【正文】 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 . 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 steadystate 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 quarature 畢業(yè)設(shè)計（中英文翻譯） 6 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 independent of the spatial waveform of the flux distribution。2 RR ? and 1239。 . 239。 IENNINNEIEk V A ???? . The argument is sound, though at first it may have seemed suspect. In fact, if the actual secondary winding was removed physically from the core and replaced by the equivalent winding and load circuit designed to give the parameters 1N , 39。XI will be found to check as 39。 RNNR ? and 22212 )/(39。39。 2? , for example, by its demagizing action on m? has caused the changes on the primary side which led to the establishment of primary leakage flux. If a low enough leading power factor is considered, the total secondary flux and the mutual flux are increased causing the secondary terminal voltage to rise with load. p? is unchanged in magnitude from the no load condition since, neglecting resistance, it still has to provide a total back . equal to 1V . It is virtually the same as 11? , though now produced by the bined effect of primary and secondary ampereturns. The mutual flux must still change with load to give a change of 1E and permit more primary current to flow. 1E has increased this time but due to the vector bination with 1V there is still an increase of primary current. Two more points should be made about the figures. Firstly, a unity turns ratio has been assumed for convenience so that 39。 alternatively, 221011 NININI ?? . At full load, the current 0I is only about 5% of the fullload current and so 1I is nearly equal to 122 /NNI . Because in mind that 2121 / NNEE ? , the input kVA which is approximately 11IE is also approximately equal to the output kVA, 22IE . The physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary , 111 ??????? mp , is shown unchanged because the total back .,（ dtdNE /111 ?? ） is still equal and opposite to 1V . However, there has been a redistribution of flux and the mutual pone nt has fallen due to the increase of 1? with 1I . Although the change is small, the secondary demand could not be met without a mutual flux and . alteration to permit primary current to change. The flux s? linking the secondary winding has been further reduced by the establishment of secondary leakage flux due to 2I , and this opposes m? . Although m? and 2? are indicated separately , they bine to one resultant in the core which will be downwards at the instant shown. Thus the secondary terminal voltage is reduced to dtdNV S /22 ??? which can be considered in two ponents, . dtdNdtdNV m // 2222 ????? or vectorially 2222 IjXEV ?? . As for the primary, 2? is responsible for a substantially constant secondary leakage inductance 222222 / ??? NiN . It will be noticed that the primary leakage flux is responsible for part 畢業(yè)設(shè)計（中英文翻譯） 2 of the change in the secondary terminal voltage due to its effects on the mutual flux. The two leakage fluxes are closely related。39。 221? . Consequently, 22212 )/(39。 222 RI must be equal to 222RI . ）222122122 /()/( NNRNNI ?? does in fact reduce to 222RI . Similarly the stored magic energy in the leakage field )2/1( 2LI which is proportional to 2239。39。2I , measurements from the primary terminals would be unable to detect any difference in secondary ampereturns, kVA demand or copper loss, under normal power frequency operation. There is no point in choosing any basis other than equal turns on primary and referred secondary, but it is sometimes convenient to refer the primary to the secondary winding. In this case, if all the subscript 1’s are interchanged for the subscript 2’s, the necessary 畢業(yè)設(shè)計（中英文翻譯） 4 referring constants are easily found。 similarly 139。 capacitance effects