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circuits still start badly, and start badly, and particularly require enormous currents at the moment of starting; hence the growth of various modified alternating systems, of which the polyphase are the best known, by the use of which it becomes possible to avoid this difficulty entirely, and to work alternating current motors in every respect as satisfactorily as those operated by direct current.
The polyphase motor has solved the problem of securing good motor service with alternating currents in a very complete manner, although at the cost of slight complications which, while generally overestimated, are, nevertheless, distasteful to central station men, especially where the use of such a system would involve an extensive and costly arrangement of the distribution service. It is the purpose of this paper to call your attention to the various modes of central station distribution involving motor service on the alternating current system, and more especially to a modified single-phase alternating system that lends itself very readily to a very simple and straightforward distribution for lighting, without sacrificing the excellent motor service which makes the true polyphase systems so desirable. For all-around central station work, the lighting service is of most fundamental importance, and convenience and economy in this particular must, in a vast majority of cases, be the first consideration. Let me invite your attention to this chart, which shows in a diagrammatic manner the general arrangement of various alternating circuits, including those already familiar and the polyphase and monocyclic systems.
First in order, both of time and extended use, comes the simple alternating system, single-phase, with an ordinary two-wire distributing circuit. In operating
the secondaries of such a system, we run immediately into limitations imposed by the feasible voltage for lamps, limitations which have acted as a brake on alternating current work, and which are responsible
ALT. 2 WIRE
for a good many difficulties in the operation of alternating plants. Opposite the diagram of this circuit, I have placed the relative amount of copper required for lighting with such a distribution.
Next comes the alternating circuit modified for Edison three-wire distribution system, which is so familiar in its character that it needs no description here. The amount of copper needed, on the suppo
ALT 3 WIRE
sition that neutral is half the size of either of the others, is, of course, enormously reduced; amounting, as you see, to less than one-third of that needed with the two-wire system; consequently the secondary main distribution, so important to economical operation, can be carried out with the greatest facility. We are justified, at present, in saying that neither of these systems lends itself to the ready operation of motors.
We may next consider the two-phase alternating system worked with four wires, giving, if desired, two separate circuits for each machine. The copper here required is the same as that used with the single-phase, and so long as the four wires are kept together motors can be freely operated. Here is assuredly a step in advance. The operation of two separate circuits, however, from a single machine, involves certain complications. In the first place, unless these circuits. are balanced as to load, the operation of the system is somewhat difficult to make satisfactory in point of regulation. The circuits themselves have absolutely no influence on each other, but they are derived from the
2PHASE 4 WIRE
same machine; and with a given exaltation of that machine, unless the load in the two circuits is the same, the voltages must necessarily be different, consequently, the generator cannot be compounded for both circuits unless the circuits are balanced. There are three ways out of this difficulty; none of them, unfortunately, are altogether unobjectionable.
In the first place, one can make the loss on the lines so small that the difference in voltage due to any reasonable difference in load is of no moment. This can sometimes be successfully done, but more often it involves a very unnecessary cost for copper and, even at its best, is likely to give regulation which, although
better than that found on most simple alternating systems, is yet not good enough to suit the requirements of the best modern engineering.
Secondly, the machine can be compounded for loss on one of the lines, a feeder regulator being used on the other. This practically means regulation of one circuit.
Or, finally, both circuits can be operated by such regulators. In any case, if motors are to be operated, the two phases must be interlinked by, at least, one wire.
A somewhat simpler two-phase distribution is that shown in the next diagram, where the two phases unite into a common three-wire circuit. The amount of
2PHASE 3 WIRE
copper here required differs very materially according to the limitations of voltage that one chooses to fix. If the maximum voltage of the system is limited to the same maximum voltage used on the single-phase, two-wire or two-phase, four-wire system, the amount of copper required is very largely increased by the use of the third wire, owing to the fact that the two circuits combined are out of phase with each other by ninety degrees. The relative amount of copper required here is, as you see, 145.5. If, on the other hand, the question of maximum voltage between wires is neglected, and the voltage is determined by the use of lamps of the same voltages as on the other systems,
then the amount of copper is materially reduced, being a trifle less than three-fourths of that required on the systems before mentioned. In this case, we neglect the fact of there being an excessive voltage between the outside wires, and connect lamps between the middle wire and the two outside ones, much in the manner of an Edison three-wire system, with the exception, of course, that the saving in copper is not anything like the same amount. Such a system allows the distribution of the whole load for lights and power in a single circuit containing only three wires. It, therefore, avoids the difficulty of regulating two separate circuits. On the other hand, it causes a new and very curious difficulty, owing to the fact that the two combined phases are unsymmetrical with regard to the inductance of the system. The voltages between the middle wire and the two outside wires are unequal, when the systems are equally loaded, by an amount depending on the inductive loss in the line. Consequently, when in balance for regulation, the system is unbalanced as regards the loss on the two sides; a condition which, although it may not lead to trouble in some cases, is still a still a menace to the successful operation of of the system, unless the distribution be skillfully engineered. There is, however, the possibility of a noticeable saving in copper.
We may next pass to the three-phase alternating system. The system which corresponds to the twophase, three-wire system, I pass by, as the use of three separate circuits would be even more troublesome than that of two, and would have no compensating advantages. The ordinary three-phase circuit is that shown in the figure, wherein the three phases are united into three wires and taken everywhere the motors and lights are wanted. In this case, for the