beyond the booster B, and therefore have the increased difference of potential, which they subdivide in two equal parts for the two sides of the system. The three machines B, M, and N are mechanically connected together by direct coupling or belting.

The field magnets in Figs. 55, 56, and 57 may all be excited by simple shunt winding connected to the brushes of each machine. respectively. It is preferable, however, to feed all the field coils from the main current supplied by the generator D, since that makes each machine less likely to aggravate variations that occur in its own portion of the circuit. The shunt coils of the compensators MN in Figs. 56 and 57 may be connected in parallel or in series to the outside wires + and -. The field coils of the booster

M

B

R

N

S

Fig. 57, Three-Wire System with Compensator and Booster.

B in Fig. 57 may also be fed from the main conductors + and in which case its voltage would be regulated by hand, using a variable resistance in the field circuit. If, however, it were provided with a series winding through which the main current of the + conductor passed, the extra pressure produced by it would automatically increase with the current as described in reference to feeder regulation in Fig. 39.

If the strength of the fields in the machines M and N (Figs 56 and 57) are capable of being independently regulated, the voltages on either side of the system may be varied to make up for differences in load, the pressure being made somewhat higher on the more heavily loaded side to counterbalance the greater drop on the conductors. This regulation can be made automatic by the arrangement represented in Fig. 58. The main generator D is assumed to be sufficiently over-compound wound to make up for the total drop on the conductors. Additional control of the vol

tage may be obtained by adjusting the variable resistance R in the shunt field circuit. The compensating machines M and N, mechanically coupled as before, are also compound wound. The effect of the series field-coils P is to cause the machine on the heavily loaded side to produce a higher voltage, and the one on the more lightly loaded side a lower voltage, whereby the difference in drop is equalized. Assuming three lamps, each taking one ampere on the side and one lamp on the side, the dynamo D will supply two amperes, as indicated by arrows and figures, the machine M will generate one ampere additional, being driven by the machine N acting as motor, and using one ampere. The current in the series field-coils P increases the E.M.F. of M, and

decreases the counter E.M.F. of N, since it flows in a reverse direction in the latter; hence the difference in drop on the wires between the compensator MN and the lamps may be equalized provided the compound winding is properly proportioned.

Conversion from Three- to Two-wire System. A three-wire system of conductors can readily be connected so that it may be used as an ordinary two-wire circuit. For this purpose the two outside wires are connected to one terminal, and the middle wire to the other terminal of the generator, as represented in Fig. 59. The lamps are fed between the middle conductor and either of the others, but the direction of current is reversed in all of those that are on one side of the system. This makes no difference in the operation of incandescent lamps, but would require the connection of arc lamps to be reversed, the same being true of storage

batteries, electroplating cells, or other electro-chemical apparatus that may be on that side of the system. Some forms of meters,

and other measuring instruments, would also be reversed in action. Motors running on either side of the circuit would not be affected, since the direction of rotation is not changed by reversing the current in both armature and field coils. But a motor or other device connected across the outside wires, which is the usual arrangement for the former, would receive no current, because these wires are of practically the same potential when used in this way.

The drop on the conductors is greatly increased by conversion to the two-wire arrangement. In a perfectly balanced three-wire system there is practically no current or drop on the middle wire; but when used as a two-wire circuit, the current and drop on this conductor is twice that in either of the others, consequently the total drop is three times as great as before. This assumes that the middle wire is the same in size as either of the others.

+

+

Fig. 59.

Conversion from Three-Wire to
Two-Wire System.

If it is only one-half as large, then the drop in it would be four times as great as in one of the outer conductors, and the total drop five times as much as in a balanced three-wire system. There is also danger of blowing the fuses on the middle wire, or overheating it, unless it is specially designed to be used in this way.

