ELECTRIC TRACTION ON MAIN LINE RAILWAYS
On tramways, automobiles, and 'third-rail' lines, the electric
current used belongs to the type described as 'continuous' or 'direct,'
because the flow is always in the same direction. The other type of
current is known as 'alternating,' as it flows backwards and forwards
many times per second. There are several kinds of alternating
current—single-phase, two-phase, three-phase, and polyphase—each
produced from generators designed in a particular way.
It is not possible to give any adequate account of these different kinds of alternating current without going rather deeply into the theory of electricity. The ultimate practical point is that in transmitting alternating currents the circuits increase in number with the phases. Thus, three-phase current requires three wires, two-phase current three or four wires, and single-phase current a single circuit like that of continuous current.
Where current has to be conveyed economically over long distances, it is generally done in the form of alternating current at high pressure. For instance, the transmission from a tramway power station to the sub-stations is almost uniformly by three-phase current at, say, 5000 volts. When it reaches the sub-station, it is 'transformed' down to the working pressure of 500 volts and 'converted' from alternating to continuous current by means of rotary machinery. The transforming is done by a stationary piece of apparatus similar in principle to the familiar induction coil. An induction coil takes current at a few volts from a battery into its primary circuit and transforms it, by induction in the secondary circuit, into current of high enough voltage to give a long spark. A transformer can be designed to 'step-up' or 'step-down' the pressure according to the requirements of the case.
So much explanation is necessary to give some account of the alternating current railways on the Continent and thence of the single-phase system on the London, Brighton and South Coast Railway. The Morecambe and Heysham section of the Midland Railway is also equipped on the single-phase system.
Most of the earliest electric railways on the Continent derived their power from waterfalls and had to transmit it for a considerable distance. Three-phase current at high pressure being adoptedfor this purpose, the Continental engineers set to work to find some means of utilising the high-pressure three-phase current directly. They did this by carrying the three wires on poles alongside the railway track, and using three 'bow' collectors (in place of trolley wheels) to convey the current to transformers on the motor cars or locomotives. In these transformers the current was brought down to working pressure and then led to motors designed for three-phase current.
An immense amount of technical ingenuity was exercised in developing this system; and when the Metropolitan Railway decided to follow the District in electrifying its lines, a three-phase system was proposed. As the Metropolitan and Metropolitan District companies share the working of the Inner Circle, it was necessary that both should adopt the same system. The result was that the question between three-phase and continuous current working had to go to arbitration. After a long discussion of masses of technical evidence, Mr Lyttelton, the arbitrator, decided that the direct current system was better suited to the conditions of traffic on an underground railway in London.
The wisdom of that decision will not be questioned now. Three-phase motors do not give the rapid acceleration which is so urgently required on suburban lines; there are complications in speed control; and the necessity of having three overhead conductors is also a serious drawback. For comparatively long-distance traffic with few stops, however, the three-phase system is quite suitable. That is to say, it is a possible solution of the main line problem.
The great simplicity and flexibility of the power supply arrangements in the case of alternating current traction encouraged engineers to find something better adapted to ordinary railway conditions than the three-phase motor. Their problem was to find an arrangement which required one overhead conductor instead of three, and also provided a motor with the high starting torque and easy speed control of the continuous-current motor. After much theoretical and experimental work, they found it in the single-phase system, using a motor which is similar in many respects to the continuous-current motor but capable of being operated by alternating current.
On the advice of Mr Philip Dawson, the London, Brighton and South Coast Railway Company decided to experiment with this system on the double line connecting London Bridge and Victoria stations, about 9 miles long. Power is supplied to each track by a single overhead conductor carrying current at 6000 volts. Transformers are placed on the trains to bring the pressure down to 300 volts; the current is then led through controllers to single-phase motors in much the usual way. The reason for using so high a pressure on the overhead line is not only economy in transmission. If lower pressures were used, the heavy currents required for train propulsion would require a thicker conductor and correspondingly heavier supports. At 6000 volts it is possible for two double sliding bows to collect sufficient current for a heavy train from a wire which is comparable in thickness to the ordinary trolley wire of a tramway.
