直流電機(jī)控制系統(tǒng)英文翻譯-展示頁
2024-11-12 23:20本頁面
【正文】 ). Because flux increase with load, speed must drop in order to maintain the balance between impressed voltage and counter . Moreover, the increased in armature current caused by increased torque is varyingspeed motor with a markedly drooping speedload characteristic. For applications requiring heavy torque overloads, this characteristic is particularly advantageous because the corresponding power overloads are held to more reasonable values by the associated speed drops. Very favorable starting characteristics also result from the increase flux with increased armature current. In the pound motor the series field may be connected either cumulatively, so that its . adds to that of the shunt field, or differentially, so that it opposes. The differential connection is very rarely used. A cumulatively pounded motor has speedload characteristic intermediate between those of a shunt and a series motor, the drop of speed with load depending on the relative number of ampereturns in the shunt and series fields. It does not have disadvantage of very high lightload speed associated with a series motor, but it retains to a considerable degree the advantages of series excitation. The application advantages of . machines lie in the variety of performance characteristics offered by the possibilities of shunt, series and pound excitation. Some of these characteristics have been touched upon briefly in this article. Still greater possibilities exist if additional sets of brushes are added so that other voltages 9 can be obtained from the mutator. Thus the versatility of . machine system and their adaptability to control, both manual and automatic, are their outstanding features. A machines is made up of two basic ponents: － The stator which is the stationary part of the machine. It consists of the following elements: a yoke inside a frame。 m=number of parallel paths through winding. And 2 aa PCK m?? （ 13） is a constant fixed by the design of the winding. The rectified voltage generated in the armature has already been discussed before for an elementary singlecoil armature. The effect of distributing the winding in several slots is shown in figure. In which each of the rectified sine wave is the voltage generated in one of the coils, mutation taking place at the moment when the coil sides are in the neutral zone. The generated voltage as observed from the brushes and is the sum of the rectified voltages of all the coils in series between brushes and is shown by the rippling line labeled ae in figure. With a dozen or so mutator segments per pole, the ripple bees very small and the average generated voltage observed from the brushes equals the sum of the average values of the rectified coil 5 voltages. The rectified voltage ae between brushes, Known also as the speed voltage, is 2 aa d m a d mPCeKm ? ? ? ???? （ 14） where aK is the design constant. The rectified voltage of a distributed winding has the same average value as that of a concentrated coil. The difference is that the ripple is greatly reduced. From the above equations, with all variable expressed in SI units, a a mei T?? （ 15） This equation simply says that the instantaneous power associated with the speed voltage equals the instantaneous mechanical power with the magic torque. The direction of power flow being determined by whether the machine is acting as a motor or generator. The directaxis airgap flux is produced by the bined . ffNi? of the field windings. The . Characteristic being the magization curve for the particular iron geometry of the machine. In the magization curve, it is assumed that the armature –. Wave is perpendicular to the field axis. It will be necessary to reexamine this assumption later in this chapter, where the effects of saturation are investigated more thoroughly. Because the armature . is proportional to flux times speed, it is usually more convenient to express the magization curve in terms of the armature . 0ae at a constant speed 0m?. The voltage ae for a given flux at any other speed m? is proportional to the speed, . 00maamee??? （ 16） There is the magization curve with only one field winding excited. This curve can easily be obtained by test methods, no knowledge of any design details being required. Over a fairly wide range of excitation the reluctance of the iron is negligible 6 pared with that of the air gap. In this region the flux is linearly pro
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