COUPLING OF DYNAMOS. When it is needful to generate a large and variable amount of electrical energy, as is the case in large installa tions and central generating stations, apart from the question of liability to breakdown, it is neither economical or desir able that the whole of the energy should be furnished from a single dynamo. Since the efficiency of a dynamo is dependent upon its output at any moment, or the load at which it is worked, the efficiency varying from 95 per cent. at full load to 80 per cent. at half load, it is obviously advisable in order to secure the greatest economy in working, to operate any dynamo as far as possible at full load. Under the above circumstances, when the whole of the output is generated by a single dynamo this can evidently not be effected, for the load will naturally fluctuate up and down during the working hours, as the lamps, motors, etc., are switched into and out of circuit; and hence, although the dynamo may be working at full load during a certain portion of the day, at other times it may probably be working below half load, and therefore the efficiency and economy in working in such an arrangement is very low. In order to secure a maximum efficiency, it is usual in such cases to divide up the generating plant inte a number of units, varying in size, so that as the load fluctu COUPLING OF DYNAMOS. ates it can either be shifted from one dynamo to another as the exigencies of the case requires; or when the load exceeds the capacity of the largest dynamo in the plant, the output of one can be added to that of another, and thus the dynamos actually at work at any moment can be operated as nearly as possible at full load. As it is necessary to take certain precautions in connecting one dynamo to another, in order that the other dynamos may not be affected by the change, and that they may work satisfactorily together, it is well to consider these in connection with the different types of machines. Series and Parallel Connections. Since the output of a dynamo is made up of two factors, viz.: the pressure and the current respectively, it follows that the output of a machine may be increased by increasing either the one or the other, or both at the same time. As, however, the systems of distribution in use at the present time involve the maintenance of either a constant current or a constant pressure in a circuit, the methods of coupling dynamos together resolve them. selves into two kinds, corresponding to the systems of distribution, viz.: parallel and series connections. In coupling two or more machines in parallel, the pressures of all the machines are kept at a constant value, while the output of the plant is increased in proportion to the current capacities of the machines in circuit. In the series coupling, the current capacity of the plant is kept at a constant value, while the output is increased in proportion to the pressures of the machines in circuit, COUPLING OF DYNAMOS. 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 COUPLING OF DYNAMOS. 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 F1 F2, and switch S, to the two main conductors or omnibus bars as represented in Fig. 99. The machine will operate when the connections are arranged in this manner, if the ends of the shunt coils are connected to the terminals of the respective machines; but a better plan is to put both the coils in series with one another, so that they form one long shunt between the two main conductors, as shown in Fig. 99. 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. Shunt Dynamos in Parallel.-The coupling of two or more shunt dynamos to run in parallel is effected without any difficulty, and is probably an operation more frequently performed than any other, it being daily practiced in central generating stations on the low tension system. Fig. 100 illustrates diagramatically the method of arranging the connec tions. The positive and negative terminals of each machine are connected respectively to two massive insulated copper bars, shown at the top of the diagram, and called omnibus bars, through the double pole switches S1 S., and the double pole fuses F1 F2. 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 1 |