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Bible Encyclopedias
Electrometallurgy and Electro Electricity Supply
1911 Encyclopedia Britannica
or "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 developments connected with the dynamo ( see 8.764), and with progress 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 statutory 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 15 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 (2-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 successful development of high-speed machinery and of reduction gearing is having a marked influence towards the higher frequency. Even 60-cycle rotary converters for traction work are becoming common. Four-pole turbo-alternators running at 1,800 revolutions 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 i,50o revolutions per minute.
Thus the alternator has been able to keep pace with the demands of the steam turbine as regards large powers at high speeds with high thermal efficiencies for the combination. Even comparatively small units of 6,000 to 7,500 kw. have shown an efficiency from the thermal units of the coal to the net kilowatthour 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 through a large 2-pole turbo-alternator of modern type is shown in fig. 1, 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 successful 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 i ' '111111L.:11111,111...: MOM Mims ?hl?
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Iln?; ?a?????? ? ? = than the frequency of 25 cycles. Where the latter frequency has been retained it has been found preferable to use mechanical reduction gearing up to capacities of about 5,00o kw. Reduction gear may indeed be said to have revolutionized turbine driving for small outputs, the loss in the gearing being more than compensated by the increased efficiency of the high-speed steam turbine. It has further to be remarked that the application of reduction gearing to electrical work is still in its infancy. The greater expense of the geared drive is considered by many to be justifiable on account of its greater reliability and the higher efficiency of the plant.
The development of the continuous-current turbo-generator could not keep pace with the demand for increased output. Though satisfactory units up to 1,000 kw. were built, continuous-current turbogenerators are seldom built at the present day, except for installation on board ship. The demands of large users of continuous-current power, such as railways, chemical works, etc., are best met either by geared generators (steam turbines driving continuous-current generators through double helical reduction gearing) for moderate outputs, or by rotary converters for large outputs. Units of 2,000 to 5,000 kw. are not uncommon.
Both machines and transformers owe much of their development to the further utilization of the means for reducing the losses which occur in the iron and the copper. The use of silicon and other elements in alloy with steel in order to increase the resistance to the flow of eddy currents in iron is the factor which has been mainly responsible for the reduced weight per kva. of transformers, whilst the devices adopted for diminishing the unequal distribution of current in machines and transformers have rendered possible many modern designs.
As an instance of a modern power station may be cited that at Zschornewitz (Golpa), which at the present time (1921) is the largest steam-driven station in the world. This was erected in 1915 during the course of the war at the instance of the German Government for the supply of power for the production of nitrate of calcium in order to ensure a sufficient home supply of nitrates for agriculture and other necessary purposes. The engine-room contains 8 steamturbine sets, each of 22,000 kva. capacity at 1,500 revolutions per minute, and the magnitude of the output may be judged from the daily consumption of about 7,000 tons of coal obtained from the lignite coal-field in the area of which the station is situated. There are 64 very large tubular boilers with 9 chimneys, each 328 ft. high, and II large cooling towers. Current is generated at 6,600 volts; of the total output 6,400 kw. are supplied at 6,000 volts to the nitrate works, while 33,000 kw. are supplied to Berlin, 95 m. distant, through a ioo,000-volt double transmission line to a receiving station at Rummelsburg. The State is erecting at Friedrichsfelde a large distributing station for Berlin and adjoining districts, and at this station the combined outputs of the power stations at Lauta (40,000 kw.) and Spremberg (20,000 kw.) and from the Golpa transmission will be dealt with, while a third generating station in the Lausitzer lignite coal-field is in contemplation.
The lay-out of the plant in modern stations has been mainly governed by principles of economy. Larger boilers, higher steam pressures, greater superheat, the substitution of a small number of large turbine-driven sets for a large number of small slow-speed sets have all helped in this direction. The design and arrangement of the switch-gear have also been matters on which much care has been bestowed, particularly in countries where high transmission pressures up to 100,000 or even 150,000 volts have been adopted. In this connexion more efficient protection against lightning, pressure surges, short circuits, faults to earth, etc., may be particularly mentioned. The transformer is now built for such large powers and high pressures that, as with the switch-gear, separate housing is essential.
