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electric circuits are interconnected, as explained with reference to Fig. 115. In this way a certain saving in the material of the iron core is effected; but they are more complicated in construction than ordinary transformers, and are seldom used in this country. A description of them may be found in Jackson's Alternating Currents, page 683. The arrangement and operation of transformers in connection with polyphase systems will be described in the next chapter.

For further information regarding the theory, construction, and operation of transformers reference may be made to the following works:

The Alternate Current Transformer, by J. A. Fleming, new edition, 2 vols. N. Y. and London, 1896.

Alternating Current Phenomena, by C. P. Steinmetz, N. Y., 1900.

Alternating Currents, by D. G. and J. P. Jackson, N. Y. and London, 1896.

The Principles of the Transformer, by F. Bedell, N. Y. and London, 1896.

CHAPTER X.

ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION.

The facility with which alternating currents may be transformed from one voltage to another gives possibilities of variation in systems of distribution that are greater than with direct currents. Adding to this the transformation from two- to three-phase, and from alternating to direct currents, or vice versa, by rectifiers and rotary converters, and the opportunity for elaboration becomes almost unlimited. There has been a tendency to yield to this temptation, and go too far in the complication of circuits and apparatus. Certain systems have become more or less standardized and generally accepted, but alternating current practice is still far less definite than direct current work. The more important methods will be classified and described in the present chapter.

Alternating Current Series Systems. — Series circuits corresponding to the direct current arrangements shown in Chapter II. may be operated by alternating currents. The principal systems that have been used are —

1. Simple series circuit with constant current alternator.

2. Series circuits supplied by constant current transformers.

3. Parallel-series circuits.

Several forms of constant current alternators have been introduced, analogous to the well-known Brush and Thomson-Houston series arc dynamos, the principal example being the Stanley machine made by the Westinghouse Company. No regulating device is required to keep the current constant; but armature reaction and self-induction are purposely exaggerated in the design, so that the current does not increase very much, even when the machine is short-circuited. The same is true to a certain extent of a constant direct current dynamo, but self-induction has a much greater effect with alternating currents. On the other hand, the voltage of a

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constant current alternator rises very high if the circuit is opened, since it is entirely relieved of armature reaction and inductance drop. This is likely to break down insulation unless it is prevented by providing a film cut-out similar to that shown on page 25, or some other device connected to the terminals of the machines, so that it will short-circuit the latter if the voltage becomes too great. Such machines have no advantage over constant direct current dynamos, except that the main current is generated without a commutator; but they require some source of direct current for field excitation. Furthermore, there are many examples of the direct current type that are very successful, hence they are generally adopted for arc-lighting on a simple series circuit, the direct current lamp being preferred when other considerations are equal. Constant current transformers have been illustrated in Figs.

[table]

Fig. 146. Constant Current Transformer System.

135, 130, and 137, and their operation described. They are not used on a true series system, since their primaries P are supplied in parallel at constant potential, as represented in Fig. 146, the secondary circuits 5 only being arranged in series fashion, and carrying constant currents which feed the lamps L. The advantage of this method is the fact that a large number of lights can be operated from the same source of current. For example, each of the circuits in E,F,G, Fig. 146, may have as many lamps as an entire dynamo in the direct current series system; so that one large alternator of 1000 k.w. capacity can supply about 2000 lights; whereas it would require 16 to 20 direct current machines, since the number of lamps that can be fed by a single dynamo is usually limited to 100 or 125. In simplicity and in economy of operation the single large alternator would have considerable advantage. The transformers T,T,T, may be of different sizes, if desired, being used to supply a larger or smaller number of lights. Lamps may also be cut out of circuit, as at J and K, the current being kept constant by the transformer in each case ; but the latter may be designed or adjusted to maintain in one circuit a different value from that in the others. The primary circuits Pare fed by the mains MN, with constant voltage from the alternator A ; hence the current in each primary is nearly proportional to the watts in the secondary. In other words, it increases as lamps are added in series. On the other hand, the current is constant in each secondary circuit, and the voltage automatically rises as the number of lamps is increased. Thus we have the interesting case of a constant potential primary and a constant current secondary circuit. This is made possible by the fact that the flux through the secondary coils varies with the load, whereas it is practically unchanged in the ordinary transformer. Hence the core loss is not a constant in the former, but the copper loss is always the same in the secondary coil, and increases in the primary circuit as the square of the load. The last fact is true of a constant potential transformer, but the first two do not apply to it.

Parallel-series systems are often operated by alternating currents, being analogous to the direct current circuits shown on page 26. Like the latter, they are used chiefly for street-lighting with series incandescent lamps. The general arrangement is similar to that represented in Fig. 9, one source of current being used to feed several circuits in parallel. Hence all are supplied with the same voltage, introducing difficulties when lamps burn out, or when it is desired to run different numbers of lamps on the various circuits. One plan consists in switching in extra or "relief" lamps L, as is done with the direct current system described on page 26. But the alternating current has an advantage over the latter in this respect, as it may be regulated by reactive coils, or auto-transformers (Frg. 145), which are more efficient and convenient than resistance coils or lamps. Several such methods have been used, in one of which variable reactive coils are placed in series with each circuit, and any reduction in the number of lamps is compensated by increasing the reactance drop in the coil. In another arrangement each lamp L is shunted with a reactive coil C, as illustrated in Fig. 147. These coils consume very little real power, but they have a certain potential difference across their terminals, thus feeding the lamps. When one of the latter burns out the continuity of the circuit is maintained, the current flowing through the coil, which also consumes about the same voltage as before. But as this last condition is only approximately fulfilled, it is necessary to have additional reactive coils R L L L in each circuit to regu- M| [L-gfcgj—ggg. late the current.

The so-called CR regulator, made by the General Electric Company, is another means of operating series incandescent lamps. It consists of an autotransformer, the connections of which are shown in Fig. 148. The primary coil AFB, and the coils BE and CD, are all wound upon the same iron core. When a plug is inserted in the contacts at R, and the switch arms H and G are in the position indicated, the voltage of the supply circuit from the switchboard

[graphic]

Fig. 147. Alternating Current Series System.

[graphic]

Secondary

Fig. 148. Regulator for Alternating Current Series Systems.

is decreased by the opposing E.M.F. produced in the portion of the coil CD included between the arms H and G. If the position of the latter is reversed, then the primary voltage is increased by the same amount. Thus the whole or part of the

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