電氣專業(yè)外文翻譯6.docx

附錄A 譯文直流電機導(dǎo)論負載運行的變壓器直流電機以其多功用性而形成了鮮明的特征。通過并勵、串勵和特勵繞組的各種不同組合，直流電機可設(shè)計成在動態(tài)和穩(wěn)態(tài)運行時呈現(xiàn)出寬廣范圍變化的伏-安或速度-轉(zhuǎn)矩特性。由于直流電機易于控制，因此該系統(tǒng)用于要求電動機轉(zhuǎn)速變化范圍寬或能精確控制電機輸出的場合。定子上有凸極，由一個或一個以上勵磁線圈勵磁。勵磁繞組產(chǎn)生的氣隙通以磁極中心線為軸線對稱分布，這條軸線稱為磁場軸線或直軸。我們知道，每個旋轉(zhuǎn)的電樞繞組中產(chǎn)生的交流電壓，經(jīng)由一與電樞連接的旋轉(zhuǎn)的換向器和靜止的電刷，在電樞繞組出線端轉(zhuǎn)換成直流電壓。換向器一電刷的組合構(gòu)成機械整流器，它產(chǎn)生一直流電樞電壓和一在空間固定的電樞磁勢波形。電刷的放置應(yīng)使換向線圈也處于磁極中性區(qū)，即兩磁極之間。這樣，電樞磁勢波形的軸線與磁極軸線相差90電角度，即位于交軸上。在示意圖中，電刷位于交軸上，因為此處正是與其相連的線圈的位置。這樣，如圖所示電樞磁勢波的軸線也是沿著電刷軸線的。（在實際電機中，電刷的幾何位置大約偏移圖例中所示位置90電角度，這是因為元件的末端形狀構(gòu)成圖示結(jié)果與換向器相連。）電刷上的電磁轉(zhuǎn)矩和速度電壓與磁通分布的空間波形無關(guān)；為了方便起見，我們假設(shè)氣隙中仍然是正弦磁密波，這樣便可以從磁場分析著手求得轉(zhuǎn)矩。轉(zhuǎn)矩可以用直軸每極氣隙磁通和電樞磁勢波的空間基波分量相互作用的結(jié)果來表示。電刷處于交軸時，磁場間的角度為90電角度，其正弦值等于1，則對于一臺P極電機 式中由于轉(zhuǎn)矩的正方向可以根據(jù)物理概念的推斷確定，因此負號已經(jīng)去掉。電樞磁勢鋸齒波的空間基波是峰值的8/。上式變換后有 式中 =電樞外部電路中的電流； =電樞繞組中的總導(dǎo)體數(shù)； =通過繞組的并聯(lián)支路數(shù)；且 其為一個由繞組設(shè)計而確定的常數(shù)。簡單的單個線圈的電樞中的整流電壓前面已經(jīng)討論過了。將繞組分散在幾個槽中的效果可用圖形表示，圖中每一條整流的正弦波形是一個線圈產(chǎn)生的電壓，換向線圈邊處于磁中性區(qū)。從電刷端觀察到的電壓是電刷間所有串聯(lián)線圈中整流電壓的總和，在圖中由標(biāo)以的波線表示。當(dāng)每極有十幾個換向器片，波線的波動變得非常小，從電刷端觀察到的平均電壓等于線圈整流電壓平均值之和。電刷間的整流電壓即速度電壓，為 式中為設(shè)計常數(shù)。分布繞組的整流電壓與集中線圈有著相同的平均值，其差別只是分布繞組的波形脈動大大減小。將上述幾式中的所有變量用SI單位制表達，有 這個等式簡單地說明與速度電壓有關(guān)的瞬時功率等于與磁場轉(zhuǎn)矩有關(guān)的瞬時機械功率，能量的流向取決于這臺電機是電動機還是發(fā)電機。直軸氣隙通由勵磁繞組的合成磁勢產(chǎn)生，其磁通-磁勢曲線就是電機的具體鐵磁材料的幾何尺寸決定的磁化曲線。在磁化曲線中，因為電樞磁勢波的軸線與磁場軸線垂直，因此假定電樞磁勢對直軸磁通不產(chǎn)生作用。這種假設(shè)有必要在后述部分加以驗證，屆時飽和效應(yīng)會深入研究。因為電樞電勢與磁通成正比，所以通常用恒定轉(zhuǎn)速下的電樞電勢來表示磁化曲線更為方便。任意轉(zhuǎn)速時，任一給定磁通下的電壓與轉(zhuǎn)速成正比，即 圖中表示只有一個勵磁繞組的磁化曲線，這條曲線可以很容易通過實驗方法得到，不需要任何設(shè)計步驟的知識。在一個相當(dāng)寬的勵磁范圍內(nèi)，鐵磁材料部分的磁阻與氣隙磁阻相比可以忽略不計，在此范圍內(nèi)磁通與勵磁繞組總磁勢呈線性比例，比例常數(shù)便是直軸氣隙磁導(dǎo)率。直流電機的突出優(yōu)點是通過選擇磁場繞組不同的勵磁方法，可以獲得變化范圍很大的運行特性。勵磁繞組可以由外部直流電源單獨激磁，或者也可自勵，即電機提供自身的勵磁。勵磁防哪個法不僅極大地影響控制系統(tǒng)中電機的靜態(tài)特性，而且影響其動態(tài)運行。他勵發(fā)電機的連接圖已經(jīng)給出，所需勵磁電流是額定電樞電流的很小一部分。勵磁電路中很小數(shù)量的功率可以控制電樞電路中相對很大數(shù)量的功率，也就是說發(fā)電機是一種功率放大器。當(dāng)需要在很大范圍內(nèi)控制電樞電壓時，他勵發(fā)電機常常用于反饋控制系統(tǒng)中。