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THREE AND FIVE WIRE SYSTEMS OF DISTRIBUTION.
The three-wire system, which was independently invented by Edison and Hopkinson, has for its object the saving of copper in distributing conductors. From the first introduction of electric lighting until about 1897, it was not considered practicable to use incandescent lamps designed for a pressure higher than 120 volts. This limited the potential at which parallel systems were operated, and demanded conductors of large size and weight, particularly when the current is transmitted any considerable distance, as already shown in the examples given. When it is attempted to supply incandescent lamps in series, difficulties immediately arise, due to the dangers of high potential, the interference between the lamps, and the imperfection of regulation, all of which were noted
in Chapter II. The principle of the three-wire arrangement may be understood by first considering two entirely distinct two-wire circuits, as represented in Fig. 41. If the lamps L and N happen to be placed in the manner shown, it is evident that they may be connected in series of two each, as illustrated in Fig. 42, in which case the intermediate wires J and K become superfluous and are omitted. But when one of the lamps is turned off or burned out, its companion will also go out ; hence a third wire, indicated in Fig. 43 by a line marked 0, is extended from the junction between the two dynamos C and D in order to avoid the difficulty. This allows any number of the lamps to be disconnected without putting out those which remain. The extra conductor is called the neutral wire, and is usually marked 0 or =t, the latter symbol representing the fact that it is positive with respect to one conductor and negative with respect to the other. The neutral wire carries no current if the system is exactly "balanced" (Fig. 43); but when the amounts of current on the two sides of the system are not the same, it supplies the difference, whatever it may be, as represented
Figs, 43 and 44. Three-Wire System.
by arrows and numbers in Figs. 44 and 45, each lamp being assumed to take one ampere.
It should be observed that the flow of current may be opposite in direction in different parts of the neutral wire (Fig. 44). Another peculiar condition in three-wire circuits is the fact that the potential at certain lamps may actually be higher than that of the dynamo on the same side of the system. This is demonstrated in the potential diagram (Fig. 46), corresponding to the arrangement
Figs. 45 and 46. Three-Wire System.
of lamps shown in Fig. 4o. Assuming the resistance of each of the three wires to be 1 ohm, the drop on the + conductor between R and M will be 4 volts with a current of 4 amperes. The drop between Ar and S on the neutral wire will be 3 volts, since 3 amperes flow back through it. With a potential of 117 volts at each dynamo, this gives 110 volts for the lamps between M and N. The drop on the — wire between P and T is 1 volt, hence the potential of the point P is 1 volt above that of T\ but as A is 3 volts higher in potential than S, it follows that the pressure between N and P is 2 volts greater than that delivered at S and T by the dynamo D. Therefore the lamps at MN receive 7 volts less pressure, and the lamp at N P is supplied with 2 volts more pressure than the potential difference at the respective generators. While this condition is possible, it is not likely to occur in practice, particularly in large systems, where the circuits are carefully balanced. In such cases, the difference in the total load on the two sides of the system is often as small as 2 or 3 per cent.
Advantages and Disadvantages of the Three-Wire System. — The sole merit of this arrangement is the fact that it saves copper, the amount of this saving being determined as follows : —
The circuit represented in Fig. 42 has two wires, while those in Fig. 41 employ four; hence the former requires one-half as much copper as the latter, assuming the size of the wires to be the same. Furthermore, the percentage of drop in Fig. 42 will only be onehalf as great as that in Fig. 41, the explanation of this fact being given in Figs. 47 and 48, which show the distribution of potential in the two cases. In the two-wire circuits (Figs. 41 and 47) there will be a drop of 4 volts on each wire, assuming 4 amperes of current and one ohm of resistance for each ; and the lamps will receive 100 volts with a pressure of 114 volts at the dynamos, the drop being iff, or 7 per cent.
The lamps on the three-wire circuits (Figs. 42 and 48) receive 110 volts with the same initial potential, i.e., 114 volts, the drop being only Tf7 or 3.5 per cent, which is one-half as much as in the previous case. It follows, therefore, if the wires in Fig. 42 have one-half the cross-section of those in Fig. 41, that the percentage of drop will be the same for both. Consequently Fig. 42 requires onehalf as many conductors of one-half the size, or only one-quarter as much copper, as Fig. 41 for the same drop. If now the neutral conductor in the three-wire system (Fig. 43) be made the same size as each of the outside wires, the weight of the copper will be 1 + I = J as much as in the two-wire circuits (Fig. 41) supplying the same number of lights at the same distance with equal drop. Since the neutral wire usually carries only a small current, it is often made (especially in feeders) one-half as large as either of the outside conductors, in which case the weight of copper becomes W that demanded by the two-wire system.
The great saving in copper, amounting ordinarily to g or 62.5 per cent, is considered of such paramount importance that the three-wire system is usually adopted in electric lighting for lowtension distribution wherever the distances are considerable. In the case of low-tension central stations this custom is very general, and even for large isolated plants the three-wire system is often selected. It is also employed in many instances for the secondary wiring in alternating-current distribution with transformers. The object in all cases is to save copper, which constitutes such a large item in the cost of nearly all electrical installations.
To offset this advantage, however, the three-wire system has the following disadvantages : —
1. It is usually necessary to operate at least two dynamos or other sources of current.
2. It is necessary to lay and to take care of three wires instead of two.
3. The switches, cut-outs, measuring instruments, etc., are also more complicated.
4. The saving of copper stated, assumes that the neutral wire carries no current.
But if all the lamps happen to be in use on one side of the system only, the copper should be the same as for a two-wire circuit.
Even though the system be kept balanced as carefully as possible, so that the current in the neutral conductor is only 10 per cent of the total, the saving of copper would be reduced from 62.5 to about 50 per cent for the same actual percentage of drop.
5. The variation in potential may be aggravated by the increase that sometimes takes place (Fig. 46), which is impossible on a two-wire circuit.
When all these objections are considered, it is somewhat doubtful if the reduction in the weight of copper makes up for them in some cases where the three-wire system is adopted. There is a strong tendency on the part of the purchaser, consulting engineer, and contractor to give too great weight to the matter of first cost, and too little heed to questions of convenience, labor involved, liability of accidents, and many other factors that make up running expense. The three-wire system is unquestionably more complicated and difficult to install or operate, and it should not be selected unless the saving that it secures is surely sufficient to pay for these disadvantages. For low-tension distribution to distances of a mile it has been considered necessary to employ it; but for isolated plants, where the length of wires is only a few hundred feet, its superiority is by no means certain, in spite of the very powerful argument which may be based upon the saving of copper.
The improvements in and applications of 220-volt incandescent lamps render the three-wire system considerably less important than formerly, since it enables a two-wire circuit to be operated at 220 volts, the copper required being only two-thirds as much as for the ordinary three-wire system. To be sure the latter can now be run at 440 volts, giving it the same relative advantage as before; in fact, such plants are now being installed in this country and abroad. Many five-wire systems are being changed to three-wire, with 220-volt lamps.
MODIFICATIONS OF THE THREE-WIRE SYSTEM.
Three-wire System with Double Dynamo. — Various arrangements have been used or proposed as substitutes for the ordinary plan of using two generators. One of these, outlined in Fig. 49,
employs a double dynamo D, having two armature windings upon the same core, connected to two separate commutators CC. This double generator is used in the same manner as the two generators in Fig. 43, and may save floor-space as well as the trouble of running two machines, but no great advantage is secured.
Bridge Arrangement of Three-wire System. — Probably the first method of operating a three-wire circuit by means of a single generator was the bridge connection devised by Edison.* The plan
Three-wire System with Double
• U. S. Patent No. 343,017, June I, 1886.