ALTERNATORS.]
DYNAMO
and thus converted into a direct current. Or both methods may be employed simultaneously. The reaction of the armature-current upon the field in an alternator is in many ways different from that of the continuousArmature current dynamo, and requires detailed consideration. reaction and agiving single-phase runningand on open a sinealternator curve ofbeE.M.F., let acircuit noninductive resistance be applied to its terminals. At the instant of closing the circuit, a current arises, and in its growth causes a corresponding self-induced flux to arise, which as it cuts the armature coils generates an E.M.F. of self-inductance; this E.M.F. delays the rise of the current, so that its instantaneous value is not in step with the instantaneous value of the impressed E.M.F., although they gradually become more and more alike in phase. The first few waves of current are therefore distorted, their exact shape depending upon the moment in the period when the circuit is closed, and it is only after some little time that the current curve reaches its final shape and phase relatively to the impressed E. M. F. Since the impressed E. M.F. is always changing in value and never becomes steady, the current-phase can never overtake that of the E.M.F., but must continue to lag behind it by some angle of lag depending on the armature inductance. If the magnetic circuit presented to the armature coils had a constant inductance and no hysteresis, the self-induced flux would vary simultaneously with, and in proportion to, the current; both would be sinusoidal, and so also would be the E.M.F. of self-inductance, which would be in quadrature with either the flux or the current. The phase of the current with reference to the impressed E. M. F. would then be given by a simple vector diagram ; since the armature current lags, the E.M.F. of self-inductance will be more than 90° behind the impressed E.M.F., and therefore will partially oppose it, so that the terminal E.M.F. will be less than the impressed E.M.F. minus the loss of volts over the armature resistance. The matter admits, however, of further analysis. The paths of the magnetic circuit presented to the armature coils vary according to the position of the coils relatively to the poles, and hence the reluctance to the self-induced flux is not constant, but varies. When the centre of a ring coil or the side of a drum coil is directly under a pole, all the ampere-turns of the coil act round a crosscircuit, or are cross ampere-turns, displacing the maximum density of the resultant field towards the trailing edges. As the coil moves forward, the self-induced flux divides, and some lines pass through a pair of field-magnet coils instead of across them. More strictly speaking, the ampere-turns are divisible now into cross and back ampere-turns, of which the latter directly reduce the strength of the symmetrical field, while the former simply distort the weakened symmetrical field. The proportion of back to cross turns gradually increases until the inductors of the coil are exactly mid-way between the poles ; later, as the coil passes under the next pole, and as soon as the current has reversed its direction, the armature turns become divisible into cross and forward ampere-turns. The latter increase the strength of the field, and their proportion relatively to the cross turns is continually increasing. Thus the effect of either the direct or the cross ampereturns during a whole period can only be calculated by taking into account both the instantaneous values of the current and the magnetizing effect which a constant current would have for the different positions of the coil at corresponding instants. E.g., if there were no difference of phase between current and impressed E.M.F., the dotted curve mm (Fig. 44) may be taken Fig. 44. to represent the direct magnetizing effect of the coil in various positions for some constant value of current, say one ampere; the current curve being that marked cc, the product of simultaneous ordinates of the two curves may be plotted as me, and will show the varying M.M.F. due to the direct ampere-turns, back ampere-turns being plotted below, and forward ampere-turns above, the base line. It is seen that in this case the periodic weakening and strengthening of the symmetrical field balances during a complete period, and the average flux is unaffected. Since, however, in our first case of an armature with self-inductance there must be some lag of current behind impressed E.M.F., Fig. 45 shows that the weakening effect lasts longer an d is greater than the strengthening effect, so that the terminal E.M.F. for the same exciting current must decrease. Similarly, the cross flux will be found periodically to vary, or, in other words, the lines will be periodi-
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I cally crowded up into one or other pole-edge, and then resume their I symmetrical distribution when either the current in the coil or its | cross effect is zero. If there be lag of the current, the displacement towards the leading pole-edge will be less marked than that towards the trailing edge. If the direct M.M.F. of the armature be taken into account in calculating the resultant flux, the smaller value of the self-induced E.M.F. which is then left simply measures the time-rate of change of the distortion, and has nothing to do with the strengthening and weakening of the field due to the lag or lead of the current. The importance of thus differentiating the armature reaction into the cross and direct magnetizing effect lies in the fact that, for a given terminal voltage under load, the actual density of lines in the magnet cores, for which allowance must be made in their sectional area, will be only that of the resultant flux due to the combined M.M.F. of the magnet winding and the direct ampereturns. Since, however, both divisions of the armature reaction produce the same class of effect, they may be combined by assigning to the armature a certain inductance, which causes an E.M.F. of self-induction in quadrature with the current. It must then be remembered that the value of the flux corresponding to the impressed E.M.F. is really that which would be due to the same excitation if there were no armature current, and never really exists except at no load. If the external circuit also has self-inductance, the lag of the current is increased and the terminal voltage is further lowered. If, on the other hand, the external circuit has sufficient condenser capacity, the current may be brought back into phase with the E.M.F., or may even be in advance of it; in this case, the above effects are reversed, and the field and terminal E.M.F. are increased for the same exciting current. Evidently, therefore, the characteristic curve connecting armature current and terminal volts will with a constant exciting current depend on the nature of the load, whether inductive or noninductive, and upon the amount of inductance already possessed by the armature itself. With an inductive load it will fall more rapidly from its initial maximum value, or, conversely, if the initial voltage is to be maintained under an increasing load, the exciting current will have to be increased more than if the load were noninductive. In practical working many disadvantages result from a rapid drop of the terminal E.M.F. under increasing load, so that between no load and full load the variation in terminal voltage with constant excitation should not exceed 15 per cent. Thus the output of an alternator is limited either by its heating or by its armature reaction, just as is the output of a continuous-current dynamo ; in the case of the alternator, however, the limit set by armature reaction is not due to any sparking at the brushes, but to the drop in terminal voltage as the current is increased, and the consequent difficulty in maintaining a constant potential on the external circuit. The joint operation of several alternators so that their outputs may be delivered into the same external circuit, is sharply distinguished from the corresponding problem in con- Tbe tinuous-current dynamos by the necessary condition cou ..,n that they must be in synchronism—i.e., not only P S must they be so driven that their frequency is the ° ffr* same, but their E.M.F.’s must be in phase or, as it is also expressed, the machines must be in step. Although in practice it is impossible to run two alternators in series unless they are rigidly coupled together—which virtually reduces them to one machine—two or more machines can be run in parallel, as was first described by Wilde in 1868, and subsequently redemonstrated by Hopkinson and Adams in 1884. Their E.M.F.’s should be as nearly as possible in synchronism, but as contrasted with series connexion, parallel coupling gives them a certain power of recovery if they fall out of step, or are not in exact synchronism when thrown into parallel. Under such circumstances a synchronizing current passes between the two machines, due to the difference in their instantaneous pressures ; and as this current agrees in phase more nearly with the leading than with the lagging machine, the former machine does work as a generator on the latter as a motor. Hence the lagging machine is accelerated and the leading machine is retarded, until their frequencies and phase are again the same. The chief use of the alternator has already been alluded to. Since it can be employed to produce very high pressures either directly or through the medium of transformers, it is specially adapted to the °f electrical transmission of energy over long dis- nators. tances.1 In the early days of electric lighting, the alternate-current system was adopted for a great number of central stations, the machines, designed to 1 In the pioneer three-phase transmission between Lauffen and Frankfort {Electrician, vol. xxvi. p. 637, and xxvii. p. 548), the three-phase current was transformed up from about 55 to 8500 volts, the distance S. III. - 75