The cooling of the machinery and transformers calls for special consideration in the lay-out of large plants. Air is still the common cooling medium for machines, but the quantities needed by modern turbo-generators are so large that special intakes and outlets have to be provided. In addition, measures have to be taken for cleaning the air, particularly near towns or industrial centres. For this purpose dry filters were first tried, but were rapidly replaced by wet filters; that a completely satisfactory solution has not been attained thereby is evident from the experiments now being made to circulate the same air through the machine and a refrigerator. With transformers the case is somewhat different; oil is here the cooling medium, and air-blast transformers are now seldom called for. With natural oil-cooling no special provision has to be made, but in larger transformers usually the oil is water-cooled either by passing water through a cooling coil immersed in the upper part of the oil or by pumping the oil through a cooling chamber.
When continuous current is required it is often customary to generate 3-phase alternating current at the pressure required at the slip rings of the rotary converters, thereby dispensing with transformers. An important feature in connexion with modern switchgear is the mistake-proof devices for preventing wrong connexions or danger to the operators.
Railway Electrification The valid reasons upon which the electrification of railways may be advocated have now become more clearly defined, and - progress has been made as these reasons have shown themselves to be applicable to specific cases. Before the World War there was a pronounced desire in certain countries to make themselves economically independent, and therefore to utilize available water-power rather than to import coal, although it was not always easy to show that any appreciable saving would accrue from electrifying railways under these conditions. The countries chiefly concerned in this way were Italy, Switzerland and Sweden. A great impetus, however, was given to this movement during the war on account of the scarcity and high price of coal, and a stage has now been reached when it is safe to say that whatever the cost of coal may be in the future, certain railway lines will no longer be worked by imported coal. Another great factor has been the difficulty of dealing with increased traffic. The introduction of the electric locomotive - by increasing the average speed, especially on inclines, and by rendering heavier train loads feasible - has in several cases proved a cheaper solution than doubling or quadrupling the track. The tunnel and terminal advantages will also be recognized.
As an indication of the importance that the electrification of main lines has assumed, reference may be made to the fact that in many countries the question has been taken up by the states concerned. The outstanding feature of all the reports and discussions that have appeared has been the debatable question of the best system. As far as can be seen at present, different countries will ultimately decide in favour of different systems. The three systems which call for discussion are: (a) The three-phase system; (b) the single-phase system; (c) the continuous-current system.
From a technical standpoint, all three systems may be said to be satisfactory. It will now be convenient to deal with the several countries separately.
Great Britain
The general electrification of railways has been discussed, but has hardly received serious consideration. In 1920, a committee was appointed to advise the Ministry of Transport, and in its interim report advocated as the standard system the continuous-current system at 1,500 volts, the mode of generation of the power to be that prevailing in the district. Up to the present, practically the only lines that have been electrified have been city and suburban railways in and around London, Liverpool, Newcastleupon-Tyne and Manchester. Until recently, the 600-volt continuouscurrent system, as used on tramways, was adopted for the railways, but with a third rail instead of an overhead conductor. There are now two exceptions - the Newport-Shilton mineral line 18 m. long at 1,500 volts with an overhead conductor, and the ManchesterBury line 10 m. long with 1,200 volts and a third rail. There are only two examples of the single-phase system - the important electrification of the suburban system of the London, Brighton and South Coast railway, with an overhead conductor at 7,000 volts and a frequency of 25 cycles per second, and the small MorecambeHeysham experimental line on the Midland railway. Extensions on the Brighton system were in progress before the World War, but these were not completed in 1921. With the exception of a few electric locomotives for hauling passenger coaches and goods trucks over the electrified sections, motor coaches are used eatirely on the English electric y railwas. Amongst recent extensions of the 600-volt system in and around + London may be mentioned the electrification of the suburban lines of the London and South-Western railway, the extension of the London and North-Western railway electrification to Watford, and the extension of the Central London railway on the Great Western railway from Shepherd's Bush to Ealing.
