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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.
The Derry Collard Co.
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
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
connection with synchronizing lamps, as shown in the diagrams; the lamps determine roughly the condition of the currents, the instrument being used to make the final adjustment. As shown in Fig. 63 the instrument has two pointers, the vertical one being stationary. The other pointer swings to one side or the other, according to the relative frequencies of the two currents. If the frequency
of the incoming machine is greater than that of the bus bars, the movable pointer will rotate to the right, and if it is slower the pointer will rotate to the left. When the two currents are at the same frequency, the movable pointer will stop at some point on the scale dependent upon the phase relation between the currents. When the two currents are in phase, the pointer will stand in the vertical position directly over the stationary pointer.
The ground detectors used with alternating currents are electro-static instruments, except occasionally in cases where low voltages are used. The ground detector made by the Westinghouse Company is shown in Fig. 67, and the way in which the instruments are connected in a threephase circuit is illustrated in Fig. 68. The tubes placed
around the three-line wires are condensers made in cylindrical form. They consist of two metallic tubes separated by a tube of insulating material. The inner tube is connected with the line conductor, and the outer tube with the instrument. It will thus be seen that there
is no direct electric connection between the circuit and the ground.
Some of the manufacturers of alternating current machinery make switchboard panels which are provided with the instruments, switches, etc., required to control the particular portion of the system they are designed for, all properly connected with each other. In building a switchboard to take care of an entire plant, these panels are placed side by side and the corresponding parts of the several panels are connected by means of bus bars. A panel of this kind, made by the General Electric Company to control one single phase generator, is shown in Figs. 69 and 70, the first showing the front and the other the back of the structure. Another generator panel, made