=

coil CD may be made to raise or lower the primary E.M.F. by moving the arms GH. Transferring the plug from R to S increases the primary E.M.F. by the voltage produced in the coil BE; and on changing the plug to P, the primary E.M.F. is diminished by that due to the coil FB. This secures a wide range of regulation; for example, the primary AB is wound for 2200 volts, FB and BE being each wound for 430, while CD produces 230 volts in steps of 23. With a plug at P, the secondary gives 2200 - 430 1770 volts, with the arms G and H in the middle; and by moving the latter the voltage may be varied from 1540 to 2000. Changing the plug connection to R, the regulation is from 1970 to 2430, and putting it in S, the range is from 2400 to 2860 volts, the total variation being 1540 to 2860. These regulators are also wound for 1100 volts, giving a range in secondary voltage from 440 to 1760. The secondary current is either 3.5 or 5.5 amperes, lamps designed for this current being connected in series, but only the two ends of the circuit are shown in Fig. 148. Several circuits, each with its own regulator, are connected to the same source of alternating current in a manner similar to that represented in Fig. 146. Each circuit is provided with an ammeter, and the attendant regulates the current by moving the switch arms. GH, when it is too high or too low. The lamps take about 1 volt per candle-power at 3.5 amperes, and are arranged with automatic cut-outs, which short-circuit them if they break, as explained on page 25.

Alternating Current Parallel Systems. The simple arrangement of lamps in parallel on a two-wire circuit may be supplied by an alternator without transformers, being analogous to the ordinary direct current system represented on page 28. This method, however, is rarely used for electric lighting alone, since the direct current has generally been adopted in such cases, including the majority of isolated or other plants in which the distances are not great. For the operation of motors, polyphase parallel systems are often used with or without transformation of voltage; and lamps are supplied from the same circuits or generators, but they are not intended primarily for electric lighting. The single-phase current is not well adapted to the running of motors for general purposes, this being the principal objection to it. It is only when the voltage is to be transformed up or down that the single-phase has any

special advantage over the direct current system, hence it is seldom used without transformers. But there is nothing to prevent the installation of two- or three-wire systems similar to those illustrated on pages 28 and 70, the direct current dynamos being replaced by alternators of equivalent voltage and current capacity. In fact, such plants have been installed in a few instances.

Single-phase parallel systems with transformers. This is the most common method of distribution with alternating currents. One alternator A (Fig. 149), or two or more alternators A and B working in parallel, supply current to the bus-bars UV, from which the lines MN and RS convey current to the primary circuits P of the various transformers T. The lamps L are connected in parallel to the secondary circuits S of the transformers. The latter operate at approximately constant potential in both primary and

Fig. 149. Constant Potential Transformer System.

secondary circuits, being of the ordinary type that has been fully discussed in the preceding chapter. In most cases the alternators generate about 1100 or 2200 volts, which is carried with a loss of about 10 per cent by the conductors MV, so that the primary coils of the transformers 7 receive about 1,000 or 2,000 volts, which is transformed down to about 100 volts for supplying the lamps L on the secondary circuits. Formerly a secondary voltage of 52 was generally employed, but at present 104 volts has become the standard in alternating current practice. This change reduces the weight of copper in the secondary wiring to one-quarter with the same percentage of drop. There is a tendency to economize still further in the secondary conductors by adopting lamps of 220 or 208 volts, or by using the three-wire system as described in the following paragraph.

Three-wire Alternating Current Systems. As already stated,

two- or three-wire parallel circuits may be supplied by single-phase generators without transformers, but they are seldom used. The two- and three-phase systems may also be operated with three wires, and will be described later. The system here referred to corresponds to the ordinary direct current three-wire circuits set forth on page 70, except that it is supplied from the secondary coils of transformers. When the alternating current was first introduced for electric lighting the secondary circuits and lamps were generally operated at 52 volts, a transformer being placed in or near each house to be lighted. But it was found that the lower efficiency and greater core-loss of a number of small transformers gave results far less economical than those obtained by the use of fewer transformers of larger size. This naturally requires that the average lengths of the secondary circuits should be increased; and

in order to avoid excessive cost in the latter, 104-volt lamps have become the standard in alternating current installations. The next step in this direction is the adoption of the three-wire system for the secondary circuits. This is easily arranged either by employing two transformers as represented in Fig. 150, or by using a transformer with two equal secondary coils as in Fig. 151. In both cases the primary is an ordinary two-wire circuit, all the primary coils being connected in parallel; but each pair of secondary coils are put in series, the neutral wire F being led from the intermediate point B. The unlike terminals must be connected at B in order to give double voltage between the outside wires A and C. If the like terminals are united at B the two sides will be in parallel, and the middle wire F must carry the sum of, instead of the difference between, the currents on the outer conductors, as explained on page 82.

A two- or three-wire network of conductors, similar to the direct current systems described in Chapter VI., is often adopted for alternating current distribution. A transformer T, or a bank of transformers, is placed at each feeding-point, the primary coils being supplied from the station generators A B by the high-voltage conductors E F G H, and the secondary coils being connected to the low-tension mains composing the network NM. These transformers are located in sub-stations or in manholes in the street. The lamps LL are fed from the network as indicated in Fig. 152. In this case a two-wire network is shown; but a three-wire system similar to that represented in Fig. 74 is also used in many places, the transformers being connected in the manner shown in Figs. 150 and 151.

Regulation of Constant-Potential Alternating Current Systems. Nearly all of the methods of regulating the voltage of direct cur

rent systems described on pages 51 to 69 are applicable to alternating current circuits. For example, the potential of an alternator may be controlled by varying its field current, using the ordinary rheostat operated by hand. In this way the voltage of the generator may be kept constant or may be increased a certain amount with rising load to make up for the drop in lines and transformers. This drop is greater for alternating than for direct currents on account of reactance, and the falling off in potential of alternators is also larger at full load than with dynamos.

Composite Wound Alternators. To make an alternator automatically maintain a constant, or a rising voltage with increase of load, it is provided with composite winding analogous to the compound winding of direct current machines.

In order that a generator may be self-regulating, the current which it produces is caused to act upon the field-magnets in order

to increase their strength in proportion to the current generated. Since an alternating current cannot be used directly for exciting the field-magnets it is necessary to rectify it for the purpose. One method is indicated in Fig. 153, the coils CC being the ordinary field winding supplied by the separate exciter E, and producing most of the magnetization. The composite coils DD are also wound upon the field-cores, and are fed through the rectifying commutator R, which is mounted upon the same shaft as the armature AA, but to avoid confusion is represented on one side in the diagram. The commutator R has as many segments as there are poles, alternate segments being connected to one terminal T of the armature winding, and the intermediate segments being connected to one of the lines M by the wire W, and brush

E

W

L

M

R

manner.

Fig. 153. Composite-wound Alternator.

on the collecting-ring at F. The other terminal S of the armature winding is connected to the second collecting-ring G. The collecting-rings FG are also mounted upon the shaft in the usual With this arrangement, the connections of the composite coils D are reversed at the brushes / and K each time that the armature current reverses, so that a unidirectional flow is established through these coils. This tends to augment the magnetization of the field as the load increases, the effect being the same as that of compound winding. It is necessary that the brushes and K should be set carefully, so that each passes from one segment to the next at the same instant that the current reverses. In this way sparking is avoided since the current is zero at that moment. A shunt shown inside of the commutator R in the diagram, and moving with it, is sometimes used when it is desired to rectify