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FIG. 3. 32,50o-kva. Combined Water-Turbine and Alternator.
labour, the tendency is to favour the development of high-head falls which require less civil engineering owing to their smaller volumes of water. The chief problem in the design of water-wheel alter- nators is in the construction of the rotor. Owing to the possibility of racing, water-turbine-driven sets have to be capable of with- standing qverspeeds of 80 to 100%. In many cases the peripheral speed is high on account of the large output, while large diameters become necessary to meet the demands for fly-wheel effect. The result is that a very rigid construction is necessary for the rotor, usually embodying some modification of the dovetail for securing the poles and field windings. The stator windings also, as in turbo-alternator, have to be securely braced in order to withstand the severe conditions of sudden short circuit. It is customary to make water-wheel al- ternators totally enclosed to reduce windage losses, to assist arti- ficial ventilation and to protect the machine against possible leaks from the turbine.
Small hydro-electric stations are now in action which are either entirely automatic and actuated by a change of water level, or set in operation by remote control in accordance with the demands for power from the network. A case of interest as involving the ex- port of energy is the hydro-electric transmission of power up to 20,000 H.P. from the power-station of Gosgen on the river Aar in Switzerland to a distributing station situated in France, where the supply is placed in parallel with the steam-driven station of Vincy. Transmissions from Norwegian waterfalls to Denmark and Sweden are also contemplated.
One reason for the comparatively small amount of power utilized in Great Britain has been the abundance of coal. In many cases the development of water-power has only become possible since coal became dear and scarce, for it must not be forgotten that hydraulic installations are frequently very costly on account of the civil en- gineering works that have to be constructed in places difficult of access, and of the long high-tension transmission lines.
In many countries water-power is now being developed in accord- ance with definite policies. Thus in Switzerland, where the linking- up of stations has been adopted on a wide scale, the low-head pow- er stations in the valleys, which utilize river energy, are designed to supply the mean power and therefore to run on practically con- stant load, while the "peak" loads are supplied by the high-head stations in the hills, which are fed from natural lakes or reservoirs in which the water is impounded by means of dams.
In Italy power is available from the Alps in summer from the melting of ice and snow, and from the Apennines in winter from rain. By linking up the several stations a continuous supply of energy is assured. In Germany the canalization of rivers is carried out hand-in-hand with the supply of electric energy by building power-stations at the weirs.
Wave-power, tidal rise and fall, and tidal currents in estuaries have all received attention, especially in France, as possible sources of power in the future, and a large scheme for utilization of the water- power available from the Severn has been proposed, but in no case have the projects advanced beyond the stage of discussion.
APPLICATIONS OF ELECTRIC MOTORS
One of the main factors in the development of electrical supply has been the extended use of electric motors for driving machinery of all kinds. In addition to the numerous class of simple, straightforward drives, the electric motor has been applied with success under more difficult conditions, demanding large starting torque, considerable powers and wide variations of speed. Along with this development has been the extension of the three-phase system, in consequence of which there has arisen a wide demand for variable-speed, alternating-current motors. Some directions of their application may be dealt with.
