电动机及速度控制外文翻译

1、英文The Electric Motor And Speed ControlEach type of motor has its particular field of usefulness. Because of its simplicity, economy, and durability, the induction motor is more widely used for industrial purposes than any other type of ac motor, especially if a high- speed drive is desired.If ac pow

2、er is available, all drives requiring constant speed should use squirrel-cage induction or synchronous motors because of their ruggedness and lower cost. Drives requiring varying speeds, such as fans, blowers, or pumps, may be driven by wound-rotor induction motors. However, if there are machine too

3、ls or other machines requiring adjustable speed or a wide range of speed control, it will probably be desirable to install dc motors on such machines and supply them from the ac system by motor-generator sets or electronic rectifiers.Almost all constant-speed machines may be driven by ac squirrel-ca

4、ge motors because these motors are made with a variety of speed and torque characteristics. When large motors are required or when power supply is limited, the wound-rotor motor is used, even to drive constant-speed machines.For varying-speed service, wound-rotor motors with resistance control are u

5、sed for fans, blowers, and other apparatus for continuous duty and are used for cranes, hoists, and other installations for intermittent duty. The controller and resistors must be properly chosen for the specific application. Synchronous motors may be used for almost any constant-speed drive requiri

6、ng about 100 hp or over.Cost is an important factor when more than one type of ac motor is applicable. The squirrel-cage motor is the least expensive ac motor of the three types considered and requires additional secondary control equipment. The wound-rotor is more expensive and requires additional

7、secondary control. The synchronous motor is even more expensive and requires a source of dc excitation, as well as special synchronizing control to apply the dc power at the correct instant. When very large machines are involved, as, for example, 1000 hp or over, the cost picture may change consider

8、ably and should be checked on an individual basis.The various types of single-phase ac motors and universal motors are used very little in industrial applications, since polyphase ac or dc power is generally available. When such motors are used, they are usually built into the equipment by the machi

9、nery manufacturer, as in portable tools, office machinery, and other equipment. These motors are, as a rule, especially designed for the specific machines with which they are used.Speed Control of D.C. MotorsThe most common requirement for a drive is that giving speed control from zero to full-load

10、speed with a load torque which is approximately constant or increasing with speed. In such an application it is necessary to supply the armature with variable voltage, and a controlled rectification configuration operating from an a. c. source is admirably suited to this application.The simplest and

11、 often the cheapest configuration is that of the half-wave rectifier applied to either a shunt or series d. c. motor in the manner shown in Fig.1. The number of components required is minimal and, in its simplest form, it is possible to dispense with current limiting circuits. The current build-up i

12、n the positive half-cycle is then limited by the armature inductance. This system suffers from the major disadvantage that the form factor is bad (at least 2:1), leading to a significant de-rating of the motor. The torque is pulsating and this may produce objectionable results, particularly at low s

13、peeds. There will also be a large a. c. component into the supply system and there will, in general, be a practical limit to the rating of such a system. In practice, this arrangement will be restricted to rating below 1h. p.Some improvement in form factor cam be made by adding a flywheel diode acro

14、ss the armature. If an inductance is based in series with the armature, its effect will be to smooth out the armature current. In practice, however, a large value of inductance is necessary making such a component large and expensive. It is often cheaper to use a full-wave arrangement.When regenerat

15、ion is not needed, it is possible to use the half-controlled bridge shown in Fig. 2. This circuit has thyristors on two arms with cathodes connected to the positive terminal and rectifiers in the other two arms. It produces full-wave rectification but has one major disadvantagte in that it is essent

16、ial to ensure that each thyristor is extinguished before the start of its positive half-cycle of the system voltage. This condition is important because it is possible for the armature current to flywheel through the conducting thyristor and its adjoining rectifier during the negative half-cycle. If

17、 this current is present when the system voltage becomes positive again, a full half-cycle of the supply voltage will appear across the motor armature, resulting in a large surge of current. This effect can be prevented by either firing the second thyristor towards the end of the negative half-cycle

