电气专业外文翻译6.docx

附录A 译文直流电机导论负载运行的变压器直流电机以其多功用性而形成了鲜明的特征。通过并励、串励和特励绕组的各种不同组合，直流电机可设计成在动态和稳态运行时呈现出宽广范围变化的伏-安或速度-转矩特性。由于直流电机易于控制，因此该系统用于要求电动机转速变化范围宽或能精确控制电机输出的场合。定子上有凸极，由一个或一个以上励磁线圈励磁。励磁绕组产生的气隙通以磁极中心线为轴线对称分布，这条轴线称为磁场轴线或直轴。我们知道，每个旋转的电枢绕组中产生的交流电压，经由一与电枢连接的旋转的换向器和静止的电刷，在电枢绕组出线端转换成直流电压。换向器一电刷的组合构成机械整流器，它产生一直流电枢电压和一在空间固定的电枢磁势波形。电刷的放置应使换向线圈也处于磁极中性区，即两磁极之间。这样，电枢磁势波形的轴线与磁极轴线相差90电角度，即位于交轴上。在示意图中，电刷位于交轴上，因为此处正是与其相连的线圈的位置。这样，如图所示电枢磁势波的轴线也是沿着电刷轴线的。（在实际电机中，电刷的几何位置大约偏移图例中所示位置90电角度，这是因为元件的末端形状构成图示结果与换向器相连。）电刷上的电磁转矩和速度电压与磁通分布的空间波形无关；为了方便起见，我们假设气隙中仍然是正弦磁密波，这样便可以从磁场分析着手求得转矩。转矩可以用直轴每极气隙磁通和电枢磁势波的空间基波分量相互作用的结果来表示。电刷处于交轴时，磁场间的角度为90电角度，其正弦值等于1，则对于一台P极电机 式中由于转矩的正方向可以根据物理概念的推断确定，因此负号已经去掉。电枢磁势锯齿波的空间基波是峰值的8/。上式变换后有 式中 =电枢外部电路中的电流； =电枢绕组中的总导体数； =通过绕组的并联支路数；且 其为一个由绕组设计而确定的常数。简单的单个线圈的电枢中的整流电压前面已经讨论过了。将绕组分散在几个槽中的效果可用图形表示，图中每一条整流的正弦波形是一个线圈产生的电压，换向线圈边处于磁中性区。从电刷端观察到的电压是电刷间所有串联线圈中整流电压的总和，在图中由标以的波线表示。当每极有十几个换向器片，波线的波动变得非常小，从电刷端观察到的平均电压等于线圈整流电压平均值之和。电刷间的整流电压即速度电压，为 式中为设计常数。分布绕组的整流电压与集中线圈有着相同的平均值，其差别只是分布绕组的波形脉动大大减小。将上述几式中的所有变量用SI单位制表达，有 这个等式简单地说明与速度电压有关的瞬时功率等于与磁场转矩有关的瞬时机械功率，能量的流向取决于这台电机是电动机还是发电机。直轴气隙通由励磁绕组的合成磁势产生，其磁通-磁势曲线就是电机的具体铁磁材料的几何尺寸决定的磁化曲线。在磁化曲线中，因为电枢磁势波的轴线与磁场轴线垂直，因此假定电枢磁势对直轴磁通不产生作用。这种假设有必要在后述部分加以验证，届时饱和效应会深入研究。因为电枢电势与磁通成正比，所以通常用恒定转速下的电枢电势来表示磁化曲线更为方便。任意转速时，任一给定磁通下的电压与转速成正比，即 图中表示只有一个励磁绕组的磁化曲线，这条曲线可以很容易通过实验方法得到，不需要任何设计步骤的知识。在一个相当宽的励磁范围内，铁磁材料部分的磁阻与气隙磁阻相比可以忽略不计，在此范围内磁通与励磁绕组总磁势呈线性比例，比例常数便是直轴气隙磁导率。直流电机的突出优点是通过选择磁场绕组不同的励磁方法，可以获得变化范围很大的运行特性。励磁绕组可以由外部直流电源单独激磁，或者也可自励，即电机提供自身的励磁。励磁防哪个法不仅极大地影响控制系统中电机的静态特性，而且影响其动态运行。他励发电机的连接图已经给出，所需励磁电流是额定电枢电流的很小一部分。励磁电路中很小数量的功率可以控制电枢电路中相对很大数量的功率，也就是说发电机是一种功率放大器。当需要在很大范围内控制电枢电压时，他励发电机常常用于反馈控制系统中。自励发电机的励磁绕组可以有三种不同的供电方式。励磁绕组可以与电枢串联起来，这便形成了串励发电机；励磁绕组可以与电枢并联在一起，这便形成了并励发电机；或者励磁绕组分成两部分，其中一部分与电枢串联，另一部分与电枢并联，这便形成复励发电机。为了引起自励过程，在自励发电机中必须存在剩磁。 在典型的静态伏-安特性中，假定原动机恒速运行，稳态电势和端电压关系为： 式中为电枢输出电流，为电枢回路电阻。在发电机中，比大，电磁转矩T是一种阻转矩。他励发电机的端电压随着负载电流的增加稍有降低，这主要是由于电枢电阻上的压降。串励发电机中的励磁电流与负载电流相同，这样，气隙磁通和电压随负载变化很大，因此很少采用串励发电机。并励发电机电压随负载增加会有所下降，但在许多应用场合，这并不防碍使用。复励发电机的连接通常使串励绕组的磁势与并励绕组磁势相加，其优点是通过串励绕组的作用，每极磁通随着负载增加，从而产生一个随负载增加近似为常数的输出电压。通常，并励绕组匝数多，导线细；而绕在外部的串励绕组由于它必须承载电机的整个电枢电流，所以其构成的导线相对较粗。不论是并励还是复励发电机的电压都可借助并励磁场中的变阻器在适度的范围内得到调节。任何用于发电机的励磁方法都可用于电动机。在电动机典型的静态转速-转矩特性中，假设电动机两端由一个恒压源供电。在电动机电枢中感应的电势与端电压间的关系为 式中此时为输入的电枢电流。电势此时比端电压小，电枢电流与发电机中的方向相反，且电磁转矩与电枢旋转方向相同。在并励和他励电动机中磁场磁通近似为常数，因此转矩的增加必须要求电枢电流近似成比例增大，同时为允许增大的电流通过小的电枢电阻，要求反电势稍有减少。由于反电势决定于磁通和转速，因此，转速必须稍稍降低。与鼠笼式感应电动机相类似，并励电动机实际上是一种从空载到满载速降仅约为5%的恒速电动机。起动转矩和最大转矩受到能成功换向的电枢电流的限制。并励电动机的突出优点是易于调速。在并励绕组回路装上变阻器，励磁电流和每极磁通都可任意改变，而磁通的变化导致转速相反的变化以维持反电势大致等于外施端电压。通过这种方法得到最大调速范围为4或5比1，最高转速同样受到换向条件的限制。通过改变外施电枢电压，可以获得很宽的调速范围。在串励电动机中，电枢电流、电枢电势和定子磁场磁通随负载增加而增加（假设铁芯不完全饱和）。因为磁通随负载增大，所以为了维持外施电压与反电势之间的平衡，速度必须下降，此外，由于磁通增加，所以转矩增大所引起的电枢电流的增大比并励电动机中的要小。因此串励电动机是一种具有明显下降的转速-负载特性的变速电动机。对于要求转矩过载很多的应用场合，由于对应的过载功率随相应的转速下降而维持在一个合理的范围内，因此，这种特性具有特别的优越性。磁通随着电枢电流的增大而增大，同时还带来非常有用的起动特性。在复励电动机中，串励磁场可以连接成积复励式，使其磁势与并励磁场相加；也可以连接成差复励式，两磁场方向相反。差复励连接很少使用。积复励电动机具有界于并励和串励电动机之间的速度-负载特性，转速随负载的降低取决于并励磁场和串励磁场的相对安匝数。这种电动机没有像串励电动机那样轻载高转速的缺点，但它在相当的程度上保持着串励方式的优点。直流电机的应用优势在于可接成并励、串励和复励等各种励磁方式，因而可提供多种性能各异的运行特性。其中有一些特性在本文中已大致提及。如果增加附加的电刷组以至于从换向器上另外可得到一些电压，那么还会存在更多的运用场合，因此直流电机系统的多用性，及其不论对人工还是自动控制的适应性，是它们的显著特性。