United States of America
In the United States where so much has been done to develop both the continuous-current and the singlephase systems, many important electrifications have been carried out on both systems; but of late years, the leading firms, the General Electric Co. and the Westinghouse Co., appear to have favoured the continuous-current system. In America a break away from 600 volts was made long ago, and electrifications with 1,200 and 1,500 volts became quite common. Of recent years, the Butte-Anaconda mineral line was equipped on the continuous-current system at 2,400 volts, and served as an experiment for the electrification of the Chicago, Milwaukee and St. Paul railway at 3,000 volts. This line, over 655 m., was in 1921 the longest in existence, but conditions on this mountainous line through the Rockies differ considerably from conditions in densely populated areas. With the possibility of one train in about every two hours, it is hard to draw comparisons with the New York Central, the Pennsylvania and the New York, New Haven and Hartford lines.
The single-phase system has also been extensively applied in the United States, particularly on the Philadelphia section of the Pennsylvania railway and on the Norfolk and Western lines, where the traffic is very heavy. There is a marked difference between the types of locomotives and of motors developed in America and those developed. in other countries, and it is possible that the direction along which designers have gone in the United States has not on the whole been the most favourable for the single-phase system. At the same time it would be wrong to assume that America as a whole is in favour of the continuous-current system. The use of 163 cycles in Europe as compared with 25 cycles in America has been much to the advantage of the former continent in single-phase work.
Italy
Italy was one of the first countries in Europe to consider and adopt the electrification of its railways.
At that time (1902) the three-phase system was practically the only one available for main lines, the position in this respect being somewhat akin to that on the Brighton railway when the single-phase system was chosen. The one serious drawback to the three-phase system is the need for two overhead wires at different potentials, which makes the overhead construction at points and crossings very complicated. Also the profile of certain tunnels renders the adoption of this system difficult. One undesirable result of the overhead complications is the limitation of the pressure to 3,000 volts. The objectionable double overhead potential and the choice of two other satisfactory systems have prevented the extension of the three-phase system to other countries. At the same time it should not be supposed that less success has been obtained with this system than with either of the others - indeed, the whole technical world must view with admiration the ability shown by the Italian engineers in carrying out the system. Many important State lines are now worked electrically, among which may be mentioned the pioneer Valtellina line (opened in 1902), the Giovi tunnel and the Mont Cenis tunnel lines. For mountain lines the three-phase system is peculiarly well adapted, because of the automatic regenerative braking action which occurs as soon as the motors run above synchronous speed. The original locomotives had two speeds obtained by the cascade arrangement of two motors; while the newer locomotives have four speeds, the cascade connexion being combined with pole-changing devices. The power for the Italian lines is obtained from hydraulic stations, the use of waterpower being important in a country without native coal.