自勵發(fā)電機的勵磁繞組可以有三種不同的供電方式。勵磁繞組可以與電樞串聯(lián)起來，這便形成了串勵發(fā)電機；勵磁繞組可以與電樞并聯(lián)在一起，這便形成了并勵發(fā)電機；或者勵磁繞組分成兩部分，其中一部分與電樞串聯(lián)，另一部分與電樞并聯(lián)，這便形成復(fù)勵發(fā)電機。為了引起自勵過程，在自勵發(fā)電機中必須存在剩磁。 在典型的靜態(tài)伏-安特性中，假定原動機恒速運行，穩(wěn)態(tài)電勢和端電壓關(guān)系為： 式中為電樞輸出電流，為電樞回路電阻。在發(fā)電機中，比大，電磁轉(zhuǎn)矩T是一種阻轉(zhuǎn)矩。他勵發(fā)電機的端電壓隨著負載電流的增加稍有降低，這主要是由于電樞電阻上的壓降。串勵發(fā)電機中的勵磁電流與負載電流相同，這樣，氣隙磁通和電壓隨負載變化很大，因此很少采用串勵發(fā)電機。并勵發(fā)電機電壓隨負載增加會有所下降，但在許多應(yīng)用場合，這并不防礙使用。復(fù)勵發(fā)電機的連接通常使串勵繞組的磁勢與并勵繞組磁勢相加，其優(yōu)點是通過串勵繞組的作用，每極磁通隨著負載增加，從而產(chǎn)生一個隨負載增加近似為常數(shù)的輸出電壓。通常，并勵繞組匝數(shù)多，導(dǎo)線細；而繞在外部的串勵繞組由于它必須承載電機的整個電樞電流，所以其構(gòu)成的導(dǎo)線相對較粗。不論是并勵還是復(fù)勵發(fā)電機的電壓都可借助并勵磁場中的變阻器在適度的范圍內(nèi)得到調(diào)節(jié)。任何用于發(fā)電機的勵磁方法都可用于電動機。在電動機典型的靜態(tài)轉(zhuǎn)速-轉(zhuǎn)矩特性中，假設(shè)電動機兩端由一個恒壓源供電。在電動機電樞中感應(yīng)的電勢與端電壓間的關(guān)系為 式中此時為輸入的電樞電流。電勢此時比端電壓小，電樞電流與發(fā)電機中的方向相反，且電磁轉(zhuǎn)矩與電樞旋轉(zhuǎn)方向相同。在并勵和他勵電動機中磁場磁通近似為常數(shù)，因此轉(zhuǎn)矩的增加必須要求電樞電流近似成比例增大，同時為允許增大的電流通過小的電樞電阻，要求反電勢稍有減少。由于反電勢決定于磁通和轉(zhuǎn)速，因此，轉(zhuǎn)速必須稍稍降低。與鼠籠式感應(yīng)電動機相類似，并勵電動機實際上是一種從空載到滿載速降僅約為5%的恒速電動機。起動轉(zhuǎn)矩和最大轉(zhuǎn)矩受到能成功換向的電樞電流的限制。并勵電動機的突出優(yōu)點是易于調(diào)速。在并勵繞組回路裝上變阻器，勵磁電流和每極磁通都可任意改變，而磁通的變化導(dǎo)致轉(zhuǎn)速相反的變化以維持反電勢大致等于外施端電壓。通過這種方法得到最大調(diào)速范圍為4或5比1，最高轉(zhuǎn)速同樣受到換向條件的限制。通過改變外施電樞電壓，可以獲得很寬的調(diào)速范圍。在串勵電動機中，電樞電流、電樞電勢和定子磁場磁通隨負載增加而增加（假設(shè)鐵芯不完全飽和）。因為磁通隨負載增大，所以為了維持外施電壓與反電勢之間的平衡，速度必須下降，此外，由于磁通增加，所以轉(zhuǎn)矩增大所引起的電樞電流的增大比并勵電動機中的要小。因此串勵電動機是一種具有明顯下降的轉(zhuǎn)速-負載特性的變速電動機。對于要求轉(zhuǎn)矩過載很多的應(yīng)用場合，由于對應(yīng)的過載功率隨相應(yīng)的轉(zhuǎn)速下降而維持在一個合理的范圍內(nèi)，因此，這種特性具有特別的優(yōu)越性。磁通隨著電樞電流的增大而增大，同時還帶來非常有用的起動特性。在復(fù)勵電動機中，串勵磁場可以連接成積復(fù)勵式，使其磁勢與并勵磁場相加；也可以連接成差復(fù)勵式，兩磁場方向相反。差復(fù)勵連接很少使用。積復(fù)勵電動機具有界于并勵和串勵電動機之間的速度-負載特性，轉(zhuǎn)速隨負載的降低取決于并勵磁場和串勵磁場的相對安匝數(shù)。這種電動機沒有像串勵電動機那樣輕載高轉(zhuǎn)速的缺點，但它在相當(dāng)?shù)某潭壬媳３种畡罘绞降膬?yōu)點。直流電機的應(yīng)用優(yōu)勢在于可接成并勵、串勵和復(fù)勵等各種勵磁方式，因而可提供多種性能各異的運行特性。其中有一些特性在本文中已大致提及。如果增加附加的電刷組以至于從換向器上另外可得到一些電壓，那么還會存在更多的運用場合，因此直流電機系統(tǒng)的多用性，及其不論對人工還是自動控制的適應(yīng)性，是它們的顯著特性。附錄 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