附录 外文原文Introduction to DC MachinesThe Transformer on loadDC machines are characterized by their versatility. By means of various combination of shunt, series, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steadystate operation. Because of the ease with which they can be controlled , systems of DC machines are often used in applications requiring a wide range of motor speeds or precise control of motor output.The essential features of a DC machine are shown schematically. The stator has salient poles and is excited by one or more field coils. The air-gap flux distribution created by the field winding is symmetrical about the centerline of the field poles. This axis is called the field axis or direct axis.As we know , the AC voltage generated in each rotating armature coil is converted to DC in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectifier, resulting in a DC armature voltage as well as an armature m.m.f. wave which is fixed in space. The brushes are located so that commutation occurs when the coil sides are in the neutral zone , midway between the field poles. The axis of the armature m.m.f. wave then in 90 electrical degrees from the axis of the field poles, i.e., in the quadrature axis. In the schematic representation the brushes are shown in quarature axis because this is the position of the coils to which they are connected. The armature m.m.f. wave then is along the brush axis as shown. (The geometrical position of the brushes in an actual machine is approximately 90 electrical degrees from their position in the schematic diagram because of the shape of the end connections to the commutator.)The magnetic torque and the speed voltage appearing at the brushes are independent of the spatial waveform of the flux distribution; for convenience we shall continue to assume a sinusoidal flux-density wave in the air gap. The torque can then be found from the magnetic field viewpoint. The torque can be expressed in terms of the interaction of the direct-axis air-gap flux per pole and the space-fundamental component of the armature m.m.f. wave . With the brushes in the quadrature axis, the angle between these fields is 90 electrical degrees, and its sine equals unity. For a P pole machine In which the minus sign has been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental of the sawtooth armature m.m.f. wave is 8/ times its peak. Substitution in above equation then gives Where =current in external armature circuit; =total number of conductors in armature winding; =number of parallel paths through winding;And Is a constant fixed by the design of the winding.The rectified voltage generated in the armature has already been discussed before for an elementary single-coil armature. The effect of distributing the winding in several slots is shown in figure ,in which each of the rectified sine waves is the voltage generated in one of the coils, commutation taking place at the moment when the coil sides are in the neutral zone. The generated voltage as observed from the brushes is the sum of the rectified voltages of all the coils in series between brushes and is shown by the rippling line labeled in figure. With a dozen or so commutator segments per pole, the ripple becomes very small and the average generated voltage observed from the brushes equals the sum of the average values of the rectified coil voltages. The rectified voltage between brushes, known also as