Switzerland
To Switzerland belongs the credit of much pioneer work in railway electrification ever since the Oerlikon Co. equipped an experimental line from Seebach to Wettingen. The piercing of the Simplon tunnel in 1907 was followed by the adoption of the three-phase system so as to utilize available plant as far as possible. This tunnel is 14 m. long (from Brigue in Switzerland to Iselle in Italy), and insulation difficulties were experienced with both overhead conductors and locomotives on account of the hot springs, which produced a very humid, warm atmosphere. On a cold day, a locomotive entering the tunnel from Brigue became rapidly covered with moisture. The earlier locomotives were provided with slip-ring induction motors, two speeds being obtained by changing the number of poles; the later locomotives have squirrel-cage rotors and are arranged for four speeds, the stators being provided with twopole changing windings. The three-phase electrification has now been extended to Sion in the Rhone valley. In 1912 the Loetschberg railway from Berne to Brigue (Simplon tunnel) was opened and from the outset this line was operated electrically. The system chosen was the single-phase system at 15,000 volts and 15 cycles. (This may be changed later to 16; cycles, the frequency used on the Federal railways.) After the initial difficulties had been overcome, both in the overhead system and in the locomotives, the Swiss Government decided to apply the same system on the St. Gothard railway. In this connexion mention may be made of the important official commission which was appointed in 1904 to study the electrification of the Swiss railways. Several reports were issued by this commission, the labours of which were concluded in 1914. It has been claimed that the economy and efficiency of the single-phase system are greater than those of other systems, and this was particularly the case on the Loetschberg railway, where the single-phase overhead line is fed directly from the single-phase generating station at Spiez at the working voltage without transformers. Not only did the commission report strongly in favour of the single-phase system, but also advocated the generation of single-phase power at railway frequency (163 cycles) rather than 3-phase generation at the industrial frequency of 50 cycles and conversion to single-phase at railway frequency. If the over-all cost of energy delivered to the locomotive, including attendance, be reckoned as unity when the current is converted from one system to another, this may be reduced to about 0.6 when conversion is dispensed with, and the latter figure can again be reduced still further when the intermediate link of transformers is eliminated. Extensions have been made on the lines adjoining the Ioetschberg line as far as Berne, and the St. Gothard line (Lucerne - Chiasso) is now working electrically from Erstfeld to Bellinzona. Several of the lines subsidized by the Canton of Berne have recently been electrified and linked up with the Loetschberg railway, while many other important projects are also under consideration. It is estimated that about 30 per cent of the Swiss railways are now worked electrically.
Doubtless one of the chief causes of the success of the single-phase system in Switzerland arises from the successful development of the single-phase commutator motor for traction work. In Europe there has always been a tendency to use fewer and larger motors and to mount them higher in the locomotive than is the case in America. Though this construction has introduced new problems with connecting and coupling rods, it has permitted the logical development of the single-phase motor. Of all the different types of commutator motor - the repulsion motor with fixed and movable brushes (Deri motor), ,the repulsion motor with phase compensation (Winter Eichberg Latour motor as used on the London, Brighton and South Coast railway), and the various forms of series repulsion motor - the successful survivor is doubtless the compensated series motor, the excitation required to give the E.M.F. to neutralize the transformer E.M.F. in the coils short-circuited by the brushes being obtained by suitable winding on auxiliary poles. Though such motors can be built for low terminal pressures only (200 to 500 volts) and therefore necessitate step-down transformers on the locomotive, advantage is taken of this t obtain economical and ample speed control by providing suitable tappings on the secondary of the transformer.
Germany
In Germany the single-phase system has also been adopted where main lines have been electrified. The chief electrified lines are the Dessau - Bitterfeld section of the Magdeburg Hall line, the Silesian mountain lines and the Wiesental railway in Baden. Early in the present century trials had been made on the BerlinZossen experimental line, and it would appear that the single-phase system at 15,000 volts, 163 cycles, will be adopted as the standard system for the German railways. The power for several of these lines is generated at 60,000 to 80,000 volts in steam stations. The electrification of the Dessau - Bitterfeld line was the alternative chosen in preference to quadrupling the tracks in order to cope with the increasingly heavy demands on this section.
Many different types of electric locomotive have been built in German y, some of which were in accordance with the specifications of the railway engineers. Much adverse criticism was raised owing to important troubles in several constructions, arising mainly from failures in the driving mechanism. Many problems, both in Germany and Switzerland, concerning vibrations set up by the natural frequency of the system, deformation of the several parts and the play in the bearings, had to be investigated before successful solutions were found. In some cases it was found that an elastic member between the driving and the driven parts proved effective in damping the oscillations.
Sweden, like Italy and Switzerland, is a country without coal but with ample water-power. The first important electrification in Sweden was the Riksgrüns railway, the most northerly railway in the world, situated entirely within the Arctic Circle. This railway extends from Lulea in the Bothnian Gulf to Narvik, an ice-free port on the Norwegian coast, and is used for transporting mineral ores to the latter place for export. Since the original electrification was carried out in 1910 extensions have been made, and it is hoped that the whole line will shortly be worked electrically.
