電子類文獻(xiàn)中英文翻譯---直流發(fā)電機(jī)

1、<p>　　ENGLISH ORIGINAL TEXT</p><p>　　DC GENENRATORS</p><p>　　1. INTRODUCTION</p><p>　　For all practical purposes, the direct-current generator is only used for special applicat

2、ions and local dc power generation. This limitation is due to the commutator required to rectify the internal generated ac voltage, thereby making largescale dc power generators not feasible.</p><p>　　Conseq

3、uently, all electrical energy produced commercially is generated and distributed in the form of three-phase ac power. The use of solid state converters nowadays makes conversion to dc economical. However, the operating c

4、haracteristics of dc generators are still important, because most concepts can be applied to all other machines.</p><p>　　2. FIELD WINDING CONNECTIONS</p><p>　　The general arrangement of brushes

5、 and field winding for a four-pole machine is as shown in Fig.1. The four brushes ride on the commutator. The positive brusher are connected to terminal A1 while the negative brushes are connected to terminal A2 of the m

6、achine. As indicated in the sketch, the brushes are positioned approximately midway under the poles. They make contact with coils that have little or no EMF induced in them, since their sides are situated between poles.&

7、lt;/p><p>　　Figure 1 Sketch of four-pole dc matchine</p><p>　　The four excitation or field poles are usually joined in series and their ends brought out to terminals marked F1 and F2. They are conn

8、ected such that they produce north and south poles alternately.</p><p>　　The type of dc generator is characterized by the manner in which the field excitation is provided. In general, the method employed to

9、connect the field and armature windings falls into the following groups (see Fig.2):</p><p>　　Figure2 Field connections for dc generators:(a)separately excited generator;(b)self-excited,shunt generator;(c)se

10、ries generator;(d)compound generator;short-shunt connection;(e)compound generator,long-shunt connection.</p><p>　　The shunt field contains many turns of relatively fine wire and carries a comparatively small

11、 current, only a few percent of rated current. The series field winding, on the other hand, has few turns of heavy wire since it is in series with the armature and therefore carries the load current.</p><p>

12、　　Before discussing the dc generator terminal characteristics, let us examine the relationship between the generated voltage and excitation current of a generator on no load. The generated EMF is proportional to both the

13、 flux per pole and the speed at which the generator is driven, EG=kn. By holding the speed constant it can be shown the EG depends directly on the flux. To test this dependency on actual generators is not very practical,

14、 as it involves a magnetic flux measurement. The flux is produ</p><p>　　The value of EG appearing at the machine terminals is measured as If is progressively increased from zero to a value well above rated v

15、oltage of that machine. The resulting curve is shown is Fig.3. When Ij=0, that is, with the field circuit open circuited, a small voltage Et is measured, due to residual magnetism. As the field current increases, the gen

16、erated EMF increases linearly up to the knee of the magnetization curve. Beyond this point, increasing the field current still further causes sat</p><p>　　Figure 3 Magnetization curve or open-circuit charact

17、eristic of a separately excited dc machine</p><p>　　The means that a larger increase in field current is required to produce a given increase in voltage.</p><p>　　Since the generated voltage EG

18、is also directly proportional to the speed, a magnetization curve can be drawn for any other speed once the curve is determined. This merely requires an adjustment of all points on the curve according to</p><p

19、>　　where the quantities values at the various speeds.</p><p>　　3. VOLTAGE REGULATION</p><p>　　Let us next consider adding a load on generator. The terminal voltage will then decrease (because

20、 the armature winding ha resistance) unless some provision is made to keep it constant. A curve that shows the value of terminal voltage for various load currents is called the load or characteristic of the generator.<

21、;/p><p>　　Figure 4 (a) directs current it to urge the generator load characteristics; (b) circuit diagram</p><p>　　Fig.4 shows the external characteristic of a separately excited generator. The dec

22、rease in the terminal voltage is due mainly to the armature circuit resistance RA. In general, </p><p>　　where Vt is the terminal voltage and IA is the armature current (or load current IL) supplied by the g

23、enerator to the load.</p><p>　　Another factor that contributes to the decrease in terminal voltage is the decrease in flux due to armature reaction. The armature current established an MMF that distorts the

24、main flux, resulting in a weakened flux, especially in noninterpole machines. This effect is called armature reaction. As Fig.4 shows, the terminal voltage versus load current curve does not drop off linearly since the i