The power distribution arrangements, it will be noticed, are very much simpler than with continuous current on the third-rail system. There are no sub-stations with rotary machinery. Power is supplied direct from the generating station to the overhead line and is transformed down by stationary plant on the train itself. Single-phase traction represents, in fact, power transmission for railway purposes reduced to its simplest elements.
The overhead construction differs, however, in some important points from the tramway standard. The supports, which are in both bridge and bracket form, are stronger; the insulators are, owing to the much higher pressure employed, more massive; and a different means of suspension has been adopted. Each conductor is hung by links from two steel cables stretched chain-wise between the supports. This method of 'catenary suspension' enables the bow to slide along the wire without the jolts which are noticeable with a tramway trolley. Such smooth running keeps the bow continuously at an even pressure on the wire—an advantage which is of great importance at high speeds. The trains are arranged on the multiple-unit system.
The full financial results obtained on this railway have not so far been made public; but it is sufficient for our purpose to note that the Company, after more than a year's full trial, extended the system to the Crystal Palace and to Croydon. Further extensions are, it is understood, contemplated over the suburban lines to Sutton and elsewhere; and in course of time the conversion of the main line to Brighton will be undertaken.
Here we touch upon the most interesting aspect of this demonstration of electric traction on the single-phase system. The system was adopted in the first instance because the third-rail system would lead to complications and dangers which could not be permitted at crowded railway termini shared by all kinds of traffic, suburban and main line. But the advisers of the Company had also in view the possibility of development beyond the range of suburban traffic. They therefore sought a system which, while comparable to the third-rail continuous current in the handling of suburban business, would be adaptable to main line conditions, where infrequent stops and long runs at high speeds are the rule.
The adoption of electric traction on such a route as the Brighton main line would be a benefit in several ways. It would lead to a faster express service, as the high overload capacity of the electric motor enables it to take small account of gradients. It would also lead to a more frequent service, as the electric system is free from the conditions which force a steam railway to try to concentrate traffic on a limited number of long trains. Further, it would, by reducing the time lost in stopping and starting, bring the average speed of stopping trains much closer to that of express trains. All these improvements—assisted, probably, by lower fares—should lead to a great increase in the volume of traffic, thus reproducing the characteristic results of electric traction on suburban lines.
It is not possible to give any adequate account of these different kinds of alternating current without going rather deeply into the theory of electricity. The ultimate practical point is that in transmitting alternating currents the circuits increase in number with the phases. Thus, three-phase current requires three wires, two-phase current three or four wires, and single-phase current a single circuit like that of continuous current.
Fig. 12. Photograph of a train on the electrified
section of the London, Brighton and South Coast Railway. The overhead
wire is suspended from cables stretched between insulators, and current
is conveyed from it to the trains through a 'bow' which slides along its
lower side. The photograph is taken from the rear part of the train.
The front and rear cars are both equipped with electric motors.
Where current has to be conveyed economically over long distances, it is generally done in the form of alternating current at high pressure. For instance, the transmission from a tramway power station to the sub-stations is almost uniformly by three-phase current at, say, 5000 volts. When it reaches the sub-station, it is 'transformed' down to the working pressure of 500 volts and 'converted' from alternating to continuous current by means of rotary machinery. The transforming is done by a stationary piece of apparatus similar in principle to the familiar induction coil. An induction coil takes current at a few volts from a battery into its primary circuit and transforms it, by induction in the secondary circuit, into current of high enough voltage to give a long spark. A transformer can be designed to 'step-up' or 'step-down' the pressure according to the requirements of the case.
So much explanation is necessary to give some account of the alternating current railways on the Continent and thence of the single-phase system on the London, Brighton and South Coast Railway. The Morecambe and Heysham section of the Midland Railway is also equipped on the single-phase system.
