电气自动化英语文献翻译.doc

ENGLISH ORIGINAL TEXT DC GENENRATORS 1. 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 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 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 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 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 Sketch 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 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-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