电信电流外文翻译外文文献英文文献直流电机导论负载运行的变压器

1、外文文献Introduction to DC Machines The Transformer on loadDC machines are characterized by their versatility. By means of various comb in ati on of shu nt, series, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for bot

2、h dynamic and steady state operation. Because of the ease with which they can be con trolled , systems of DC machi nes are ofte n used in applicati ons requiri ng a wide range of motor speeds or precise con trol of motor output.The essential features of a DC machine are shown schematically. The stat

3、or 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.The magn etic torque and the speed voltage appeari ng at the brushes ar

4、e in depe ndent of the spatial waveform of the flux distributi on; 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 in teracti on of the direct-axis ai

5、r-gap flux per pole d and the space-fundamental component Fa1 pole mach inefun dame ntalFa1 of the sawtooth armaturem.m.f. wave is 8/2 timesitsIn which the minus sig n has bee n dropped because the positive direct ionphysical reasoning. The spaceof the torque can be determined frompeak. Substituti o

6、n in above equati on the n givesPCa id ia2 mWhere ia =current in external armature circuit;Ca=total nu mber of con ductors in armature wi nding; m =nu mber of parallel paths through winding;AndPCa2 mIs a con sta nt fixed by the desig n of the winding.The rectified voltage gen erated in the armature

7、has already bee n discussed before for an elementary single-coil armature. The effect of distributi ng the winding in several slots is show n in figure ,in which each of the rectified sine waves is the voltage gen erated in one of the coils, commutation taking place at the moment when the coil sides

8、 are in the n eutral zone. The gen erated 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 ea in figure. With a dozen or so commutator segme nts per pole, the ripple becomes very small and t

9、he average gen erated voltage observed from the brushes equals the sum of the average values of the rectified coil voltages. The rectified voltageea between brushes, known alsoas the speed voltage, isPCadWmKa dWmWhere Ka is the design constant. The rectified voltage of a distributed winding has the

10、same average value as that of a concentrated coil. The differe nee is that the ripple is greatly reduced.From the above equations, with all variable expressed in SI units:ea i aTwmThis equation simply says that the instantaneouselectric powerassociated with the speed voltage equals the in sta ntan e

11、ous mecha ni cal power associated with the magn etic torque , the direct ion of power flow being determ ined by whether the mach ine is acting as a motor or gen erator.Nfif ea0at a constant speed wm0. The voltage ea for a given flux at any other speed Wm is proportional to the speed,i.e.wmwmoFigure

12、shows the magn etizati on curve with only one field winding excited. This curve can easily be obta ined by test methods, no kno wledge of any desig n details being portionality being the direct-axis air-gap permeanee.The outsta nding adva ntages of DC machi nes arise from the wide variet

13、y of operat ing characteristics which can be obta ined by selectio n 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., themach ine may supply its own excitati on. The method of excitati on pr

14、ofo un dly in flue nces not on ly the steady-state characteristics, but also the dyn amic behavior of the mach ine in con trol 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

15、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 whe n con trol of the armature voltage over a wide range is required.The field wind

16、ingsof self-excited generators may be supplied in threediffere ntresult ingways. The field may be connected in series with the armature, in a shunt generator, or the field may be in two sections, one ofwhich is connectedin series and the other in shunt with the armature, resulting in a compound gene

17、rator. 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 relati on betwee n the steady-state gen eratedEa and the termi

18、nal voltageVt isEa IaRaWhere Ia is the armature current output and Ra is the armature circuit resista nee. In a gen erator, Ea is large tha n Vt ; and the electromag netic torque T is a coun tertorque oppos ing rotati on.Ea gen erated in the armature and theterminal voltage Vt isVtEa laRaWhere I a E

19、a is now smallerthan the terminal voltageVt, the armature current is in theopposite directi on to that in a motor, and the electromag netic torque is in the directi on to sustai n rotati on of the armature.motor is therefore a vary in g-speed motor with a markedlydroop ingspeed-load characteristic.

20、Forapplicati ons requiri ng heavy torque overloads,this characteristic is particularlyadvantageous because the correspondingpower overloads are held to more reas on able values by the associated speed drops. Very favorable starti ng characteristics also result from the in crease in flux with in crea

21、sed armature curre nt.In the compound motor the series field may be connected eitherso that it opposes. The differentialconnection is very rarely used. Acumulatively compoundedmotor has speed-load characteristic in termediatebetween those of a shuntand a series motor, the drop of speed with loaddepe

22、nding on the relativenumber of ampere-turnsin the shunt and seriesfields.It does not havethe disadvantage of very high light-loadspeedThe applicationadvantages of DC machines lie in the variety ofperforma nee characteristics offered by the possibilities of shunt, series, and compo und excitati on. S

23、ome of these characteristics have bee n touched upon briefly in this article. Still greater possibilities exist if additional sets of brushes are added so that other voltages can be obtained from the commutator.Thus the versatilityof DC machine systems and theiradaptability to control, both manual a

24、nd automatic, are their outstanding features.外文翻译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

25、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

26、矩。转矩可以用直轴每极气隙磁通 d和电枢磁势波的空间基波分量 Fa1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Fa!是峰值的8/ 2。上式变换后有PCa id ia2 m式中ia =电枢外部电路中的电流;Ca=电枢绕组中的总导体数; m =通过绕组的并联支路数;PCaTm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

27、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ea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ea即速度电压，为PCaeaa dWm Ka dWm2 m式中Ka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，有色 ia TWm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

28、Nfif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Wm0下的电枢电势eao来表示磁化曲线更为方便。任意转速Wm时，任一给定磁通下的电压ea与转速成正比，即WmWmo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

29、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

30、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 Ea和端电压Vt关系为:Vt E a I a Ra式中la为电枢输出电流，Ra为电枢回路电阻。在发电机中，Ea比Vt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