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is indicated in Fig. 50, and consists simply in connecting a resistance RR across the outside conductors + and –, the neutral wire 0 being brought to a point on the resistance through the movable switch-arm S. The objections to this method are, first, the continuous loss of energy that occurs through the Fig. 50. Bridge Arrangement of Three-Wire
System. resistance RR, and second, the fact that the arm S must be adjusted for any change in load, in order to equalize the pressures on the two sides of the circuit.
Three-wire System with Storage Battery. — This modification, illustrated in Fig. 51, requires only a single dynamo, D, generating the total pressure for both sides of the system, which is usually about 230 volts. A storage battery, AB, is connected between the two outside wires + and –, the neutral wire 0 being led to the middle point of the battery. In cases where it is advantageous to employ a battery to equalize the load on the engines, or for other reasons, this plan is a convenient one, since it only necessitates the running of one dynamo. The potential of the neutral wire 0 may be varied to make up for differences in load on the two sides of the system by shifting the point at which it is connected to the battery. If the difference of potential between the two outside conductors is greater than the E.M.F. of the battery, the latter will be charged, and vice verså, the same as in a two-wire system. With a three-wire circuit it is also possible for one part, A, of the
battery to be discharging while the other part, B, is charging. This may occur if there are a great many lamps on the + side, and very few on the – side. The function of the battery is to act as an equalizer, taking or giving current, as required, and keeping the potential of the neutral wire approximately half way between the potentials of the + and – conductors. The fall in voltage during discharge and the rise during charge, amounting to three-tenths of a volt or more, being nearly 15 per cent, and the difference between the charging and discharging pressures, make it necessary to employ extra cells and switching-devices, or means of regulation, such as are described in Volume I., page 397. A differential booster may also be used to automatically generate the extra voltage required to charge the battery.
The storage battery represented in Fig. 51 can be placed at a distance from the generator, and connected to it by two feeders, three wires being required only for the local distribution. This arrangement also enables the current on the feeders to be made more uniform, the battery being charged during periods of light load, and discharged when the demands for current are great. This permits feeders of smaller size to be used, and also reduces the variations in load on the generating plant, so that the latter operates more efficiently, and can be designed for less capacity than the maximum load. But extra expenses for attendance, rent, etc., are involved at the sub-stations.
Three-wire System with Three-brush Dynamo. — Fig. 52 indicates another three-wire arrangement that can be operated with only one generator, D, the neutral wire being connected to a third brush, F, placed half way between the main brushes E and G, to
which the outside wires + and
– are respectively attached. With most types of dynamo the brush F would spark excessively because it short-circuits the armature coils when they are generating the maximum E.M.F. There are sev
eral ways to avoid this difficulty, Fig. 53. Three-Wire System with One Dynamo. one of which consists in employ
ing a four-pole dynamo, shown in Fig. 53, having two adjacent north poles, N and N, the other two being south poles, S and S. The machine thus becomes in effect a bipolar dynamo with each pole divided, the armature coils shortcircuited by the brush F being in the space between the two halves S and S of the south pole, where they generate little or no E.M.F. A dynamo to be used in this way requires a field-magnet ring of sufficient cross-section between the N and S poles (i.e., at the top
and bottom in Fig. 53) to carry the total magnetic flux of one field core. In an ordinary multipolar machine with alternate N and S poles, this ring need have only one-half as much sectional area. Since practically no Aux passes through the two sides of the field ring they might be greatly reduced in size, but this is limited by considerations of strength and appearance. The radial depth of the armature core must also be sufficient for the total flux of one field core. The extra quantity and less favorable disposition of material in the generator is not a very serious matter, however ; and this plan of operating a three-wire system would often be a very practical and convenient one for small plants.
There is, however, with this arrangement, the difficulty that armature reaction tends to increase the flux in the lower S pole, and reduce it in the other, hence with heavy loads the voltage on the + side of the system would be less that on the – side. This can be counteracted by compound winding on the upper S pole, and differential winding on the lower S pole. Another brush could be placed between the two N poles, and connected in parallel with F; but since the neutral wire only carries a comparatively small current, one brush would ordinarily be sufficient, the main portion of the current being supplied through the brushes E and G.