the speed voltage, is Where is the design constant. The rectified voltage of a distributed winding has the same average value as that of a concentrated coil. The difference is that the ripple is greatly reduced. From the above equations, with all variable expressed in SI units: This equation simply says that the instantaneous electric power associated with the speed voltage equals the instantaneous mechanical power associated with the magnetic torque , the direction of power flow being determined by whether the machine is acting as a motor or generator.The direct-axis air-gap flux is produced by the combined m.m.f. of the field windings, the flux-m.m.f. characteristic being the magnetization curve for the particular iron geometry of the machine. In the magnetization curve, it is assumed that the armature m.m.f. wave is perpendicular to the field axis. It will be necessary to reexamine this assumption later in this chapter, where the effects of saturation are investigated more thoroughly. Because the armature e.m.f. is proportional to flux times speed, it is usually more convenient to express the magnetization curve in terms of the armature e.m.f. at a constant speed . The voltage for a given flux at any other speed is proportional to the speed,i.e. Figure shows the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods, no knowledge of any design details being required.Over a fairly wide range of excitation the reluctance of the iron is negligible compared with that of the air gap. In this region the flux is linearly proportional to the total m.m.f. of the field windings, the constant of proportionality being the direct-axis air-gap permeance.The outstanding advantages of DC machines arise from the wide variety of operating characteristics which can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external DC source, or they may be self-excited; i.e., the machine may supply its own excitation. The method of excitation profoundly influences not only the steady-state characteristics, but also the dynamic behavior of the machine in control systems.The connection diagram of a separately excited generator is given. The required field current is a very small fraction of the rated armature current. A small amount of power in the field circuit may control a relatively large amount of power in the armature circuit; i.e., the generator is a power amplifier. Separately excited generators are often used in feedback control systems when control of the armature voltage over a wide range is required. The field windings of self-excited generators may be supplied in three different ways. The field may be connected in series with the armature, resulting in a shunt generator, or the field may be in two sections, one of which is connected in series and the other in shunt with the armature, resulting in a compound generator. With self-excited generators residual magnetism must be present in the machine iron to get the self-excitation process started.In the typical steady-state volt-ampere characteristics, constant-speed prime movers being assumed. The relation between the steady-state generated e.m.f. and the terminal voltage is Where is the armature current output and is the armature circuit resistance. In a generator, is