18、 or by using a flywheel diode across the armature so that an alternative low impedance path is provided for the current.It is, however, possible to rearrange the components in the bridge in the manner shown in Fig. 3, to remove the problem of failure to commutate the armature current from the conduc

19、ting thyristor. In the arrangement shown in Fig. 3, the thyristors are connected to a supply terminal so that the armature current circulates around the rectifiers. In these circumstances the thyristors will be extinguished by reversal of the system voltage. Thus it is possible that the current in t

20、he rectifiers will exceed the current in the thyristors and that the armature current will exceed the mains current. It then becomes necessary to use larger rectifiers or to use protective current limiting operated from the armature circuit. The circuit shown in Fig. 2has the advantage that, if the

21、rectifier bridge is completed, a supply is available for the motor field.An alternative full-wave configuration with some useful features is shown in Fig. 4. This arrangement consists of a full-wave uncontrolled rectifier supplying a single thyristor which can now conduct on both half-cycles of the

22、system voltage. Its original merit in the days when thyristors were very expensive was that it made full use of single thyristor. It also has the advantage that a supply for the motor field is readily available and that, for a multi-machine system, only one main uncontrolled rectifier is necessary.

23、Only one thyristor firing circuit is necessary and this ensures an even balance of current pulses between alternate half-cycles of the supply. Its major disadvantage is that some reliable method of extinguishing the thyristor at each zero of the mains voltage must be found so that the thyristor can

24、enter a blocking mode to regain control during the next half-cycle.All the circuits so far described have operated from a single-phase supply with either line to line voltage. The normal acceptable loading limit for a single-phase system is about 7KW and for ratings above this, it is necessary to us

25、e a 3-phase supply with the corresponding thyristor configuration.The simplest 3-phase arrangement is the half-controlled bridge with thyristors in one half and rectifiers in the other half of the bridge, connected in the manner shown in Fig.5. Such an arrangement suffers from the same commutation t

26、rouble as the single-phase bridge. In these circumstances either a minimum firing angle unit must be used or a flywheel diode must be fitted across the armature.In some applications, a d. c. motor will be used to speed control an overhauling load such that, under certain circumstances, power transfe

27、r from the load to the motor is possible. Under these conditions the motor will act as a generator and thyristor power converter capable of transferring power into the a. c. mains must be used, The basic circuit of such a regenerative system is shown in Fig 6. Under regenerative condition current mu

28、st flow out of the positive motor terminal into the supply system and the thyristor TH1 in Fig. 6controls the amount of regenerative power returned to the mains while TH2 controls the system under motoring conditions. Care must obviously be taken to ensure that both thyristors circuit of this type i

29、t is usual to ensure that the net circuit e. m. f. always acts in a direction such that the rate of change of current is negative and to ensure that the e. m. f. is present until zero current is reached.This simple circuit suffers from severe restriction must be placed on the value of the firing ang

30、le during regeneration if uncontrolled conduction is to be avoided. The type of restriction is to be overcome by the use of the bi-phase, half-wave system illustrated in Fig. 7, for which continuous conduction is possible with the current being forcibly transferred from one thyristor to the other be

31、cause the cathode voltage of the incoming device is lower than that of the outgoing device at the instant of firing.Speed Control of A.C. MotorsMost a. c. motors operate at constant speed and speed control can be obtained by varying the frequency of the applied voltage. In many cases the magnitude o

32、f the applied voltage will also be varied in direct proportion to the frequency in order to maintain the flux in the machine at a constant value. In general, a static power converter producing a variable frequency, variable magnitude polyphase output-voltage from fixed polyphase a. c. mains is requi

33、red and this can be achieved in one of two days. Firstly, a direct conversion (a. c to a. c) can be made using the so-called cycloconverter principle. The second and more common way is to convert the fixed a. c. to variable d. c. and to then reconvert this d. c. voltage to the required variable a. c

34、. system; such a method uses a d. c. link. In this case negative anode-cathode voltage does not occur naturally in the d. c. to a. c. thyristor converter and the process of forced commutation must be used. Two basic methods of forced commutation inverters, the so-called parallel inverter and the pul

