element touches the open contact, the current flow bypasses the operating coil through a small resistor and causes the relay open. The resistor is usually built into the relay and serves to prevent a short circuit.FIG.3-14 Starter coil(M) is controlled by thermostat relayThe thermostat contacts must not overlap or be adjusted too closely to one another as this may result in the resistance unit being burned out. It is also advisable to compare the inrush current of relay with the current rating of the thermostat.3.4 TIMING RELAYS3.4.1 IntroductionA timing relay is similar to a control relay, except that certain of its contacts are designed to operate at a present time interval, or time lag, after the coil is energized, or de-energized.Many industrial control applications require timing relays that can provide dependable service and are easily adjustable over the timing ranges. The proper election of timingrelays for a particular application can be made after a study of the service requirements and with the knowledge of the operating characteristics inherent in each available device. A number of timing devices are manufactured with features suitable for a wide variety of applications.3.4.2 Fluid Dashpot Timing RelaysMagnetically operated, oil dashpot timing relay may be used on voltages up to 800 volts ac or dc. The contacts are operated by the movement of the iron core. The magnetic field of a solenoid coil lefts the iron core against retarding force of a piston moving in an oil-filled dashpot. This type of relay is not very accurate. The piston must be allowed to settle back down to the bottom of the dashpot between successive timing periods. If the piston is not allowed to make a full return, the timing is erratic. The dashpot timing relay provides time delay after the magnet is energized. The contact may be normally open or normally closed for different applications.Unlike the magnetic overload relay, the dashpot timing relay operates with a potential coil connected across the line through contacts or switches. The overload relay operates with a current coil that is affected by the motor current load. Fluid dashpot timing relays are used for a number of applications: to control accelerating contactors of motor starters to time the closing or opening of valves on refrigeration equipment for any application where the operating sequence a delay.It is necessary that the elapsed time of the delay be extremely accurate.These relays use a silicone dashpot fluid which is not an oil. The fluid helps to eliminate the effect of varying viscosity on the timing due to changes in ambient temperature. The silicone fluid operates successfully in an ambient temperature range of48.9 to 34.4. The timing range can be adjusted easily from two seconds to 30seconds.Multicontact dashpot timing relays are used for dc motor starting. When the coil is energized on this type of timing relay, the contacts close in succession with a time lag between each closing.3.4.3 Pneumatic TimersThe construction and performance features of the pneumatic (air) timer make it suitable for the majority of industrial applications. Pneumatic timers have the following characteristics: unaffected by normal variations in ambient temperature or atmospheric pressure adjustable over a wide range of timing periods good repeat accuracy available with a variety of contact and timing arrangements.This type of relay has a pneumatic time-delay unit that is mechanically operated by a magnet structure. The time-delay function depends upon the transfer of air through arestricted orifice by the use of reinforced synthetic rubber bellows or diaphragm. The timing range is adjusted by positioning a needle valve to vary amount of orifice or vent restriction. The process of energizing or deenergizing pneumatic timing relays can be controlled by pilot devices such as push buttons, limit switches, or thermostatic relays. Since the power drawn by a timing relay coil is small, sensitive control devices may be used to control the operating sequence.Pneumatic timing relays are used for motor acceleration and in automatic control circuits.Automatic control is necessary in applications where repetitive accuracy is required, such as controls for machine tools and control of sequence operations, industrial process operation, and conveyor lines.3.4.4 Magnetic Time Limit RelayIf current is increasing in a coil, then the emf due to self-induction acts in a direction to oppose the increase of current in that coil. If a current is decreasing in a coil, the emf due to self-induction in the coil acts in a direction to oppose the decrease of current in the coil.One type of magnetic time limit relay has a single coil wound on a hollow copper cylinder containing an iron core. Other time limit relays have copper -jacketed coils. For either type of relay, time delay is provided when the relay drops out.When the electric circuit in the coil is disconnected, the current quickly falls to zero. Therefore, as the flux in the coil decreases, it cuts the short-circuited copper cylinderand induces a voltage in the cylinder. This induced voltage sends a current through the copper cylinder. As a result of this current, a flux is produced that holds the magnet armature up for a period of time after the coil circuit is broken. The delay time is limited by the number of turns on the coil and the amount of iron in the circuit. Figure3-15 is a sketch of a magnetic time limit relay. When the relay is energized, the armature(M)FIG.3-15 Dc magnetic time limit relayis drawn against the core (N). At the same time, the tension of the spring tends todraw the armature away from the core. As the flux maintained by the current in the copper sleeve dies away, the time at which the armature is released depends to some degree on the tension of the spring. The release time also can be varied by inserting a bronze shim in the gap between M and N, the thicker the shim, the lower the flux, and the sooner the armature is released.The magnetic time limit relay is used to short out resistance steps in the start-up of dc motors. With this type of relay armature, pickup is instantaneous. The time delay atdropout is obtained by the use of a nonmagnetic armature shim and the adjustment spring.3.4.5 Capacitor Time limit RelayAssume that a capacitor is charged by connecting it momentarily across a dc line and then the capacitor dc is discharged through a relay coil. The current induced in the coil will decay slowly, depending on the relative values of capacitance, inductance, and resistance in the discharge circuit.If a relay coil and a capacitor are connected in parallel to a dc line, the capacitor is charged to the value of the line voltage and a current appears in the coil. If the coil and capacitor combination is now removed from the line, the current in the coil will start todecrease along the curve shown in Figure 3-16.If the relay is adjusted so that the armature is released at current i1, a time delay of t1 is obtained. The time delay can be increased to a value of t2 by adjusting the relay so