Compound Dynamos in Parallel.--Machines of this type will not run satisfactorily together in parallel unless all the series coils are connected together by an equalizing connection, as in series dynamos. The method of arranging the connections as adopted in practice, being illustrated in [fig. 690]. By means of it idle machines are completely disconnected from those at work.
Ques. How is the equalizer connected?
Ans. The equalizer is connected direct to the positive brushes of all the dynamos, a three pole switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working. The two contacts of the switch are respectively connected to the positive and negative conductors, while the central contact is connected to the equalizer.
Fig. 690.--Diagram showing method of coupling compound dynamos in parallel.
Switching a Compound Dynamo Into and Out of Parallel.--If the characteristics of all the dynamos be similar, and the connections arranged as in figs. 690, or 691, the only precaution to be observed in switching a new machine into parallel is to have its voltage equal, or nearly equal to that of the bus bars previous to closing the switch. If this be the case, the new machine will take up its due share of the load without any shock.
Fig. 691.--Diagram showing another and better method of coupling compound dynamos in parallel. With this arrangement the idle machines are completely disconnected from those at work. The same reference letters are common in both diagrams. S, S' are switches; F, F' fuses; A, A' ammeters, which indicate the total amount of current generated by each of the machines; AC, AC', automatic switches, arranged for automatically switching out a machine in the event of the pressure at its terminals being reduced through any cause; R, R,' are hand regulators, inserted in the shunt circuits of each of the machines, by means of which the pressures of the individual machines may be varied and the load upon each adjusted. The pressure at the bus bars is given by the voltmeter V, one terminal of which is connected to each of the bars; a second voltmeter may be used, to give the pressure of any individual machine, by connecting "voltmeter keys" to the terminals of each of the machines, or a separate voltmeter may be used for each individual machine. The only essential difference between figs. 690 and 691 is, that in [fig. 690] the equalizer is connected direct to the positive brushes of all the dynamos, while in [fig. 691] the equalizer is brought up to the switchboard and arranged between the two bus bars, a switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working.
Ques. How is a compound dynamo, running in parallel, cut out of circuit?
Ans. The load is first reduced to a few amperes, as in the case of shunt dynamos, either by easing down the engine, or by cutting resistance into the shunt circuit by means of the hand regulator, and then opening the switch. Previous to this, however, it is advisable to increase the voltage at the bus bars to a slight extent, as while slowing down the engine the load upon the outgoing dynamo is transferred to the other dynamo armatures, and the current in their series coils not being increased in proportion, the voltage at the bus bars is consequently reduced somewhat.