【正文】 ould be disconnected, there will be small voltage Ef generated due to this remanent magism, 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。39。shortshunt connection。(c)series generator。 1 ENGLISH ORIGINAL TEXT① DC GENENRATORS 1. 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 largescale 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, since their sides are situated between poles. 2 Figure 1 Sketch of fourpole 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 excitation is provided. In general, the method employed to connect the field and armature windings falls into the following groups (see ): Figure2 Field connections for dc generators:(a)separately excited generator。(b)selfexcited,shunt generator。(d)pound generator。(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, 3 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 magic 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 magic 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 magization 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 magism. As the field current increases, the generated EMF increases linearly up to the knee of the magization curve. Beyond this point, increasing the field current still further causes saturation of the magic structure to set in. Figure 3 Magization 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 magization 4 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 nnxEE GG39。 ? 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 curre nts is called the load or characteristic of the generator. Figure 4 (a) directs current it to urge the generator load characteristics。(b)load characteristic 6 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 buildup process is readily seen to be cumulanve. That is, more voltage increases the field current, which 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