that the armature will not be released until the current is reduced to a value of i2.A potentiometer is used as an adjustable resistor to vary the time. Thisresistance-capacitance (RC) theory is used in industrial electronic and solid-state controls also.This timer is highly accurate and is used in motor acceleration control and in many industrial processes.FIG.3-16 Charged capacitor discharging through a relay coil. The graph at the right illustrates thecurrent decrease in the coil3.4.6 Electronic TimersElectronic timers use solid-state components to provide the desired time delays. A elementary diagram for this timer is shown in Figure 3-17. The timer has a light-emitting diode (LED) that is off when the timing relay is deenergized, flashes while timing, and is on while energized. The unit fits standard industrial control relay mounting.3.4.7 Selecting A Timing RelayIn selecting a timing relay for a specific application, the following factors should be carefully considered. Length of time delay required Timing range required Allowable error Cycle or operation and reset time Cost Additional requirementsFIG.3-17 Typical elementary diagram for a solid-state timerLength of Time Delay RequiredThe length of time delay required is determined by the type of machine or process that the timer will control. The time delay will range from a fraction a second to as several minutes.Timing Range RequiredThe phrase timing range means the various time intervals over which the timer can be adjusted. Timers are available which can be set for a time delay of 1 second, and 100 seconds,or any value of delay between 1 and 100 seconds. When selecting a timer for use a machine or process, the range should be wide enough to handle the various time-delay periods that may be required by the machine or process.The exact timing value for any position within the timing the range must be found by trial and error. A scale provided with a timer is intended primarily to permit a quick reset of the timer to the timing position previously determined to be correct for a given operation.Allowable ErrorAll timers are subject to some error, that is, there may be a plus or minus time variation between successive timing operations for the same setting. The amount of error varies with the type of timer and the operating conditions. The error is usually stated as some percentage of the time setting.The percentage of error for any timer depends on the type of timer, the ambienttemperature(especially low temperature), coil temperature, line voltage, and the length of time between operations.Cycle of Operation Required and Reset TimeFor one type of timer, the timer becomes operative when a electrical circuit opens or closes. A Time delay then occurs before the application process begins. As soon as the particular process action is complete, the timer circuit resets itself. The circuit must be energized or deenergized each time the timing action is desired. A Second type of timer is called a process timer. When connected into a circuit, the time provides control for a sequence of events, one after another. The cycle is repeated continuously until the circuit is deenergized.An important consideration in the selection of a timer is the speed at which the timer resets. Reset time is the time required for the relay mechanism to return to its original position. Some industrial processes require that the relay reset instantaneously. Other processes require a slow reset time. The reset time varies with the type of timing relay and the length of the time delay.CostWhen there are several electromagnetic timers that meet the requirements of a given application. It is advisable to select the timer with the smallest number of operatingparts. In other words, select the simplest timer. In most cases, this timer will probably be the lowest in cost.3.5 TWO-WIRE CONTROLSA two-wire control may be toggle switch, pressure switch, float switch, limit switch, thermostat, or any other type of switch having definite on and off positions. As indicated, devices of this type generally are designed to handle small currents. Two-wire control devices will not carry sufficient current to operate large motors. In addition, 230-volt motors and three-phase motors require more contacts than one contact usually provided on two-wire devices.Two-wire controls may be connected to operating coils of magnetic switches, as shown in Figure 3-18. When the switch is closed, the control circuit is completed through the coil(M). When the coil is energized, it closes the contacts at M and runs the motor. Whenthe switch is opened, the coil is deenergized and the contacts open to stop the motor. In the case of an overload contact in the thermal heaters open the overload contacts in the control circuit and deenergized the coil, thus stopping the motor. Two-wire control provides no voltage (or law voltage) release. When the start is wired. as shown in Figure 3-18, it will operate automatically in response to the control devices. A human operator is not required. the control maintaining contact 2-3(show in the wiring diagram) is furnished with the starter. However, this contact is not used in two-wire control. For simplicity, this contact is omitted from two-wire elementary diagram. The motor starter in Figure 3-18 is a line voltage, or across-the-line, starter.FIG.3-18(A) Basic two-wire control circuitelementary diagramFIG.3-18(B) Basic two-wire control circuitwiring diagram3.6 THREE-WIRE AND SEPARATE CONTROLS3.6.1 Three-Wire ControlsA three-wire control circuit uses momentary contact, start-stop stations and holding circuit interlock connected in parallel with the start button, to maintain the circuit. In general, three-wire devices are connected, as shown in Figure 3-19. Although the arrangement of the various parts may vary from one manufacturers switch to another, the basic circuit remains the Same.FIG.3-19 Basic three-wire control circuitThe sequence of operation for this circuit is follows: when the start button is pushed, the circuit is completed through the coil (shown as M) and the contacts at M close. The power circuit contacts to the motor also close (not shown). When the start button is released, the holding contact at M keeps this auxiliary contact on. When the starter is used in this manner, it is said to be “maintaining” or “sealing”. With the holding contact closed, the circuit is still complete through the coil. If the stop button is pushed, the circuit is broken, the coil loses its energy, and the contacts at M open. When the stop button is released, the circuit remains open because both the holding contact and the start button must be pushed again to complete the circuit. The operation of the overload protection opens the control circuit, resulting in the same effect.If the supply voltage fails, the circuit is deenergized. When the supply voltage returns, the circuit remains open until the start button is pushed again. This arrangement is called no-voltage protection and protects both operator and equ
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