COUPLING OF DYNAMOS. Series Dynamos in Parallel.—Simple series wound dynamos not being well adapted for the purpose of maintaining a SERIES COILS SERIES COILS constant pressure, are seldom in practice coupled in parallel; the conditions of working, however, derive importance from R2 FIG. 102. Ri COUPLING OF DYNAMOS. the fact that compound dynamos, being provided with series coils, are subject to similar conditions when working in parallel, which is frequently the case. In coupling two or more plain series dynamos in parallel, the same procedure cannot be followed as in the case of plain shunt dynamos, for the reason that if the voltage of the new dynamo is exactly equal to that of the bus bars when connected in parallel, the combination will be unstable. If from any cause the pressure at the terminals of one of the dynamos falls below that of the others, it immediately takes a smaller proportion of the load; as a consequence, the current in its field coils is at once reduced, and a further fall of pressure immediately takes place owing to this circumstance. This increased fall of pressure again causes the dynamo to relinquish a portion of its load, and again occurs a further fall of pressure. Thus the process goes on, until finally the dynamo ceases to supply current, and the current from the other dynamos flowing in its field coils in the reverse direction reverses its magnetism, and causes it to run as a motor against the driving power in the opposite direction to that in which it previously ran as a dynamo. Under such circumstances the armature is liable to be destroyed if the fuse is not immediately blown, and in any case is subjected to a very detrimental shock. This tendency to reversal in series dynamos can be effectually prevented by the simple expedient of connecting the field coils of all the dynamos in parallel, as was first suggested by Gramme. This is effected in practice by connecting the ends of all the S series coils where they join on to the armature circuit by a third connection, called the "equalizing connection," or equalizer," as shown in Fig. 103. The immediate effect of COUPLING OF DYNAMOS. this equalizing connection is to cause the whole of the current generated by the plant to be divided among the series coils of the several dynamos in the inverse ratio of their resistance, without any regard as to whether this current comes from one armature, or is divided among the whole, The fields of the several dynamos being thus maintained constant, or at any rate caused to vary equally, the tendency for the pressure of one dynamo to fall below that of the others is much diminished, reversal of polarity is entirely obviated, and the machines will run together under all conditions of load. Coupling Compound Dynamos in Series —Since compound dynamos may be regarded as a combination of the shunt and series wound machines, and as no special difficulties are experienced in running these latter in series, analogy at once leads to the conclusion that compound dynamos under similar circumstances may be coupled together with equal facility. This is found to be the case in practice, it being only necessary, in order that two compound dynamos may run satisfactorily together in series, to connect the series coils of each together, as represented in Fig. 102; the shunt windings must be connected as a single shunt, as in Fig. 99, which may either extend simply across the outer brushes of the machines, so as to form a double short shunt, or may be a shunt to the bus bars or external circuit, so as to form a double long shunt. Compound Dynamos in Parallel.--Compound dynamos will not run satisfactorily together in parallel, unless all their COUPLING OF DYNAMOS. series coils are connected together by an equalizing connec tion, as in series dynamos. The method of arranging the connections as adopted in practice, being illustrated in Fig. 104. By means of it idle machines are completely discon nected from those at work. The reference letters indicate as follows-S1 S, are switches; F1 F2 fuses; A1 A, are ammeters, which indicate the total amount of current generated by each of the machines; AC, AC, are automatic switches, ar |