Fig. 685.--Diagram showing method of coupling series dynamos in series. R and R' are two hand regulators which are placed in shunt across the coil terminals to regulate the pressure and output of the machine.
Ques. Why is this?
Ans. If, from any cause, the pressure at the terminals of one of the dynamos fall below that of the others, it immediately takes a smaller proportion of the load; as a consequence, the current in its field coils is reduced, and a further fall of pressure immediately takes place. This 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 be not immediately blown, and in any case it is subjected to a very detrimental shock. This tendency to reverse in series dynamos can be effectually prevented by connecting the field coils of all the dynamos in parallel.
Fig. 686.--Diagram showing method of coupling series dynamos in parallel. In the diagram A, A', are ammeters; F, F', fuses; S, S', switches.
Ques. How are the field coils of all the dynamos connected in parallel?
Ans. This is effected in practice by connecting the ends of all the series coils where they join on to the armature circuit by a third connection, called the "equalizing connection," or "equalizer," as shown in [fig. 686].
Ques. What is the effect of the equalizer?
Ans. The immediate effect 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 being caused to vary equally, the tendency for the pressure of one dynamo to fall below that of the others is diminished.