1、外文文獻introduction to dc machines the 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 dy

2、namic and steady state 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 sal

3、ient 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

4、 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.

5、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 brus

6、hes 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 schem

7、atic 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.

8、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 i

9、s 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 a

10、bove 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

11、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 vo

12、ltage 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 b

13、rushes 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 differenc

14、e 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

15、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 magnetiz

16、ation 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

17、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 obt

18、ained 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 prop

19、ortionality 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 d

20、c 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 i

21、s 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 f

22、eedback 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 sectio

23、ns, 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,

24、 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 rot

25、ation. 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 volt

26、age 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 actio

27、n 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

28、 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 consta

29、nt-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 e

30、lectromagnetic 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 count

31、er 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 fr

32、om 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 w

33、ill, 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 impress

34、ed 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 mai

35、ntain 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 motor with a markedly drooping speed-load characterist

36、ic. for applications requiring heavy torque overloads, this characteristic is particularly advantageous because the corresponding power overloads are held to more reasonable values by the associated speed drops. very favorable starting characteristics also result from the increase in flux with incre

37、ased armature current.in the compound motor the series field may be connected either cumulatively, so that its.m.m.f.adds to that of the shunt field, or differentially, so that it opposes. the differential connection is very rarely used. a cumulatively compounded motor has speed-load characteristic

38、intermediate between those of a shunt and a series motor, the drop of speed with load depending on the relative number of ampere-turns in the shunt and series fields. it does not have the disadvantage of very high light-load speed associated with a series motor, but it retains to a considerable degr

39、ee the advantages of series excitation.the application advantages of dc machines lie in the variety of performance characteristics offered by the possibilities of shunt, series, and compound excitation. some of these characteristics have been touched upon briefly in this article. still greater possi

40、bilities exist if additional sets of brushes are added so that other voltages can be obtained from the commutator. thus the versatility of dc machine systems and their adaptability to control, both manual and automatic, are their outstanding features.外文翻譯直流電機導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

41、組的各種不同組合，直流電機可設計成在動態和穩態運行時呈現出寬廣范圍變化的伏-安或速度-轉矩特性。由于直流電機易于控制，因此該系統用于要求電動機轉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，每個旋轉的電樞繞組中產生的交流電壓，經由一與電樞連接的旋轉的換向器和靜止的電刷，在電樞繞組出線端轉換成直流電壓。換向器一電刷的組合構成機械整流器，它產生一直流電樞電壓和一在空間固定的電樞磁勢波形。電刷的放置應使換向線圈也處于磁極中性區，即兩磁極之間。這樣，電樞磁勢波形的軸線與磁極軸線相

42、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成圖示結果與換向器相連。）電刷上的電磁轉矩和速度電壓與磁通分布的空間波形無關；為了方便起見，我們假設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矩。轉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 式中由于轉矩的正方向可以根據物理概念的推斷確定，因此負號已經去

43、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； =電樞繞組中的總導體數； =通過繞組的并聯支路數；且 其為一個由繞組設計而確定的常數。簡單的單個線圈的電樞中的整流電壓前面已經討論過了。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生的電壓，換向線圈邊處于磁中性區。從電刷端觀察到的電壓是電刷間所有串聯線圈中整流電壓的總和，在圖中由標以的波線表示。當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 式中為設計常數。分布繞組的整流電壓與集中線圈有著相同的

44、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的瞬時功率等于與磁場轉矩有關的瞬時機械功率，能量的流向取決于這臺電機是電動機還是發電機。直軸氣隙通由勵磁繞組的合成磁勢產生，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生作用。這種假設有必要在后述部分加以驗證，屆時飽和效應會深入研究。因為電樞電勢與磁通成正比，所以通常用恒定轉速下的電樞電勢來表示磁化曲線更為方便。任意轉速時，任一給定磁通下的電壓與轉速成正比，即 圖中表示只有一

45、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，不需要任何設計步驟的知識。在一個相當寬的勵磁范圍內，鐵磁材料部分的磁阻與氣隙磁阻相比可以忽略不計，在此范圍內磁通與勵磁繞組總磁勢呈線性比例，比例常數便是直軸氣隙磁導率。直流電機的突出優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中電機的靜態特性，而且影響其動態運行。他勵發電機的連接圖已經給出，所需勵磁電流是額定電樞電流的很小一部分。勵磁電路中很小數量的功率可以控制電樞電路中相對很大數量的功率，也就是說發電機是一種功率放大器。當需要在很大范圍內控制電樞電壓時，他勵發電機