The high price and great scarcity of coal towards the end of the war, and afterwards, made the consideration of the utilization of water-power extremely urgent. The expert commission appointed to study the question confined its attention to the problem of immediate urgency - the Stockholm - Gothenburg line. A careful comparison was made between the continuous-current system at 3,000 volts and the single-phase system at 15,000 volts, and it was shown that the latter was slightly better from an economic standpoint, in addition to which the Swedish railway administration and manufacturing firms were fairly well acquainted with the actual working of the single-phase system. The proposals for this scheme were accepted by the Riksdag in 1920.
France
A commission was also set up in this country to study the electrification of the French railways. Before the war certain short sections had been electrified on the single-phase system, but as a result of a post-war visit to the United States, the commission appeared to be whole-heartedly in favour of the continuous-current system, at a pressure of 1,500 volts - in this respect agreeing with the findings of the British advisory committee. It is intended to make use of the waterfalls for supplying energy to the railways.
Austria
Prior to the war, the Mittenwald railway between Austria and Bavaria had been electrified, and it has now been decided to adopt electrification on a general scale. The system adopted is the single-phase at 15,000 volts and 163 cycles. Locomotives were ordered in 1920, and it was hoped to commence running in 1925. General. - As general problems connected with electric traction on railways may be mentioned interference with communication circuits, regenerative braking and speed control.
In most countries telegraph and telephone lines run alongside the track, and all systems have created disturbances in these circuits from electromagnetic or electrostatic influence. Some of these disturbances are periodic and traceable to harmonics in the current in the power circuit; others, perhaps the most violent, arise from pressure surges, earths, short circuits, etc. Numerous remedies have been adopted, most of which are more or less costly. Thus the avoidance of close parallels by removing the communication circuits to a distance or placing them in underground cables is an expensive expedient. To say, as is usual, that the single-phase system causes worse disturbances than the continuous-current system could not be accepted as a general statement; some of the most troublesome cases have occurred in continuous-current systems fed from rotary converters. However, the causes are now better understood and successful remedies are in sight. It may be mentioned that, while the French commission in their decision in favour of the continuous-current system were largely influenced by the interference question, the Swedish commission regarded it as no better in this respect than the alternating-current single-phase system.
FIG. 2.-300-Ampere Mercury-Vapour Rectifier.
All three systems used for traction are capable of regenerative braking, by which is meant the use of the electric machine as a generator absorbing the mechanical energy from the train and returning it to the supply system as electrical energy. In this respect the three-phase system is simplest, for all that is necessary here is that the speed should exceed synchronous speed, in which case the induction machines act as generators. Obviously the method is not suited for bringing trains to rest. With the other two systems special devices are requisite, and though regenerative braking was first developed for continuous-current traction, successful solutions have now been developed and applied on single-phase locomotives in Switzerland, which enable the train to be brought to rest by regenerative braking. Hitherto, in the matter of regenerative braking, economy in power has usually been of less importance than the saving in wear and tear of tires, brake blocks and rails. On the lines where such braking is applied, it is frequently impossible to utilize the returned energy, which is accordingly consumed in resistance. Speed control can be obtained with all systems. With a continuous-current supply series, parallel connexion and field
weakening together provide a limited number of economical running speeds. It must be borne in mind, however, that weakening the field reduces one of the torque-producing factors, which may entail serious increase in armature heating when the torque rises rapidly with the speed. With three-phase supply two or four speeds are obtained by cascade connexion or pole-changing devices. The single-phase system, by means of a variable-ratio transformer, provides most easily a large number of economical speeds.