There are two cases in which the conversion from the threeto the two-wire system is commonly practiced. First, a small station, or isolated plant, which is operated on the three-wire plan when heavily loaded, and on the two-wire plan for light loads, one dynamo being sufficient in the latter case, and the drop on the conductor then being quite small. Second, an isolated plant, which is supplied by its own generator most of the time, but is connected to the three-wire "street circuit" (i.e., central station conductors) during certain portions of the day or night, or in case its machinery is disabled. On account of the latter contingency the name "breakdown switch" is applied to the device which connects the wiring inside the building with the outside conductors. This switch may be so made that it simultaneously opens the connections with the local generator.

OPERATION OF THREE-WIRE SYSTEMS.

The general arrangement of three-wire feeders and mains may be made substantially the same as already described for the twowire system, the same methods and care being used in regulating the voltage. The feeders may consist of three conductors of the same size; but usually the neutral feeder is made or as large as either of the others, and if storage batteries or equalizing machines are placed at the outer ends of the feeders, the neutral conductor may be omitted, as previously explained (Figs. 51 and 56). For the mains and leads the three wires are generally made the same in size. The important point in connection with the threewire system is the necessity for carefully balancing it; that is, keeping the currents on the two sides approximately equal. To accomplish this the lamps and other devices requiring current are divided between the two sides of the system, so that the loads shall be as nearly as possible the same for full capacity or any fraction of it. For this reason all three wires should be carried to any point where energy is required, unless the amount is extremely small. This applies to every building, even though it contains only a few lamps, and in fact to almost every room that is to be supplied with current. In this way the chance of having any considerable difference in load is reduced to a minimum.

Nevertheless, it is possible that a great many lamps might happen to be lighted on one side of the system and very few on the other side, in which case the drop in voltage would have about twice its normal value for the larger number of lamps, while the pressure might be raised for the smaller number, as already explained (Fig. 46). The likelihood of this happening is small, however, particularly in large systems, provided the lamps are carefully divided in wiring them. In case many lamps are to be lighted at the same time, they should be controlled by three-pole switches, which connect them to the two sides equally, or they should be divided into groups that are thrown on the sides alternately.

It is not sufficient in a three-wire system to have equal numbers of lamps on the two sides, they must also be distributed in approximately the same manner. For example, with a group of lamps requiring 100 amperes connected between the + and 0

wires at one point and an equal load between the 0 and wires some distance away, there would be a current of 100 amperes flowing on the 0 wire between those points. This involves considerable extra drop, although the system would appear at the generating station to be perfectly balanced. In practice this local unbalancing of the three-wire system is one of the chief causes of variations in voltage upon it, and should be made as small as possible by carefully distributing the load on both sides of the system. It is this fact which renders it desirable to carry all three wires to almost every place where current is required, even though a fair balance in the total load might be obtained by supplying buildings alternately from the two sides of the system.

Grounding the Neutral Conductor. A question that has aroused much discussion is the advisability of purposely grounding the neutral conductor of a three-wire system. The two principal arguments in favor of this plan are: First, it practically limits the potential between any point on the system and the earth to about 110 volts; second, it reduces the drop on the neutral conductor, since the current can also flow through the earth. In regard to the first of these reasons, it is a fact that the potential of the positive wire may rise to 220 volts if the negative wire becomes grounded, or vice versa when the neutral wire is insulated. But

it can hardly be said that trouble would be avoided if the neutral were grounded, as an accidental ground connection on either of the other sides would make a short-circuit and blow the fuse, thus putting out the lamps on that portion of the circuit. To be sure this locates the trouble, and calls for immediate attention, which may be a simple, but is also a crude way to keep the circuits clear of faults. If an accidental ground connection exists on one of the conductors when the neutral wire is not grounded, no trouble results until another ground occurs on one of the other two conductors. In the meantime an opportunity is afforded to correct the fault before any interruption of service or difficulty of any kind is experienced.

Unfortunately it is very troublesome to detect and locate a ground connection even on a two-wire circuit, and still more so with three wires. Nevertheless, there are methods which will accomplish this result; and if these were more generally used, they would be found to afford reasonably practical and convenient