Page:EB1922 - Volume 30.djvu/1005

Rh Eisner was the author of various books and pamphlets, which display considerable literary faculty. They include Psychopathia Spiritualis (1892); Eine Junkerrevolte (1899); Wilhelm Liebknecht (1900); Fesle der Festlosen (1903), and Die Neue Zeit (1919).

(G. S.) ELECTRICAL ENGINEERING (see 9.193). In the articles on ELECTRICITY SUPPLY, ELECTROMETALLURGY AND ELECTRO- CHEMISTRY, TELEGRAPHY AND TELEPHONY, PYROMETRY, ELEC- TRIC LIGHTING, WIRELESS TELEGRAPHY AND TELEPHONY, various important applications of Electrical Engineering, as developed since 1910, are separately dealt with. This article deals with devel- opments connected with the dynamo (see 8.764), and with prog- ress as regards power stations and electric traction generally.

LARGE ELECTRIC SUPPLY STATIONS

Technical advances on the generation side of the electrical industry have been mainly in connexion with the wider use of the steam turbine on the one hand and with alternating-current transmission on the other. Thus the large turbo-alternator has become the standard machine for all important central stations dependent on steam. A further factor in this development has been the tendency towards the linking-up of supply stations in large areas in order to obtain increased economy a matter which has so much importance for industry as to call for the appointment in Great Britain in 1919 of special Electricity Commissioners to deal with it. In other countries also the statu- tory regulation of electric supply has been seriously discussed and in Germany state control has been adopted.

Perhaps the most important feature which affects linking-up problems and standard lines of manufacture is the question of the system, or rather of the frequency, to be adopted. In the course of natural development, the 3-phase alternating current system at a frequency of 50 cycles per second has been more and more widely used until it can now be regarded as the standard throughout Europe. On the Continent, apart from traction work for which 50/3 or 1 5 cycles per second have been adopted, a few stations only still operate at 42 cycles per second. In Great Britain the chief exceptions are to be found in the use of 40 cycles in the N.E. coast area, and of 25 cycles in Birmingham and the Clyde valley, the 3-phase system being still retained. With 50 cycles as the standard the turbo speeds become fixed at 3,000 revolutions per minute (a-pole machines) and 1,500 revolutions per minute (4-pole machines). Units up to 20,000 kva. have been built at the former speed, and at the latter up to 40,000 kva. In the United States the standard frequencies are

60 and 25 cycles per second, the latter being essentially used for traction purposes. The higher frequency makes the construction of large 2-pole units more difficult, but nevertheless the success- ful development of high-speed machinery and of reduction gear- ing is having a marked influence towards the higher frequency. Even 6o-cycle rotary converters for traction work are becoming common. Four-pole turbo-alternators running at 1,800 revolu- tions per minute to give a frequency of 60 have been built up to a capacity of 33,333 kva. Steam-turbine units of as much as 60,000 kw. are in use, but in this case the high-pressure and two low-pressure turbines each drive a separate 20,000 kw. generator at 1,500 revolutions per minute.

Thus the alternator has been able to keep pace with the de- mands of the steam turbine as regards large powers at high speeds with high thermal efficiencies for the combination. Even com- paratively small units of 6,000 to 7,500 kw. have shown an ef- ficiency from the thermal units of the coal to the net kilowatt- hour of 18 per cent. It is possible that the normal units of the future will be in the neighbourhood of 25,000 rather than of 50,000 kw. if an output of 100,000 to 150,000 kw. should come to be regarded as the maximum desirable for any one station.

A longitudinal section throitgh a large 2-pole turbo-alternator of modern type is shown in fig. I, wherein will be seen the channels provided for air to ventilate both rotor and stator. A fan is attached to each end of the rotor to blow air through the stator channels, and the heated air is discharged at the top of the outer casing.

The design of large turbo-alternators presents many difficult problems. The rotor (particularly at 3,000 revolutions per minute) is commonly of the cylindrical type made from a solid steel forging, the exciting winding being accommodated in slots and the coil ends secured by means of covers forged from special alloy steels. It is only by the most rigid construction that suc- cessful rotors can be made to withstand the enormous stresses set up at peripheral velocities in the neighbourhood of 25,000 ft. per minute. The adequate ventilation of such rotors is not easily obtained, and, while both air and water ducts are used, there is a strong tendency to dispense with ducts altogether and rely on non-combustible insulation (mica) for preventing injury from high temperature. The stator also needs especial care not only is the cooling problem difficult, but the bracing of the coil ends has to be such that no movement of the conductors is possible even under conditions of sudden short circuit.

It has doubtless been due to the rapidly increasing demands for large powers and high speeds, and the success achieved therewith, that the frequency of 50 cycles has come to be more widely adopted

FIG. I. Longitudinal Section of Large 2-Pole Turbo-Alternator (Metropolitan-Vickers Electrical Co., Ltd.).