【正文】 ltage tV is aaat RIEV ?? Where aI the armature is current output and aR is the armature circuit resistance. In a generator, aE is large than tV 。 And mPCK aa ?2? 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 waves is the 9 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 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 voltages. The rectified voltage ae between brushes, known also as the speed voltage, is mdamdaa WKWmPCe ??? ?? 2 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: maa Twie ? This equation simply says that the instantaneous electric power associated with the speed voltage equals the instantaneous mechanical power associated 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 . ffiN? 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 10 times speed, it is usually more convenient to express the magization curve in terms of the armature . 0ae at a constant speed 0mw . The voltage ae for a given flux at any other speed mw is proportional to the speed,. 00 amma ewwe ? Figure shows 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 pared with that of the air gap. In this region the flux is linearly proportional to the total . of the field windings, the constant of proportionality being the directaxis airgap permeance. The outstanding advantages of DC machines arise from the wide variety of operating characteristics which can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external DC source, or they may be selfexcited。 aC =total number of conductors in armature winding。2V ,or (b) Viewed from the secondary as a source of constant voltage 1V with internal drops due to 1Re and 1Xe . The magizing branch is sometimes omitted in this representation and so the circuit reduces to a generator producing a constant voltage 1E (actually equal to 1V ) and having an internal impedance jXR? (actually equal to 11Re jXe? ). In either case, the parameters could be referred to the secondary winding and this may save calculation time. 7 The resistances and reactances can be obtained from two simple light load tests. Introduction to DC Machines DC machines are characterized by their versatility. By means of various bination of shunt, series, and separately excited field windings they can be designed to display a wide variety of voltampere or speedtorque characteristics for both dynamic and steady state operation. Because of the ease with which they can be controlled, systems of DC machines are often used in applications requiring a wide range of motor speeds or precise control of motor output. The essential features of a DC machine are shown schematically. The stator has salient poles and is excited by one or more field coils. The airgap flux distribution created by the field winding is symmetrical about the centerline of the field poles. This axis is called the field axis or direct axis. As we know, the AC voltage generated in each rotating armature coil is converted to DC in the external armature terminals by means of a rotating mutator and stationary brushes to which the armature leads are connected. The mutatorbrush bination forms a mechanical rectifier, resulting in a DC armature voltage as well as an armature . wave which is fixed in space. The brushes are located so that mutation occurs when the coil sides are in the neutral zone, midway between the field poles. The axis of the armature . wave then in 90 electrical degrees from the axis of the field poles, ., in the quadrature axis. In the schematic representation the brushes are shown in quadrature axis because this is the position of the coils to which they are connected. The armature . wave then is along the brush axis as shown.. (The geometrical position of the brushes in an actual machine is approximately 90 electrical degrees from their position in the schematic diagram because of the shape of the end connections to the mutator.) The magic torque and the speed voltage appearing at the brushes are 8 independent of the spatial waveform of the flux distribution。2V to 2V .All calculations of internal voltage and power losses are made before this ideal transformati