电子类文献中英文翻译发电机.docx

1、电子类文献中英文翻译发电机ENGLISH ORIGINAL TEXTDC GENENRATORS1. INTRODUCTION For all practical purposes, the direct-current generator is only used for special applications and local dc power generation. This limitation is due to the commutator required to rectify the internal generated ac voltage, thereby making

2、 largescale dc power generators not feasible. Consequently, 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 characteristics of dc generat

3、ors are still important, because most concepts can be applied to all other machines.2. FIELD WINDING CONNECTIONS The general arrangement of brushes 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

4、A1 while the negative brushes are connected to terminal A2 of the machine. 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.Figure 1 Sketc

5、h of four-pole dc matchine The four excitation or field poles are usually joined in series and their ends brought out to terminals marked F1 and F2. They are connected such that they produce north and south poles alternately. The type of dc generator is characterized by the manner in which the field

6、 excitation is provided. In general, the method employed to connect the field and armature windings falls into the following groups (see Fig.2):Figure2 Field connections for dc generators:(a)separately excited generator;(b)self-excited,shunt generator;(c)series generator;(d)compound generator;short-

7、shunt connection;(e)compound generator,long-shunt connection.The shunt field contains many turns of relatively fine wire and carries a comparatively small 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

8、 the armature and therefore carries the load current. 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 flux per pole and the speed

9、 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, as it involves a magnetic flux measurement. The flux is produced by the ampere-turns of the field coils: in

10、 turn, the flux must depend on the amount of field current flowing since the number of turns on the field winding is constant. This relationship is not linear because of magnetic saturation after the field current reaches a certain value. The variation of EG versus the field current If may be shown

11、by a curve known as the magnetization curve or open-circuit characteristic. For this a given generator is driven at a constant speed, is not delivering load current, and has its field winding separately excited. The value of EG appearing at the machine terminals is measured as If is progressively in

12、creased from zero to a value well above rated voltage 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 generated EMF increases linearly up

13、to the knee of the magnetization curve. Beyond this point, increasing the field current still further causes saturation of the magnetic structure to set in.Figure 3 Magnetization curve or open-circuit characteristic of a separately excited dc machine The means that a larger increase in field current

14、 is required to produce a given increase in voltage. Since the generated voltage EG 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 where the qu

15、antities values at the various speeds.3. VOLTAGE REGULATION Let us next consider adding a load on generator. The terminal voltage will then decrease (because the armature winding ha resistance) unless some provision is made to keep it constant. A curve that shows the value of terminal voltage for va

16、rious load currents is called the load or characteristic of the generator.Figure 4 (a) directs current it to urge the generator load characteristics; (b) circuit diagramFig.4 shows the external characteristic of a separately excited generator. The decrease in the terminal voltage is due mainly to th

17、e armature circuit resistance RA. In general, where Vt is the terminal voltage and IA is the armature current (or load current IL) supplied by the generator to the load. Another factor that contributes to the decrease in terminal voltage is the decrease in flux due to armature reaction. The armature

18、 current established an MMF that distorts the 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 iron behaves nonlinear. Because arm

19、ature reaction depends on the armature current it gives the curve its drooping characteristic.4. SHUNT OR SELF-EXCIITED GENRATORS A shunt generator has its shunt field winding connected in parallel with the armature so that the machine provides its own excitation, as indicated in Fig.5. The question

20、 arises whether the machine will generate a voltage and what determines the voltage. 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 have seen in Sect

21、ion 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-excitation, this small voltage will be applied to the shunts field and drive a small current thro

22、ugh the field circuit. If this resulting small current in the shunt field is of such a direction that it weakens the residual flux, the voltage remains near zero and the terminal voltage does not build up. In this situation the weak main pole flux opposes the residual flux.Figure 5 Shunt generator:(

23、a)circuit;(b)load characteristic If the connection is such that the 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, which in turn inc

24、reases 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 current to flow that is just sufficient to develop a flux required for the generated EMF that cau

25、ses the field current to flow. 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 field current depends on the generated voltage in accordance with Ohms law. It should be evident th

26、at on a new machine or one that has lost its residual flux because of a long idle period, some magnetism 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.Series Generators As

27、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 wire. At no load, the generated voltage is small due to residual field flux only. When a load is added, the f

28、lux increases, and so does the generated voltage. Fig.7 shows the load characteristic of a series generator driven at a certain speed. The dashed line indicates the generated EMF of the same machine with the armature open-circuited and the field separately excited. The difference between the two cur

29、ves is simply the IR drop in the series field and armature winding, such thatwhere RS is the series field winding resistance. Figure 7 Series generator: (a)circuit diagram;(b)load characteristicsCompound Generators The compound generator has both a shunt and a series field winding, the latter windin

30、g wound on top of the shunt winding. Fig.8 shows the circuit diagram. 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. The shunt connection illustrated in Fig.8 is called a long shunt connectio

31、n. 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 consequence, since the shunt field winding carries a small current compared to the full-load current. Furthermore, the numbe

32、r of turns on the series field winding. This implies it has a low resistance value and the corresponding voltage drop across it at full load is minimal. 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 increased 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 be flatcompounded. If the number of series turns