25、ron behaves nonlinear. Because armature reaction depends on the armature current it gives the curve its </p><p>　　4. SHUNT OR SELF-EXCIITED GENRATORS</p><p>　　A shunt generator has its shunt fie

26、ld winding connected in parallel with the armature so that the machine provides its own excitation, as indicated in Fig.5. The question arises whether the machine will generate a voltage and what determines the voltage.&

27、lt;/p><p>　　For voltage to “build up” as it is called, there must be some remanent magnetism in the field poles. Ordinarily, if the generator has been used previously, there will be some remanent magnetism. We

28、have seen in Section 3 that if the field would be disconnected, there will be small voltage Ef generated due to this remanent magnetism, provided that the generator is driven at some speed. Connecting the field for self-

29、excitation, this small voltage will be applied to the shunts field and drive a sma</p><p>　　Figure 5 Shunt generator:(a)circuit;(b)load characteristic</p><p>　　If the connection is such that the

30、 weak main pole flux aids the residual flux, the induced voltage increases rapidly to a large, constant value. The build-up process is readily seen to be cumulanve. That is, more voltage increases the field current, whic

31、h in turn increases the voltage, and so on. The fact that this process terminates at a finite voltage is due to the nonlinear behavior of the magnctic circuit. In steady state the generated voltage is causes a field curr

32、ent to flow that is just s</p><p>　　The circuit carries only dc current, so that the field current depends only on the field circuit resistance, Rf. This may consist of the field circuit resistance Rf, the f

33、ield current depends on the generated voltage in accordance with Ohm’s law.</p><p>　　It should be evident that on a new machine or one that has lost its residual flux because of a long idle period, some magn

34、etism must be created. This is usually done by connecting the field winding only to a separate dc source for a few seconds. This procedure is generally known as flashing the field.</p><p>　　Series Generators

35、</p><p>　　As mentioned previously, the field winding of a series generator is in series with the armature. Since it carries the load current the series field winding consists of only a few turns of thick wir

36、e. At no load, the generated voltage is small due to residual field flux only. When a load is added, the flux increases, and so does the generated voltage. Fig.7 shows the load characteristic of a series generator driven

37、 at a certain speed. The dashed line indicates the generated EMF of the same machin</p><p>　　where RS is the series field winding resistance.</p><p>　　Figure 7 Series generator: (a)circuit diagr

38、am;(b)load characteristicsCompound Generators</p><p>　　The compound generator has both a shunt and a series field winding, the latter winding wound on top of the shunt winding. Fig.8 shows the circuit diagra

39、m. The two windings are usually connected such that their ampere-turns act in the same direction. As such the generator is said to be cumulatively compounded.</p><p>　　The shunt connection illustrated in Fig

40、.8 is called a long shunt connection. If the shunt field winding is directly connected across the armature terminals, the connection is referred to as a short shunt. In practice the connection used is of little consequen

41、ce, since the shunt field winding carries a small current compared to the full-load current. Furthermore, the number of turns on the series field winding. This implies it has a low resistance value and the corresponding

42、voltage drop across i</p><p>　　Curves in Fig.9 represents the terminal characteristic of the shunt field winding alone. By the addition of a small series field winding the drop in terminal voltage with incre

43、ased loading is reduced as indicated. Such a generator is said to be undercompounded. By increasing the number of series turns, the no-load and full-load terminal voltage can be made equal; the generator is then said to

44、be flatcompounded. If the number of series turns is more than necessary to compensate for the voltage dr</p><p>　　Figure 9 Terminal characteristics of compound generators compared with that of the shunt gene

45、rator</p><p>　　The overcompounded generator may be used in instances where the load is at some distance from the generator. The voltage drops in the feeder lines are the compensated for with increased loadin

46、g. Reversing the polarity of the series field in relation to the shunt field, the fields will oppose each other more and more as the load current increase. Such a generator is said to be differentially compounded. It is

47、used in applications where feeder lines could occur approaching those of a short circuit</p><p>　　As illustrated, the full-load terminal voltage can be maintained at the no-load value by the proper degree of

48、 compounding. Other methods of voltage control are the use of rheostats, for instance, in the field circuit. However, with changing loads it requires a constant adjustment of the field rheostat to maintain the voltage. A

49、 more useful arrangement, which is now common practice, is to use an automatic voltage regulator with the generator. In essence, the voltage regulator is a feedback contro</p><p>　　TRANSFORMER</p><

