Shunt Dynamos in Series.--The simplest operation in connection with the coupling of dynamos, and the one used probably more frequently in practice than any other, is the coupling of two or more shunt dynamos to run either in series or in parallel. When connected in series, the positive terminal of one machine is joined to the negative of the other, and the two outer terminals are connected through the ammeter A, fuses F, F', and switch S, to the two main conductors or omnibus bars as represented in [fig. 687]. The machine will operate when the connections are arranged in this manner, if the ends of the shunt coils be connected to the terminals of their respective machines.
Shunt Dynamos in Parallel.--The coupling of two or more shunt dynamos to run in parallel is effected without any difficulty. This method of coupling dynamos is one that is very frequently used. [Fig. 688] illustrates diagrammatically the method of arranging the connections. The positive and negative terminals of each machine are connected respectively to two massive insulated copper bars, shown at the top of the diagram, called omnibus bars, through the double pole switches, S, S', and the double pole fuses F, F'. Ammeters, A, A' are inserted in the main circuit of each machine, and serve to indicate the amount of current generated by each. An automatic switch or cutout, Ac, Ac', is also shown as being included in the main circuit of each of the machines, although this appliance is sometimes dispensed with. The pressure of each of the machines is regulated independently by means of the hand regulators R, R', inserted in series with the shunt circuit.
The shunt circuits are represented as being connected to the positive and negative terminals of the respective machines, but in many cases where the load is subjected to sudden variations, and when a large number of machines is connected to the bus bars, the shunt coils are frequently connected direct to these. In such circumstances this method is preferable, as by means of it the fields of the idle dynamos can be excited almost at once direct from the bus bars by the current from the working dynamos; hence, if a heavy load come on suddenly, no time need be lost in building up a new machine previous to switching it into parallel. The pressure of the lamp circuit is given by a voltmeter whose terminals are placed across the bus bars; and the pressure at the terminals of each of the machines is indicated by separate voltmeters or pilot lamps, the terminals of which are connected to those of the respective machines.
Fig. 687.--Diagram showing method of coupling shunt dynamos in series. The ends of the shunt coils may be connected to the terminals of their respective machine, or they may be connected in series as shown.
Ques. Describe a better method of parallel connection.
Ans. Better results are obtained by connecting both the shunt coils in series with one another, so that they form one long shunt between the two main conductors, the same as in [fig. 687].
When arranged in this way, the regulation of both machines may be effected simultaneously by inserting a hand regulator (R) in series with the shunt circuit as represented.
Fig. 688.--Diagram showing method of coupling shunt dynamos in parallel.