Зображення сторінки
PDF
ePub

The other dynamo, C, runs at a higher voltage, and is connected to the auxiliary 'bus bar E for supplying the longer or more heavily loaded feeders represented by A. The scope for regulation is still further increased by varying the pressure at either or both of the 'bus bars. This may be accomplished by hand or automatically, rheostats in the shunt field circuits of the dynamos, compound winding, or other means for controlling the E.M.F. of dynamos, being employed for the purpose. In this connection it should be

[blocks in formation]

noted that the introduction of resistance in a shunt or separately excited field circuit involves far less loss of energy than when it is put in the main circuit, since the current in the former is only about one to three per cent of the latter. It is evident that any number of 'bus bars may be used, being supplied with current by dynamos running at different voltages; and two or more dynamos may be operated in parallel on any 'bus bar, in accordance with the demands for current. It is also obvious that several feeders may be connected to one 'bus bar. Each feeder is provided with switches that connect it to any particular 'bus bar according to the load upon it..

A Transfer 'Bus Bar is used to enable a feeder to be gradually shifted from one 'bus bar to another, without the sudden variation in potential which would occur if it were thrown over directly by means of the switches mentioned above. This arrangement is indicated in Fig. 38, being similar to that shown in Fig. 37, but having a transfer 'bus bar P in addition to the main and auxiliary bars F and E. The latter is connected to the resistance coils of a rheostat, the movable arm V of which is connected to the transfer bar P. The operation of shifting the feeder B from the main 'bus bar F to the auxiliary E, when its load becomes large, is as follows: The feeder B is connected to the transfer bar at P, the rheostat V being previously open circuited. The arm V is then moved clockwise until it comes in contact with the extremity of the resistance. This connects the feeder B with the auxiliary bar E through the whole of the resistance, and allows a certain current to flow from E into B, the former having a higher potential than F, to which B still remains connected. The resistance is then gradually cut out by the further movement of the arm V until the current supplied from the auxiliary E to the feeder B is equal to the load carried by the latter, when the connection between B and the main 'bus bar F is opened. The remainder of the resistance is then cut out, which directly connects the feeder B with the auxiliary bar E, the transfer having been made without any disturbance of the system. In performing this operation the

Hot

Fig. 38. Feeder Regulation with Transfer 'Bus Bar.

amounts of current may be ascertained by having an amperemeter inserted between the rheostat and E in addition to the amperemeter that should always be placed in circuit with each feeder. The difference of potential between the feeders, 'bus bars, etc., should also be indicated by voltmeters. The necessary instruments are assumed to be present in every case, but are omitted from the diagrams to avoid confusion, as they perform no active part in the operation of the system, their only function being to give information.

Where two or more feeders run to the same mains, one of them may be transferred from one 'bus bar to another by simply opening the switch connecting it with one bar and closing an instant later the switch that connects it to the other. Both switches must never be closed at the same time, as it would short-circuit the potential difference between the two 'bus bars. The drop on a feeder being usually only a few volts, its circuit may be opened

with practically no greater flash than is produced by any other circuit having a voltage equal to this drop, provided the remaining feeders and the mains are sufficient to carry the current without materially increasing the fall of potential upon them.

Feeder Regulation by Means of “ Boosters” is represented in Fig. 39, in which D is the main dynamo generating the greater part of the electrical energy, and R and S are two small auxiliary dynamos called “ boosters,” connected in series with the dynamo D and the two feeders A and B respectively. Assuming that the main dynamo D generates a constant voltage, the variation in pressure required to regulate the feeders in accordance with the changing loads upon them is obtained by controlling the potential of the boosters R and S. This is usually accomplished by exciting the field magnets of the boosters from the dynamo D, a rheostat being

[merged small][ocr errors][ocr errors][merged small]

inserted in each field circuit. Another plan is to provide the boosters with series-wound field magnets, in which case the voltage generated by each increases with the current flowing through it. With field magnets designed to work below magnetic saturation, this gives an automatic regulation that is almost perfect ; since the extra pressure produced by the booster may be made to exactly overcome the drop on the corresponding feeder for all loads, or it may be designed to also make up for some or all of the drop on the mains and leads. The principle of this last-described method is quite similar to that of compound-wound dynamos; but it acts upon the feeders individually, instead of upon the system as a whole. It corresponds to a case where each feeder is supplied by its own compound generator, which is a possible but hardly practicable arrangement with a large number of feeders, twenty or thirty being an ordinary number. In any system the main generator and the boosters may all be driven by one or more steam-engines or other prime movers, or the boosters may be operated by electric motors supplied with current from the main dynamos.

Similar machines are sometimes used to reduce or “crush ” the voltage instead of raising it, in which case the main dynamo may be run at the average pressure required by the feeders, the potential in those that are heavily loaded being increased, and being depressed in the lightly loaded ones. A “crusher” which lowers the voltage by generating a counter E.M.F. acts as a motor, and tends to develop power; consequently, if it is coupled with a booster, it will drive the latter as a dynamo, provided that the energy absorbed by the former is slightly greater than that produced by the latter, in order to make up for mechanical and electrical losses in both machines. By thus arranging the machines in pairs, they run each other, and no external driving-power is needed..

Instead of having a separate booster for each feeder, as indicated in Fig. 39, two or more feeders requiring approximately equal voltages may be supplied from the same booster. In this way a large number of feeders may be regulated with only a few boosters, which are run at different potentials, the feeders being divided among them according to the extra pressure required. This is practically equivalent to a system having several auxiliary 'bus bars supplied with different voltages. In Fig. 37, for example, the dynamo C might be omitted, and the higher voltage required for the auxiliary bus 'bar E could be obtained by connecting a booster between E and F. This is often preferable; since it would only be necessary to run one main dynamo, the booster being driven by a motor fed with current from the dynamo.

The arrangement represented in Fig. 40 combines several of the methods of feeder regulation already described. The generators C and D operating in parallel are connected to the 'bus bars F and H, to which they supply the ordinary voltage required by the feeders. The conductor E, called the high auxiliary 'bus bar, is maintained at a higher potential than F by means of the booster A. The conductor G receives its current from the main 'bus F through the resistance B, consequently its pressure is less than that of F; and it is designated as the low auxiliary 'bus bar. The feeders J, K, and L are connected to E, F, or G according to the voltage that they require, the longest and most heavily loaded being fed by the high auxiliary E, which is often 10 or 20 per cent and sometimes 40 or 50 per cent higher in pressure than the main 'bus F. The amount of this extra voltage is regulated by means of a rheostat in the field circuit of the booster A, usually excited from the 'bus bars F and H. If the dynamo C be connected to E and not to F, the higher pressure may be generated by it, and the booster A can be dispensed with, as already shown in Fig. 37 ; but it is often more convenient to operate all the main

[merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small]

generators in parallel on the same pair of 'bus bars, and obtain the higher and lower potentials, if required, by means of boosters and resistances. The latter, on account of their simplicity, are applicable when the differences in pressure are small, the voltage of the low auxiliary 'bus G in Fig. 40 being between 5 and 10 per cent less than that of F, for example. But with greater reductions the loss of energy becomes too large an item, and it is more economical either to run another generator or a “crusher” (with counter E.M.F.) to supply G.

« НазадПродовжити »