35、se-width modulated inverter are in common use.A simplified diagram representing a single-phase parallel inverter is given in Fig.8 in which the inductance L between the source and the thyristors acts as a current limiter. With thyristor TH1 in Fig.8 conducting the supply voltage V appears across one

36、 half of the primary of the output transformer T and load current flows. The voltage across the whole of the transformer primary winding is then 2V and the capacitor C is charged to the voltage 2V. When thyristor TH2 is fired, the capacitor discharges through the two thyristors and TH1 is reverse bi

37、ased until it turns off. Thyristor TH2 is then in a conducting state and the supply voltage V appears across the other half of the transformer primary in an opposite sense. The output voltage across the secondary then reverses and is thereforce an approximate square wave whose frequency is controlle

38、d by the firing pulse applied to the two thyristors. Filtering can be introduced at the output if a sinusoidal output voltage is required. The general principles of pulse-width modulation are illustrated by the circuit of Fig.9.Thyristor TH1 is the main circuit device and thyristor TH2, capacitor C

39、and resistor R from the commutating circuit for the main thyristor .The presence of the diode D is now essential in that it provides a path for load current and allows the capacitor to discharge. Initially neither thyristor is conducting so that both points A and B in Fig.9 are at the potential of t

40、he negative rail. If TH1 is fired the point B in Fig.9 will now be at the potential of the positive rail. Current will build up in the load on an exponential of time constant L/R and, at the same time, the capacitor C will be charged to a voltage +V, through the resistor R, with the potential at poi

41、nt B positive with respect to that at point A. If after some convenient time, thyristor TH2 is fired, the potential at point A will rise to +V volts, the presense of the charged capacitor will result in the potential at point B, in Fig.9, rising to +2V volts. The main thyristor TH1 will then be reve

42、rse biased and can turn off. Load current must still be flowing, because of the presence of load inductance and this current is supplied by the charge stored in the capacitor. The capacitor will thus be discharged resonantly until point B goes to a negative potential, at which point the diode D will

43、 conduct, thereby clamping the load voltage at approximately zero and also providing a flywheel path for the load current. Capacitor C will now have a voltage of +V across it with point A positive and the thyristor TH1 will be conducting with a small current passing through the resistor R. The load

44、current will fallexponentially until thyristor TH2 is fired again. This will cause the potential at point B to rise to +V so that the potential at point A rises to +2V. The thyristor TH1 is thus extinguished and the capacitor C recharges through R with the point B again at the positive potential, re

45、ady for the next cycle of operation.More elegant forms of the basic circuit of Fig. 9 can be used to generate a sinusoid of output voltage with a superimposed high frequency ripple. The ripple frequency is controlled by the maximum permissible switching rate for the thyristors and the allowable comm

46、utation loss which takes place as switching occurs. The frequency of the output voltage is readily controlled by control of the pulse repetition rate of the gate signal.译文电动机及速度控制每种型号的电机都有其特定的使用范围。与其它型号的交流电动机相比，感应电动机因其简单、 经济和耐用而更广泛地用于工业，特别是需要高速驱动时更是如此。因为鼠笼式感应电动机或同步电动机经久耐用且成本低，若有交流电源可用，所有需要恒速驱 动的装置应使

47、用这两种电机。需要变速驱动的装置，如电扇、鼓风机或水泵可由绕线式感应电动机 驱动。然而，对于那些需要调速或速控范围较广的机床或其它机器而言，安装直流电动机，并通过 电动机-发电机组或者电子整流器从交流电系统向其提供电能是较为理想的做法。因为鼠笼式电机具有多种速度和转矩特性，几乎所有的恒速机器均可由交流鼠笼式电机驱动。1当需要大功率电动机或电源受限制时，则可使用线绕转子电动机，即使是驱动恒速机器。32对于变速作业，有电阻控制的线绕转子电机可用于驱动电风扇、鼓风机和其它装置的持续作业，5也可用于起重机、卷扬机等机械和设备的间歇作业。某些特殊应用时，须挑选适当的控制器和电阻 器。几乎任何一种需要10

