【正文】 loss in the referred secondary winding must be the same as in the original secondary otherwise the primary would have to supply a different loss power. 39。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。XI will be found to check as 39。39。 22 XI . The referred secondary 2212221222 )/()/(39。39。 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。2R , 39。2X and 5 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 referring constants are easily found。 . 239。1 RR ? , 2139。 XX ? 。 similarly 139。2 RR ? and 1239。 XX ? . The equivalent circuit for the general case where 21 NN ? except that mr has been added to allow for iron loss and an ideal lossless transformation has been included before the secondary terminals to return 39。2V to 2V .All calculations of internal voltage and power losses are made before this ideal transformation is applied. The behavior of a transformer as detected at both sets of terminals is the same as the behavior detected at the corresponding terminals of this circuit when the appropriate parameters are inserted. The slightly different representation showing the coils 1N and 2N side by side with a core in between is only used for convenience. On the transformer itself, the coils are, of course, wound round the same core. Very little error is introduced if the magizing branch is transferred to the primary terminals, but a few anomalies will arise. For example, the current shown flowing through the primary impedance is no longer the whole of the primary current. The error is quite small since 0I is usually such a small fraction of1I . Slightly different answers may be obtained to a particular problem depending on whether or not allowance is made for this error. With this simplified circuit, the primary and referred secondary impedances can be added to give: 6 221211 )/(Re NNRR ?? And 221211 )/( NNXXXe ?? It should be pointed out that the equivalent circuit as derived here is only valid for normal operation at power frequencies。 capacitance effects must be taken into account whenever the rate of change of voltage would give rise to appreciable capacitance currents, dtCdVIc /? . They are important at high voltages and at frequencies much beyond 100 cycles/sec. A further point is not the only possible equivalent circuit even for power frequencies .An alternative , treating the transformer as a threeor fourterminal work, gives rise to a representation which is just as accurate and has some advantages for the circuit engineer who treats all devices as circuit elements with certain transfer properties. The circuit on this basis would have a turns ratio having a phase shift as well as a magnitude change, and the impedances would not be the same as those of the windings. The circuit would not explain the phenomena within the device like the effects of saturation, so for an understanding of internal behavior. There are two ways of looking at the equivalent circuit: (a) viewed from the primary as a sink but the referred load impedance connected across 39。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 whic