【正文】 (e)pound generator ,longshunt connection.The shunt field contains many turns of relatively fine wire and carries a paratively 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 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 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 ampereturns of the field coils: in 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 by a curve known as the magnetization curve or opencircuit 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 increased from zero to a value well above rated voltage of that machine. The resulting curve is shown is . 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 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 opencircuit characteristic of a separately excited dc machine The means that a larger increase in field current 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 quantities 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 various load currents is called the load or characteristic of the generator. shows the external characteristic of a separately excited generator. The decrease in the terminal voltage is due mainly to the 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 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 shows, the terminal voltage versus load current curve does not drop off linearly since the iron behaves nonlinear. Because armature reaction depends on the armature current it gives the curve its drooping characteristic.4. SHUNT OR SELFEXCIITED 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 . The question 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 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 selfexcitation, this small voltage will be applied to the shunts field and drive a small current through 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:(a)circuit。(c)series generator。譯自 College English Reading For Students Of Electric Reading附錄5DC GENENRATORS1. INTRODUCTION For all practical purposes, the directcurrent generator is only used for special applications and local dc power generation. This limitation is due to the mutator required to rectify the internal generated ac voltage, thereby making large scale dc power generators not feasible. Consequently, all electrical energy produced mercially is generated and distributed in the form of threephase ac power. The use of solid state converters nowadays makes conversion to dc economical. However, the operating characteristics of dc generators 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 fourpole machine is as shown in . The four brushes ride on the mutator. The positive brusher are connected to terminal 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, si