Large mercury-vapour rectifiers have recently been constructed and put into commercial use; these entail further auxiliary apparatus as vacuum and water pumps, and their relative advantage or disadvantage as an alternative to the rotary converter for traction work remains to be decided in the future. Fig. 2 shows a small 30o-ampere rectifier as made by Messrs. Power Rectifiers, Ltd., which can supply the rectified current at any voltage up to 750 volts. The arc operates in the lower chamber A, between the mercury cathode D and anodes C, of which there are usually six connected to the six-phase secondary of a transformer. The neutral point of the secondary is brought out and forms the negative pole of the continuous-current system, the cathode being the positive pole of that system. The arc is struck by means of the ignition anode E, which is connected by a long rod with the solenoid mounted on the top of the condensing chamber B. This solenoid is controlled by a push-button ignition switch, and the connexions are so arranged that when the anode E touches the mercury a portion of the current which was previously flowing through the solenoid coil is diverted; this allows a spring acting in opposition to the solenoid to raise again the ignition anode. The rectifier is cooled by water circulated through the base of the cathode, through a jacket round the arc chamber, and thence through the plate in which the anodes are mounted and the jacket round the condensing chamber. Larger sizes dealing with 600 and i,000 amperes are manufactured, and for larger outputs two or more rectifier cylinders are placed in parallel and connected to a single transformer.
| Country | Available | Developed | Per Cent | U.S.A.. .. . Canada A " B | 28,100,000 18,803,000 8,094,000 | 7,000,000 1,735,000 1,725,000 | 24'9 9.2 21.3 | Austria-Hungary | 6,460,000 | 566,000 | 8.8 | France. .. . | 5,587,000 | 1,100,000 | 11.6 | Norway | 5,500,000 | 1,120,000 | 20.4 | Spain. ... . | 5,000,000 | 440,000 | 8.8 | Sweden. ... . | 4,500,000 | 704,000 | 15.6 | Italy. ... . | 4,000,000 | 976,000 | 24'4 | Switzerland . | 2,000,000 | 511,000 | 25.5 | Germany | 1,425,000 | 618,100 | 43'4 | Great Britain | 963,000 | 80,000 | 8.3 Hydraulic Electric Stations Probably in no direction has greater progress been made of recent years than in the utilization of water-power. In all civilized countries throughout the world plants have been installed and projects drawn up for utilizing this natural source of energy. An idea of what is possible and of what has been done in this direction is obtained from the following approximate table, taken from a paper by E. M. Bergstrom (Inst. Mech. Eng. 1920): - B.H.P. Low, medium and high falls, ranging from 4 ft. (e.g. on the river Main) to 2,700 ft. of head (e.g. at Luchon on the French Pyrenees) have all been brought into service. To take one instance only, the modern water-power station on the river Dal, about 80 m. from Stockholm, contains four turbines, each of 10,000 H.P. coupled directly to dynamos at 125 revolutions per minute, and larger sets up to 20,000 H.P. are not uncommon. The latest (1920) station of the Southern Power Co., operating in S. Carolina, U.S.A., has been installed on the Wateree river for 90,000 H.P. and contains five turbines, directly coupled to generators each of 14,000 kva. The extension of station No. 3 of the Niagara Falls Power Co., developing an additional 100,000 H.P. at Niagara, is noteworthy for the inclusion of 32,500 kva. 12,000-volt three-phase alternators running at 150 revolutions per minute and a frequency of 25 cycles per second. One of these, manufactured by the Allis Chalmers Mfg. Co., is shown in fig. 3.  For high falls Pelton wheels are employed, and in the case of Luchon, quoted above, each Pelton wheel develops 6,200 H.P. at the high speed of 1,500 revolutions per minute. Still higher heads are being utilized, and owing to the high costs of material and ?r?,? ?G ? ?'?k???.. N " ??'.???51 ..?? ?,:: ??=C'ila: ' 'i? ?1'N.??? .'... - FIG. 3.-32,500-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 alternators is in the construction of the rotor. Owing to the possibility of racing, water-turbine-driven sets have to be capable of withstanding overspeeds of 80 to too %. 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 alternators totally enclosed to reduce windage losses, to assist artificial 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 export 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 engineering 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 accordance with definite policies. Thus in Switzerland, where the linkingup of stations has been adopted on a wide scale, the low-head power stations in the valleys, which utilize river energy, are designed to supply the mean power and therefore to run on practically constant 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 waterpower 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. Considering first of all continuous-current motors, it may be said from a theoretical standpoint that the possibilities of continuouscurrent motors are almost unlimited. The speed of such motors may be economically regulated by