Ques. What capacity of single lamp must be used?
Ans. It must be good for twice the voltage of either machine.
Fig. 2,847.—Brilliant lamp method of synchronizing. The synchronizing lamps are connected as shown, and must be of the alternator voltage. When the voltages are equal and the machines in phase, the difference of pressure between a and a given point is the same as that between a' and the same point; this obtains for b and b'. Accordingly, a lamp connected across a b' will burn with the same brilliancy as across a' b; the same holds for the other lamp. When the voltages are the same and the phase difference is 180° the lamps are dark, and as the phase difference is decreased, the lamps glow with increasing brightness until at synchronism they glow with maximum brilliancy. Hence the incoming alternator should be thrown in at the instant of maximum brilliancy.
Ques. What modification of the synchronizing methods shown in the accompanying illustrations is necessary when high pressure alternators are used?
Ans. Step down transformers must be used between the alternators and the lamps to obtain the proper working voltages for the lamps.
Fig. 2,848.—Synchronizing with high pressure alternators; dark and brilliant lamp methods. In both methods the primaries of the transformers are connected in the same way across the terminals of the alternators as shown. In the dark lamp method, the connections between the secondary coils of the transformers must be made so that when each is subjected to the same conditions the action of the one coil opposes that of the other as in the dark lamp method; then, if the transformers be both of the same design, there will be no voltage across the lamps when the alternators are in phase with each other. If the ratio of each transformer is such as to give, for example, 100 volts across its secondary terminals, then the two incandescent lamps since they are joined together in series must each be designed for 100 volts. One 200 volt lamp could be used in either method in place of the two 100 volt lamps. When, therefore, the alternators are directly opposite in phase to each other, both the lamps will burn brightly; as the alternators come together in phase the lamps will produce less and less light, until when the machines are exactly in phase no light will be emitted at all, at which instant the incoming alternator should be thrown in. It must be evident, if the transformer secondary connections are arranged as in the brilliant lamp method, so that they do not oppose each other, the lamps will be at maximum brilliancy when the alternators are in phase and dark when the phase difference is 180°, assuming of course equalized voltage.