DISCOVERY 



19 



good, especially for main lines, and is at present in 

 use on the London, Brighton and South Coast Railway, 

 where the system was adopted with a view to the 

 ultimate conversion of the whole system after the 

 suburban sections had been electrified. On the 

 Brighton railwaj' the overhead line pressure is 1,500 

 volts, but on the Continent, where longer distances 

 and sparser traffic are the rule, the pressure used is in 

 some cases ten times this amount. 15,000 volts is 

 the pressure used on the main lines of the Swiss Federal 

 Railways, the whole of which are being gradually 

 converted to electric traction. 



As an example of up-to-date practice in the field of 

 single-phase traction, we are able, through the courtesy 

 of Messrs. Brown, Boveri & Co., Ltd., to illustrate 

 in^Fig. I one of a group of single-phase locomoti\es 

 which they have supplied to the Rhretian Railway in 

 Switzerland, a narrow gauge line of severe gradients 

 and curves including the lines connecting Coire and 

 Dissentis with St. Moritz, Davos, etc., on the whole of 

 which electric traction will shortly be used exclusively. 

 This locomotive comprises two trucks, each driven 

 by one large motor mounted on the main frame and 

 geared to a countershaft from which a ffe.xible drive 

 to the wheels is obtained by outside connecting and 

 coupling rods — a system of drive much favoured in 

 Continental locomotives. To give an idea of size, it 

 may be mentioned that the overall length is about 

 43 ft., and the total weight about 66 tons, of which 

 about 28 tons consist of electrical equipment. The 

 overhead line pressure varies from 7,500 to 11,000 

 volts, and the motors are fed from a single transformer 

 on the locomotive at pressures up to 520 volts. The 

 two motors, together capable of 950 h.p. continuously, 

 or 1,140 h.p. for one hour, can give a maximum drawing 

 effort of 13 tons at starting and a maximum speed of 

 28 miles per hour. The motors are kept cool by forced 

 ventilation. 



There is one further system of alternating- current 

 traction which was used before the single-phase 

 traction motor was developed. This is the " three- 

 phase " system, which requires two overhead contact 

 lines as well as the earth return, to provide for the 

 currents. Such three-phase current permits of the 

 use of a class of motor, known as the induction motor, 

 which is in many respects simpler than the continuous- 

 current or the single-phase commutator motor. This 

 advantage of simplicity and robust construction is 

 counterbalanced bj' the fact that it cannot conveniently 

 exert so great a pull at starting and cannot give power 

 above a certain fi.xed speed called the " synchronous " 

 speed, depending upon the frequency of the alterna- 

 tions of the current. This last feature, however, is a 

 very great advantage for certain cases, such as mountain 

 lines, because if an induction motor be driven above 



its synchronous speed, say when the train is running 

 down hill, it automatically becomes a generator 

 which, like a brake, limits the speed of descent to a 

 few per cent, above that corresponding to its synchro- 

 nous speed, but without wasting power in heat hke a 

 mechanical brake, as the energy developed is returned 

 into the line. This regenerative braking effect, as 

 it is called, renders the three-phase system excellent 

 for mountain lines, be they of the adhesion or rack 

 type, and in no other system can locomotives be built 

 with so low a weight per horse-power. This system 

 was first used on the mountain lines in Switzerland 

 and has extended to some of the main lines of northern 

 Italy, where steep gradients are plentiful. It is in 

 use on the line approaching and passing through the 

 Simplon tunnel. For ordinary railway work, however, 

 the three-phase motor has disadvantages in that speed 

 regulation below the maximum speed presents difficul- 

 ties, because such a motor, even with the most com- 

 plicated control gear, will only run economically at a 

 few submultiples of its synchronous speed. It will be 

 remembered that the three-phase system was at one 

 time seriously proposed for the Metropolitan Railway, 

 and it was only after an arbitration that uniformity of 

 system with the District Railway was attained. 



Recently a system has been developed to combine 

 the advantages of single-phase, single-wire distribution 

 with three-phase motors, by employing an apparatus 

 which changes one kind of current to the other on the 

 train, but the system is more or less experimental at 

 present, and would appear to suffer the disabilities in 

 speed regulation already mentioned. In the earlier 

 days of single-phase working advantage was taken of 

 the fact that the single-phase commutator motor could 

 be made to run also on continuous-current circuits, so 

 that the same motors could run in country zones on a 

 single-phase circuit and in urban areas as continuous- 

 current motors. But this compromise was not a 

 happy one, since a motor that is well designed for one 

 kind of current is badly designed for the other. 



Although the single-phase system is very suitable 

 for main-line working, and is adopted as the standard 

 for the purpose in Central Europe, it cannot be affirmed 

 that the single-phase motor is as good for use on ordinary 

 railway lines as the continuous-current motor. The 

 equipment to be carried on the train with its trans- 

 formers, etc., is heavier for its power than the 

 continuous-current apparatus. And in all alterna- 

 ting-current systems there are difficulties in avoiding 

 interference with neighbouring telegraph and tele- 

 phone circuits. 



High-pressure Continuous Currents 

 Meanwhile improvements in design made both on 

 the continuous-current traction motor and on sub- 



