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Synchronizers.

direction indicated by the arrows, and the current induced in the secondary will flow downward through the left side a lamp, causing it to light up. Let the main switch of generator 2 be open and the switch connecting H' closed in the direction indicated, then current will pass from the bus bars through the primary coil of H' in the direction indicated by the arrows, and this will induce a current in the secondary of H' in the direction of the solid arrows, causing the right side a lamp to light up. With the current flowing in the two secondary coils in the direction of the solid arrows, there will be no current passing through the b b lamps. If, however, the connections of the primary of H' are reversed, the current in its secondary will flow in the direction of the dotted arrows and then the lamps b b will light up as well as lamps a a. Suppose that we now swing over the switch in the circuit of H' so as to connect with the generator 2 below the main switch, then if at a given instant the two generators are generating currents that flow in the same direction, the current of generator 2 will flow through H' in the same direction it would if it came from generator 1, as is clearly indicated by the arrows; hence, if the connection of the primary coil of H' is such that the b b lamps will remain dark when both transformers are traversed by current from generator 1, they will remain dark when the current comes from generator 2, providing the two machines are generating currents that are in phase. If the connections of H' are reversed, so that b b will light up when the current comes from 1, then they will light up if it comes from 2, providing the two machines are in phase. This is what will occur if the two generators are

Synchronizers.

running in synchronism. If generator 2 is running faster or slower than 1, the lamps b b will light up and go out periodically, as often as the current of 2 comes into phase with 1. If generator 2 is running slower than I the lamp a on the right will not be as bright as the one on the left. If 2 is running faster than I the right side a lamp will be brighter than the other. As the velocities of the two generators approach equality, the fluctuations in the b b lamps will come at longer intervals, and when both machines reach the same speed the fluctuations will stop and the lamps will remain dark or bright continuously, according to how the primary of H' is connected, providing the currents are in phase or nearly so. When this stage is reached, the main switch of 2 can be closed, and thus the generator is connected with the bus bars in parallel with 1. After the two generators are connected the switches of their transformers can be opened, or they can be turned so as to connect with the generators below the main switches.

Fig. 61 shows the synchronizing arrangement for two two-phase generators. As will be noticed, it is simply a duplicate of Fig. 73. The arrangement of lamps, however, is different, the a a lamps being omitted. As will be seen, these last-named lamps are not actually necessary, for if generator I is running and the switches of H' K' are open, the currents passing through the primaries of H and K will generate currents in the secondaries that will light up the lamps LL L'L', and if the switches of H'K' are now closed, the currents generated in the secondaries of these transformers will be either in phase or in opposition with the currents from the secondaries of H K, so that they will either help to light

Synchronizers.

up the lamps, and make them brighter by doubling the e. m. f. or they will extinguish the lamps by neutralizing the e. m. f.

Fig. 62 shows the synchronizing arrangements for two three-phase generators, and as the principle of action is the same as in the previous diagrams, an extended explanation will not be necessary. In this diagram it will be noticed that the primaries of the transformers can be connected with the generators below the main switches only. It is not necessary to provide a switch that will connect on either side of the main switch, because the upper connections are only required to ascertain the direction in which the transformer primaries are connected, and this has only to be found out when the switchboard is first tried, or when through some circuit changes doubt may arise as to the actual connection. In all such cases, however, if the upper connection with double throw switches are not provided, temporary connections can be made in any convenient manner.

The generators shown in Figs. 57 to 59 are separately excited only, but in almost every case alternators are made so that the field is magnetized in part by a current derived from an independent source and in part by a current derived from the armature, this current being rectified by means of a commutator. The object of this arrangement is to provide means whereby the magnetism of the field may increase as the current increases, so as to maintain the e. m. f. as nearly constant as possible. There are many ways in which the armature and independent currents utilized for magnetizing the field are adjusted, and also in the way in which the armature current is obtained. In some cases it is taken directly from

Three phase Switchboard.

the armature through commutators, and in others it is derived from transformers. The latter arrangement is commonly used if the voltage of the generator is high.

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Switchboard for three; three phase generators as used by
General Electric Co.

Fig. 63 shows three three-phase generators, provided with field coils f' energized by current derived from the armature, through commutators, and field coils f energized by current obtained from the exciter E. This is an arrangement used by the General Electric Company. It will be noticed that equalizing bus bars are used, and that these

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Synchroscopes.

connect in parallel the field coils f' which are energized by the armature currents. Each generator has an independent regulating rheostat R, which is connected so as to shunt the field coil. The synchronizing transformers are connected with each other through the bus bars B, the main line busses being at A, and the equalizing busses at C. In many cases rheostats are inserted in the circuit of the f field coils so that the current in these coils may be regulated independently. In alternating current generators the coil f acts the same as the shunt coil in continuous current machines, and the f' coil performs the office of the series, or compounding coil.

From the time when alternating currents came into use up to the present day, the lamp synchronizing devices have been used for connecting generators in parallel. While it answers the purpose, it is far from perfect, because the lamps do not indicate closely whether the currents are in phase or not. If the difference in phase is forty or fifty degrees, the lamps will indicate, but small angular differences cannot be detected. When generators are connected in parallel it is necessary that the currents be in phase as well as in synchronism to avoid the passing of strong cross currents between the machines. During the past two years the Westinghouse Company have been manufacturing a synchronizing instrument that indicates correctly the phase of the currents. This instrument, which is called a "Synchroscope," is shown in Fig. 64, and Figs. 65 and 66 show the manner in which it is connected in two and three phase circuits, respectively. The instrument will not indicate with certainty the relation of the currents if the difference in their frequencies is more than about ten per cent., and on that account it is used in

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