48、0马力以上的恒速驱动装置都可使用同步电动机。当不只一种交流电动机可供挑选使用时，成本便成了很重要的考虑因素。鼠笼式电动机是可供 选择的三种型号中价格最低廉的，而且几乎不需要控制装置。线绕式转子电动机昂贵一些，且需要 另加次级控制。同步电动机则更贵，而且需要直流励磁电源和专门的同步控制器以便在适当的瞬间 接入直流电源。当涉及到1000马力以上的大功率机器时，成本情况会大大改变，需针对各自情况逐 个核算。因为通常有多相交流电源或直流电源可用，因此，在工业上很少使用各种类型的单相交流电动 机和交直流两用电动机。如需使用这类电动机，机械制造商常把它们安装在设备中，如便携式工具、 办公机械及其它设备。总

49、的来说，这些电动机是为使用它们的专用机器特别设计的。直流电机速度控制最平常的驱动需要是提供从零到全负载速度，该过程伴随着近似恒定或随速度增加的负载力矩。 在次应用中必须为电枢提供可变电压，而采用交流电源的控制整流结构极为适合该应用。最简单、廉价的结构是可用于并联或串联直流电机的半波整流器，如图1所示。它所需要部件 数量最少，如果是最简单的形式还可省却限流电路。这样，在正半周期内电流受电枢电感限制。该 系统的主要缺点是波形因子较差（至少2： 1），导致电机显著减效。尤其是在速度较低时，力矩的 不停搏动可能产生令人讨厌的后果。电枢电流中还存在大量交流成分。该系统的供应系统注入了支 流成分，通常将对

50、系统的效率产生实际的限制。实际应用中该结构仅限于1马力内。添加一横穿电枢的续流二极管可是波形因子得到一定的改善。若电感是基于与电枢串联，则 其效果是平滑电枢电流。然而，实际上大量电感应是必须的，这会使该部件变得庞大而昂贵。通常使用全波结构更便宜。1图3若无须再生（电流），还可使用如图2的半控制电桥。该电路两臂上有硅控整流管，其阴极与正 极和另两臂上的整流器相连。它能产生全波整流，但也有明显的缺点，即务必确保系统电压正半周 期启动前各硅控整流管为关闭状态。这一点至关重要，因为这样在负半周期内电枢电流才有可能通 过导向硅控整流管及其邻接整流器。若系统电压在度变正时该电流存在，则电机电枢中会出现完整

51、 的半周供电压，从而产生一大股电流。次效应可以防止，一种方法是在负半周期结束是激发第二个 硅控整流管；另一种是使用横穿电枢的续流二极管来为电流提供一交变低阻抗通路。当然，也可用如图3所示的方法调整电桥上的组件来解决对流出导向硅控整流管的电枢电流整 流实效的问题。硅控整流管连接于供电极，这样电枢电流就会在整流器间循环。在次情况下硅控整 2流管会因系统电压反向而关闭。从而整流器中的电流有可能超过硅控整流管中的电流，电枢电流有 可能超过干线电流。这时就需要使用更大的整流器或使用电枢电路限流保护。图8-2中的电路优点 在于：若整流电桥完成，则电机场可获得供电。图4显示了具有一些使用特征的交变全波结构。该结构含一全波非控制整流器，提供在系统电 压正、负半周期均可传导的单一硅控整流器。在硅控整流器十分昂贵的时代，它最初的优势在于很 好的使用了单一硅控整流管。其优势还在于对电机场可随时供电，并且对多机器系统而言仅需一个 非控制主整流器。只需一个硅控整流管触发电路并确保了供电交变半周期间电流脉冲的均衡。其主 要缺点是必须找到干线电压每次为零时关闭硅控整流管的一些可靠方法，从而使硅控整流管能进一 步阻塞模式，以便在下一周期重获控制权。以上讨论的电路均单向供电来操作，即可线接地