50、;p>　　1. INTRODUCTION</p><p>　　The high-voltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage mus

51、t be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles

52、and applications.</p><p>　　2. TOW-WINDING TRANSFORMERS</p><p>　　A transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. The coils are said to be mu

53、tually coupled because they link a common flux.</p><p>　　In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotation

54、al losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher va

55、lues applying to the larger power transformers.</p><p>　　The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, whi

56、ch varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing; therefore, it induces a voltage in the secondary by electrom

57、agnetic induction in accordance with Lenz’s law. Thus the primary receives its power from the source while the secondary supplies </p><p>　　3. TRANSFORMER PRINCIPLES</p><p>　　When a sinusoidal v

58、oltage Vp is applied to the primary with the secondary open-circuited, there will be no energy transfer. The impressed voltage causes a small current Iθ to flow in the primary winding. This no-load current has two functi

59、ons: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and φm, where φm is the maximum value of the core flux; and (2) it provides a component to account for the hysteresis and eddy c

60、urrent losses in the core. There combined</p><p>　　The no-load current Iθ is usually few percent of the rated full-load current of the transformer (about 2 to 5%). Since at no-load the primary winding acts a

61、s a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90º. It is readily seen that the current component Im= I0sinθ0, called the magnetizing current, is 90º in pha

62、se behind the primary voltage VP. It is this component that sets up the flux in the core; φ is therefore in phase with Im.</p><p>　　The second component, Ie=I0sinθ0, is in phase with the primary voltage. It

63、is the current component that supplies the core losses. The phasor sum of these two components represents the no-load current, or</p><p>　　I0 = Im+ Ie</p><p>　　It should be noted that the no-loa

64、d current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material.</p><p>　　If it is assumed that there are no other losses in the transformer, the induced voltage I

65、n the primary, Ep and that in the secondary, Es can be shown. Since the magnetic flux set up by the primary winding，there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, E=NΔφ/

66、Δt. This same flux also links the primary itself, inducing in it an EMF, Ep. As discussed earlier, the induced voltage must lag the flux by 90º, therefore, they are 180º out of phase with the</p><p&g

67、t;　　Eavg = turns×</p><p>　　which is Faraday’s law applied to a finite time interval. It follows that</p><p>　　Eavg = N = 4fNφm</p><p>　　which N is the number of turns on the wi

68、nding. Form ac circuit theory, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage; thus</p><p>　　E = 4.44fNφm</p><p>　　Since the same flux links wi

69、th the primary and secondary windings, the voltage per turn in each winding is the same. Hence</p><p>　　Ep = 4.44fNpφm</p><p><b>　　and</b></p><p>　　Es = 4.44fNsφm</p&

70、gt;<p>　　where Ep and Es are the number of turn on the primary and secondary windings, respectively. The ratio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a,

71、it is seen that</p><p><b>　　a = = </b></p><p>　　Assume that the output power of a transformer equals its input power, not a bad sumption in practice considering the high efficiencie

72、s. What we really are saying is that we are dealing with an ideal transformer; that is, it has no losses. Thus</p><p><b>　　Pm = Pout</b></p><p><b>　　or</b></p><

73、;p>　　VpIp × primary PF = VsIs × secondary PF</p><p>　　where PF is the power factor. For the above-stated assumption it means that the power factor on primary and secondary sides are equal; there

74、fore</p><p>　　VpIp = VsIs</p><p>　　from which is obtained</p><p><b>　　= ≌ ≌ a</b></p><p>　　It shows that as an approximation the terminal voltage ratio eq

75、uals the turns ratio. The primary and secondary current, on the other hand, are inversely related to the turns ratio. The turns ratio gives a measure of how much the secondary voltage is raised or lowered in relation to

76、the primary voltage. To calculate the voltage regulation, we need more information.</p><p>　　The ratio of the terminal voltage varies somewhat depending on the load and its power factor. In practice, the tra

77、nsformation ratio is obtained from the nameplate data, which list the primary and secondary voltage under full-load condition.</p><p>　　When the secondary voltage Vs is reduced compared to the primary voltag

78、e, the transformation is said to be a step-down transformer: conversely, if this voltage is raised, it is called a step-up transformer. In a step-down transformer the transformation ratio a is greater than unity (a>1.