varying either the applied pressure or the exciting current. In the case of a constant-voltage supply, the usual method of varying the supply voltage consists in the use of series-parallel connexion. This involves the use of at least two motors and finds its commonest application in traction. Occasionally, however, some form of the Ward-Leonard system of control is adopted. This, however, entails the use of a variablevoltage generator, which in turn needs an electric motor or a prime mover to drive it. Since each conversion of energy is associated with loss, such systems are not only costly but eventually become more or less wasteful. This is particularly the case when the WardLeonard control is used on an alternating-current system of comparatively small power (e.g. that of a private installation) in order to drive, say, a rolling-mill or a winding-engine where the peak load is much in excess of the mean load. Here it often becomes needful to supply a fly-wheel converter set consisting of an induction motor with slip regulator (see below), a variable-voltage generator and a fly-wheel, in addition to the driving motor, the armature of which has often to be divided into two or three parts in order to reduce inertia when rapid reversals are necessary. The function of the slip regulator is to allow the speed of the induction motor to fall when a heavy load comes on, and so to permit the excess load to be taken up by the stored energy in the fly-wheel. Such sets often have to deal with peaks of 20,000 H.P. and may give an overall efficiency of 50 -70%. Where the supply systems are sufficiently large, as on the Rand, the fly-wheel can be dispensed with, but the induction motor must then be able to cope with the peaks. The electric winder affords a good example of the problems that have to be met in many cases in order to replace a steam-engine drive. A much simpler method of controlling the speed of a continuouscurrent motor is to vary the exciting current. This can be done automatically or manually, and it may be made dependent on or independent of the load; but in every case a single machine only is necessary. The usual continuous-current motor for different speeds is the shunt motor; in this, with a given excitation, the speed is practically independent of the load; but by increasing or decreasing the exciting current the speed is lowered or raised respectively. By this method of shunt control it is possible to obtain speed ranges as high as 1.5 or 1.6. Such wide ranges, however, make the design difficult. At the lowest speed the ventilation is usually very poor, while the exciting current is highest, but fans built on the shaft of the armature can usually overcome any difficulty arising therefrom. It is at the higher speeds that the design becomes a serious problem. In addition to the high peripheral speeds of armature and commutator the very weak field may render the motor unstable, while the commutating poles - which are essential to prevent sparking - may produce hunting. It becomes necessary therefore to provide such motors with compensating windings in order to neutralize armature reaction. Thus despite the economy of this method, the motors become costl y when wide speed ranges are demanded. Series motors in which the exciting winding is in series with the armature winding, and in which in consequence the speed becomes a function of the load, are widely used for drives where there is no danger of the load being removed - e.g. for traction or for fans, cranes, etc., but the only common application of voltage and field control of series motors is for traction work. The compound-wound motor combines the shunt and series characteristics in varying degree, according to requirements. If a series characteristic is required with merely a limiting top speed, it is only necessary to provide the motor with a small shunt winding in addition to the series winding in order to prevent racing. When, however, an increased torque at starting or a fall in speed in the case of overloads is demanded, a small series winding is added to the shunt winding. In the former case the series turns may be shortcircuited if desired after a definite speed has been reached. Except in cases where a variable voltage is applied to the motor, starting resistances are necessary with continuous-current motors, so that continual starting becomes wasteful. For general speed control the continuous-current motor is doubtless unrivalled, and where circumstances justify the outlay conversion from alternating to continuous current is the best solution. A typical case would be a factory in which several variable-speed motors are installed. Coming to the alternating-current side, mention must first be made of the question of power-factor rectification. The alternatingcurrent, three-phase system having established itself as the standard method of transmission, vigorous attempts are being made in every country to keep the power-factor of such systems as high as possible, in order to secure the minimum outlay in transmission and generation. Obviously, with three-phase supply it becomes highly important to employ wherever practicable three-phase motors, but in