large than ; and the electromagnetic torque T is a countertorque opposing rotation. The terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally because of the voltage drop in the armature resistance. The field current of a series generator is the same as the load current, so that the air-gap flux and hence the voltage vary widely with load. As a consequence, series generators are not often used. The voltage of shunt generators drops off somewhat with load. Compound generators are normally connected so that the m.m.f. of the series winding aids that of the shunt winding. The advantage is that through the action of the series winding the flux per pole can increase with load, resulting in a voltage output which is nearly constant. Usually, shunt winding contains many turns of comparatively heavy conductor because it must carry the full armature current of the machine. The voltage of both shunt and compound generators can be controlled over reasonable limits by means of rheostats in the shunt field. Any of the methods of excitation used for generators can also be used for motors. In the typical steady-state speed-torque characteristics, it is assumed that the motor terminals are supplied from a constant-voltage source. In a motor the relation between the e.m.f. generated in the armature and the terminal voltage is Where is now the armature current input. The generated e.m.f. is now smaller than the terminal voltage , the armature current is in the opposite direction to that in a motor, and the electromagnetic torque is in the direction to sustain rotation of the armature.In shunt and separately excited motors the field flux is nearly constant. Consequently, increased torque must be accompanied by a very nearly proportional increase in armature current and hence by a small decrease in counter e.m.f. to allow this increased current through the small armature resistance. Since counter e.m.f. is determined by flux and speed, the speed must drop slightly. Like the squirrel-cage induction motor ,the shunt motor is substantially a constant-speed motor having about 5 percent drop in speed from no load to full load. Starting torque and maximum torque are limited by the armature current that can be commutated successfully.An outstanding advantage of the shunt motor is ease of speed control. With a rheostat in the shunt-field circuit, the field current and flux per pole can be varied at will, and variation of flux causes the inverse variation of speed to maintain counter e.m.f. approximately equal to the impressed terminal voltage. A maximum speed range of about 4 or 5 to 1 can be obtained by this method, the limitation again being commutating conditions. By variation of the impressed armature voltage, very wide speed ranges can be obtained.In the series motor, increase in load is accompanied by increase in the armature current and m.m.f. and the stator field flux (provided the iron is not completely saturated). Because flux increases with load, speed must drop in order to maintain the balance between impressed voltage and counter e.m.f.; moreover, the increase in armature current caused by increased torque is smaller than in the shunt motor because of the increased flux. The series motor is therefore a varying-speed mo