79、0), while for a step-up transformer it is smaller than unity (a<1.0). In the event that a=1, the transformer secondary voltage equals the primary voltage. This is a special type of transformer used in instances w</

80、p><p>　　As is apparent, it is the magnetic flux in the core that forms the connecting link between primary and secondary circuit. In section 4 it is shown how the primary winding current adjusts itself to the s

81、econdary load current when the transformer supplies a load.</p><p>　　Looking into the transformer terminals from the source, an impedance is seen which by definition equals Vp / Ip. From = ≌ ≌ a , we have

82、 Vp = aVs and Ip = Is/a.In terms of Vs and Is the ratio of Vp to Ip is</p><p><b>　　= = </b></p><p>　　But Vs / Is is the load impedance ZL thus we can say that</p><p>　　

83、Zm (primary) = a2ZL</p><p>　　This equation tells us that when an impedance is connected to the secondary side, it appears from the source as an impedance having a magnitude that is a2 times its actual value.

84、 We say that the load impedance is reflected or referred to the primary. It is this property of transformers that is used in impedance-matching applications.</p><p>　　4. TRANSFORMERS UNDER LOAD</p>&l

85、t;p>　　The primary and secondary voltages shown have similar polarities, as indicated by the “dot-making” convention. The dots near the upper ends of the windings have the same meaning as in circuit theory; the marked

86、terminals have the same polarity. Thus when a load is connected to the secondary, the instantaneous load current is in the direction shown. In other words, the polarity markings signify that when positive current enters

87、both windings at the marked terminals, the MMFs of the two windings a</p><p>　　Since the secondary voltage depends on the core flux φ0, it must be clear that the flux should not change appreciably if Es is t

88、o remain essentially constant under normal loading conditions. With the load connected, a current Is will flow in the secondary circuit, because the induced EMF Es will act as a voltage source. The secondary current prod

89、uces an MMF NsIs that creates a flux. This flux has such a direction that at any instant in time it opposes the main flux that created it in the first p</p><p>　　In general, it will be found that the transfo

90、rmer reacts almost instantaneously to keep the resultant core flux essentially constant. Moreover, the core flux φ0 drops very slightly between n o load and full load (about 1 to 3%), a necessary condition if Ep is to fa

91、ll sufficiently to allow an increase in Ip.</p><p>　　On the primary side, Ip’ is the current that flows in the primary to balance the demagnetizing effect of Is. Its MMF NpIp’ sets up a flux linking the prim

92、ary only. Since the core flux φ0 remains constant. I0 must be the same current that energizes the transformer at no load. The primary current Ip is therefore the sum of the current Ip’ and I0.</p><p>　　Becau

93、se the no-load current is relatively small, it is correct to assume that the primary ampere-turns equal the secondary ampere-turns, since it is under this condition that the core flux is essentially constant. Thus we wil

94、l assume that I0 is negligible, as it is only a small component of the full-load current.</p><p>　　When a current flows in the secondary winding, the resulting MMF (NsIs) creates a separate flux, apart from

95、the flux φ0 produced by I0, which links the secondary winding only. This flux does no link with the primary winding and is therefore not a mutual flux.</p><p>　　In addition, the load current that flows throu

96、gh the primary winding creates a flux that links with the primary winding only; it is called the primary leakage flux. The secondary- leakage flux gives rise to an induced voltage that is not counter balanced by an equiv

97、alent induced voltage in the primary. Similarly, the voltage induced in the primary is not counterbalanced in the secondary winding. Consequently, these two induced voltages behave like voltage drops, generally called le

98、akage reactanc</p><p><b>　　中文翻譯</b></p><p><b>　　直流發(fā)電機(jī)</b></p><p><b>　　1.介紹</b></p><p>　　對于所有實(shí)際目的來說，直流發(fā)電機(jī)僅用于特殊場合和地方性發(fā)電廠。這個(gè)局限性是由于換向器要把

99、發(fā)電機(jī)內(nèi)部的電壓整流為直流電壓，因此使大規(guī)模直流發(fā)電不能實(shí)行。</p><p>　　結(jié)果，所有大規(guī)模生產(chǎn)的電能都以三相交流電的形式生產(chǎn)和分配。今天固態(tài)轉(zhuǎn)換器的應(yīng)用使交流變直流成為可能。而且，直流發(fā)電機(jī)的操作特性一直重要，因?yàn)榇蟛糠值睦碚撃鼙粦?yīng)用到所有其它機(jī)器上。</p><p><b>　　2.勵(lì)磁繞組連接</b></p><p>　　對于一個(gè)