【正文】 e addition of the secondary resistance and leakage reactance. Practically all transformers have a turn’s ratio different from unity although such an arrangement is sometimes employed for the purposes of electrically isolating one circuit from another operating at the same voltage. To explain the case where 21 NN ? the reaction of the secondary will be viewed from the primary winding. The reaction is experienced only in terms of the magizing force due to the secondary ampereturns. There is no way of detecting from the primary side whether 2I is large and 2N small or vice versa, it is the product of current and turns which causes the reaction. Consequently, a secondary winding can be replaced by any number of different equivalent windings and load circuits which will give rise to an identical reaction on the primary .It is clearly convenient to change the secondary winding to an equivalent winding having the same number of turns 1N as the primary. 4 With 2N changes to 1N , since the are proportional to turns, 2212 )/(39。39。 NINI ? must be equal to 22NI .. 2122 )/( INNI ? . For impedance, since any secondary voltage V bees VNN )/( 21 , and secondary current I bees INN )/( 12 , then any secondary impedance, including load impedance, must bee IVNNIV /)/(39。 221? . Consequently, 22212 )/(39。 XNNX ? . If the primary turns are taken as reference turns, the process is called referring to the primary side. There are a few checks which can be made to see if the procedure outlined is valid. For example, the copper 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。 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。39。2R , 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。1 RR ? , 2139。 similarly 139。 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。 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。 for convenience we shall continue to assume a sinusoidal fluxdensity wave in the air gap. The torque can then be found from the magic field viewpoint. The torque can be expressed in terms of the interaction of the directaxis airgap flux per pole d? and the spacefundamental ponent 1aF of the armature . wave . With the brushes in the quadrature axis, the angle between these fields is 90 electrical degrees, and its sine equals unity. For a P pole machine 12)2(2 adFPT ??? In which the minus sign has been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental 1aF of the saw tooth armature . wave is 8/ 2? times its peak. Substitution in above equation then gives adaada iKimPCT ??? ?? 2 Where ai =current in external armature circuit。 m=number of parallel paths through winding。 ., the machine may supply its own excitation. The method of excitation profoundly influences not only the steadystate characteristics, but also the dynamic behavior of the machine in control systems. The connection diagram of a separately excited generator is given. The required field current is a very small fraction of the rated armature current. A small amount of power in the field circuit may control a relatively large amount of power in the armature circuit。 and the electromagic torque T is a counter torque opposing rotation. The terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally because of the voltage drop in the armature resistance. The field current of a series generator is the same as the load current, so that the airgap flux and hence the voltage vary widely with load. As a consequence, series generators are not often used. The voltage of shunt generators drops off somewhat with load. Compound generators are normally connected so that the . of the series winding aids that of the shunt winding. The advantage is that through the action of the series winding the flux per pole can increase with load, resulting in a voltage output which is nearly constant. U