Dobrowolsky Three-wire System. — Another method of operating a three-wire system by means of a single dynamo invented by
Fig. 54. Dobrowolsky system with Self-Induction Coil. von Dolivo-Dobrowolsky * is represented diagrammatically in Fig. 54. It consists of an ordinary direct current generator, the armature A and pole pieces N and S of which are shown. A self-induction coil, D, is connected to two diametrically opposite points of the winding of the armature A. The coil D may be carried by and
* V. S. Patent, No. 513,006, Jan. 14, 1894.
revolve with the armature; but in the construction represented it is stationary, being connected to the armature winding through the brushes CC, rings and wires JJ. The middle point of the selfinduction coil D is connected to the neutral conductor 0 of the three-wire system, the outside conductors + and – being supplied from the brushes BB in the usual manner. The E.M.F at the terminals of the coil D is alternating; hence the latter, on account of its self-induction, does not act as a short-circuit to the armature. Furthermore, the inductances of the two halves of the coil D being equal, the potential of the neutral wire 0 is kept midway between the potentials of the outside wires + and -. When the two sides of the system are unbalanced in load, the difference in current carried in one direction or the other by the neutral wire passes freely through the coil D, since the current is steady, or varies slowly, and is therefore unimpeded by the self-induction. It is evident that the ohmic resistance of D should be as low and its self-induction as high as possible, in order that the loss of energy and the difference in voltage on the two sides of the system shall be as small as possible under all conditions.
Three-wire System with Auxiliary Generator.— In the threewire system represented in Fig. 55 the neutral wire 0 is connected
to an auxiliary machine H which supplies a potential one-half as great as that of the main dynamo D. The machine H acts as a generator when the – side requires more current than the + side, but it runs as a motor when the current on the + side is greater. Hence it should be belted to or directly coupled with the dynamo D, in order to save its power when acting as a motor. The machine H, being intended to carry only the difference between the currents on the two sides of the system, may have only 5 or 10 per cent of the capacity of the dynamo D. This is sufficient as long as the sides are fairly well balanced, but is entirely inadequate if the difference becomes great, which may easily occur by accident. The ordinary three-wire arrangement, or that shown in Fig. 52, has the advantage of being able to operate, if necessary, with a full load on one side and none on the other, which might occur if there was an open circuit on one of the outside wires, due to the blowing of a fuse or to some other cause.
The same statements apply to the storage battery in Fig. 51, which may be designed to have a capacity equal to the full load or only a fraction of it.
Three-wire System with « Compensators" is represented in Fig. 56, in which the two auxiliary machines M and N are mechanically coupled together, and each generates one-half as much pressure as the main dynamo D. These machines are called compensators or equalizers, and serve to equalize the pressure and load, the one on the more lightly loaded side running as a motor, and driving the other as dynamo. Hence they are capable of operating with a difference in power on the two sides of the circuit equal to their combined capacity. When the system is perfectly balanced, both machines run as motors without load, and consume very little energy.
This combination involves three machines in place of the two dynamos required in the ordinary three-wire system ; nevertheless, it is very commonly and successfully used, being in many cases decidedly preferable. The two machines M and N are entirely self-acting, driving each other mechanically and maintaining equal voltages, with very little attention or likelihood of trouble. They are in most cases more easily operated than a second dynamo, and the friction as well as other losses are usually less.
. If both armature windings are upon the same core, armature reaction is neutralized, and the tendency to sparkling greatly reduced. A still more important advantage is the fact that the double machine MN can be placed at any desired distance from the generating plant and connected to it by two feeders, three wires being required only for the local distribution, as already stated in reference to Fig. 51. It is also possible to run a “booster” or small auxiliary dynamo by means of the compensating machines M and N, in order to raise the pressure of the circuit a certain amount, and make up for drop on the conductors. An arrangement of this kind, illustrated in Fig. 57, requires only one “booster," B, for both sides of the system. The compensating machines M and N are connected to the outside conductors at the points R and S