any such application the power-factor must not be overlooked. Broadly speaking, the user does not stand to gain by ignoring this question, for whether the rectification is achieved by him or by the power company, or is not done at all, the consumer has to pay. Though with alternating current there are more types of notors available than with continuous current, speed control presents a more difficult problem. From the point of view of power-factor correction, the synchronous motor can be regarded as ideal, but here speed control is not available, while there are the additional difficulties of providing facilities for starting and for separate (continuous-current) excitation. Where the conditions at starting do not call for a large amount of torque, it is often possible to bring the motor up to speed as an induction motor by means of eddy currents induced in the pole shoes or by using the damping winding as a squirrel-cage winding. The next stage consists in the provision of a starting motor in the form of an induction motor with two poles less than the synchronous motor. For severe starting conditions, such a starting motor would become too costly, and the present solution is being sought by building the synchronous motor itself as an induction motor. The machine then runs up to speed as an induction motor, is excited by continuous current and pulls into synchronism, whence it continues running as a synchronous motor. In addition to meeting severe starting conditions, this arrangement is also replacing the induction motor where power-factor correction is important. By its simplicity the induction motor is doubtless the alternating-current motor that finds most favour. Where repeated starting or where speed control is necessary the motor is uneconomical, because the input to an induction motor depends on the torque, and is independent of the speed. Nevertheless it is often preferable to incur this waste rather than to install converting sets. It is possible, however, to obtain economical speed control with an induction motor by changing the number of poles or by connecting two induction motors in cascade - in each case, however, with a certain sacrifice in power-factor as well as through the extra cost incurred. There are numerous ways of effecting a change in the number of poles - e.g. by regrouping the coils, by varying the number of phases, by using two or more windings, etc. - and generally it becomes needful to employ a squirrel-cage rotor. Such a rotor, however, does not necessarily mean a low starting torque, for some of the locomotives used on the Simplon tunnel railway have such windings. Generally speaking, it is not usual to obtain more than six speeds with induction motors, while two and four are more usual. The commutator motor offers theoretically the best solution for obtaining speed control with alternating current, and the possibilities here are the same as with continuous current. Actually, however, the limitations are more severe, because not only do commutation conditions limit the pressure as in the continuouscurrent motor, but the transformer pressure induced by the alternating flux in the coils undergoing short-circuit imposes further limitations which result in a comparatively small output per pole. The reduced commutator pressure usually entails a transformer between supply and motor, but where speed control is required advantage can be taken of this to vary the applied pressure by using a variable-ratio transformer. The real trouble occurs when the E.M.F. in the short-circuited coils depends upon synchronism, as in three-phase commutator motors and single-phase commutator motors of the repulsion and shunt types. The practical result is that the speed of such motors never varies greatly from synchronous speed, and that their limiting output is a few hundred horse-power. On the other hand, types like the single-phase series commutator motor, free from this restriction, have been successfully built for outputs of over 1,000 H.P. and speed ranges up to four or five times that of synchronism. Despite limitations, alternating-current commutator motors are becoming more widely used, particularly for small outputs; while as cascade or auxiliary motors they have been successfully applied for utilizing the slip energy of large induction motors. Variable-speed sets of this kind will probably be more widely developed in the future, particularly when the properties of alternating-current commutator motors come to be better understood. Authorities.-AS additional authorities may be consulted: Miles Walker, The Specification and Design of Dynamo-Electric Machinery (1915); Hawkins, Smith and Neville, Papers on the Design of Alternating Current Machinery (1919); Alexander Gray, Electrical Machine Design (1913); A. T. Dover, Electric Traction (1917), and G. Klingenberg, Bau grosser Elektrizitatswerke (1920). (C. C. H.; S. P. S.) Copyright Statement These files are public domain. Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Electrometallurgy and Electro Electricity Supply'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/e/electrometallurgy-and-electro-electricity-supply.html. 1910.  Ads FreeProfile |