Ans. It is advisable to close the switch when the machines are approaching synchronism rather than when they are receding from it, that is to say, the instant the lamp becomes dark.
Fig. 2,845.—Method of synchronizing with two lamps; dark lamp method. The two synchronizing lamps are connected as shown, and each must be designed to supply its rated candle power at the normal voltage developed by the alternators. Now since the alternators are both running under normal field excitation the left hand terminals of each of them will alternately be positive and negative in polarity, while the right hand terminals are respectively negative and positive in polarity. If, however, the alternators be in phase with each other, the left hand terminals of both of them will be positive while the right hand terminals are negative, and when the left hand terminals of both machines are negative the right hand terminals will be positive. Hence, when the machines are in phase there will be no difference of pressure between the left hand terminals or between the right hand terminals of the two machines. Hence, if the synchronizing lamps be connected as shown, both will be dark. The instant there is a difference of phase, both lamps will glow attaining full candle power when the difference of phase has reached a maximum. As the alternators continue to come closer in step, the red glow will gradually fade away until the lamps become dark. Then the switch may be closed, thereby throwing the two machines in parallel. If the intervals between the successive lighting up of the lamps are of short duration it is advisable to wait until these become longer even though the other conditions are satisfied, because where the phases pass each other rapidly there is a greater possibility of not bringing them together at the proper instant. An interval of not less than five seconds should therefore be allowed between the successive lighting up of the lamps, before closing the switch.
Fig. 2,846.—Inductor type synchroscope. This type is especially applicable where pressure transformers are already installed for use with other meters. As it requires only about ten apparent watts it may be used on the same transformers with other meters. There are three stationary coils, N, M and C, and a moving system, comprising an iron armature, A, rigidly attached to a shaft suitably pivoted and mounted in bearings. A pointer is also attached to the shaft. The moving system is balanced and is not subjected to any restraining force, such as a spring or gravity control. The axes of the coils N and M are in the same vertical plane, but 90 degrees apart, while the axis of C is in a horizontal plane. The coils N and M are connected in "split phase" relation through an inductive resistance P and non-inductive resistance Q, and these two circuits are parallel across the bus bar terminals 3 and 4 of the synchroscope. Coil C is connected through a non-inductive resistance across the upper machine terminals 1 and 2 of the synchroscope. In operation, current in the coil C magnetizes the iron core carried by the shaft and the two projections, marked A and "iron armature." There is however, no tendency to rotate the shaft. If current be passed through one of the other coils, say M, a magnetic field will be produced parallel with its axis. This will act on the projections of the iron armature, causing it to turn so that the positive and negative projections assume their appropriate position in the field of the coil M. A reversal of the direction in both coils will obviously not affect the position of the armature, hence alternating current of the same frequency and phase in the coils C and M cause the same directional effect upon the armature as if direct current were passed through the coils. If current lagging 90 degrees behind that in the coils M and C be passed through the coil N, it will cause no rotative effect upon the armature, because the maximum value of the field which it produces will occur at the instant when the pole strength of the armature is zero. The two currents in the coils M and N produce a shifting magnetic field which rotates about the shaft as an axis. As all currents are assumed to be of the same frequency, the rate of rotation of this field is such that its direction corresponds with that of the armature projections at the instant when the poles induced in them by the current in the coil C are at maximum value, and the field shifts through 180 degrees in the same interval as is required for reversal of the poles. This is the essential feature of the instrument, namely, that the armature projections take a position in the rotating magnetic field which corresponds to the direction of the field at the instant when the projections are magnetized to their maximum strength by their current in the coil C. If the frequency of the currents in the coils which produce the shifting field be less than that in the coil which magnetized the armature, then the armature must turn in order that it may be parallel with the field when its poles are at maximum strength.
Ques. What are the objections to the one lamp method?
Ans. The filament of the lamp may break, and cause darkness, or the lamp may be dark with considerable voltage as it takes over 20 volts to cause a 100 volt lamp to glow.