Most of the earliest electric railways on the Continent derived their power from waterfalls and had to transmit it for a considerable distance. Three-phase current at high pressure being adoptedfor this purpose, the Continental engineers set to work to find some means of utilising the high-pressure three-phase current directly. They did this by carrying the three wires on poles alongside the railway track, and using three 'bow' collectors (in place of trolley wheels) to convey the current to transformers on the motor cars or locomotives. In these transformers the current was brought down to working pressure and then led to motors designed for three-phase current.
An immense amount of technical ingenuity was exercised in developing this system; and when the Metropolitan Railway decided to follow the District in electrifying its lines, a three-phase system was proposed. As the Metropolitan and Metropolitan District companies share the working of the Inner Circle, it was necessary that both should adopt the same system. The result was that the question between three-phase and continuous current working had to go to arbitration. After a long discussion of masses of technical evidence, Mr Lyttelton, the arbitrator, decided that the direct current system was better suited to the conditions of traffic on an underground railway in London.
The wisdom of that decision will not be questioned now. Three-phase motors do not give the rapid acceleration which is so urgently required on suburban lines; there are complications in speed control; and the necessity of having three overhead conductors is also a serious drawback. For comparatively long-distance traffic with few stops, however, the three-phase system is quite suitable. That is to say, it is a possible solution of the main line problem.
The great simplicity and flexibility of the power supply arrangements in the case of alternating current traction encouraged engineers to find something better adapted to ordinary railway conditions than the three-phase motor. Their problem was to find an arrangement which required one overhead conductor instead of three, and also provided a motor with the high starting torque and easy speed control of the continuous-current motor. After much theoretical and experimental work, they found it in the single-phase system, using a motor which is similar in many respects to the continuous-current motor but capable of being operated by alternating current.
On the advice of Mr Philip Dawson, the London, Brighton and South Coast Railway Company decided to experiment with this system on the double line connecting London Bridge and Victoria stations, about 9 miles long. Power is supplied to each track by a single overhead conductor carrying current at 6000 volts. Transformers are placed on the trains to bring the pressure down to 300 volts; the current is then led through controllers to single-phase motors in much the usual way. The reason for using so high a pressure on the overhead line is not only economy in transmission. If lower pressures were used, the heavy currents required for train propulsion would require a thicker conductor and correspondingly heavier supports. At 6000 volts it is possible for two double sliding bows to collect sufficient current for a heavy train from a wire which is comparable in thickness to the ordinary trolley wire of a tramway.
The power distribution arrangements, it will be noticed, are very much simpler than with continuous current on the third-rail system. There are no sub-stations with rotary machinery. Power is supplied direct from the generating station to the overhead line and is transformed down by stationary plant on the train itself. Single-phase traction represents, in fact, power transmission for railway purposes reduced to its simplest elements.
The overhead construction differs, however, in some important points from the tramway standard. The supports, which are in both bridge and bracket form, are stronger; the insulators are, owing to the much higher pressure employed, more massive; and a different means of suspension has been adopted. Each conductor is hung by links from two steel cables stretched chain-wise between the supports. This method of 'catenary suspension' enables the bow to slide along the wire without the jolts which are noticeable with a tramway trolley. Such smooth running keeps the bow continuously at an even pressure on the wire—an advantage which is of great importance at high speeds. The trains are arranged on the multiple-unit system.
The full financial results obtained on this railway have not so far been made public; but it is sufficient for our purpose to note that the Company, after more than a year's full trial, extended the system to the Crystal Palace and to Croydon. Further extensions are, it is understood, contemplated over the suburban lines to Sutton and elsewhere; and in course of time the conversion of the main line to Brighton will be undertaken.
Here we touch upon the most interesting aspect of this demonstration of electric traction on the single-phase system. The system was adopted in the first instance because the third-rail system would lead to complications and dangers which could not be permitted at crowded railway termini shared by all kinds of traffic, suburban and main line. But the advisers of the Company had also in view the possibility of development beyond the range of suburban traffic. They therefore sought a system which, while comparable to the third-rail continuous current in the handling of suburban business, would be adaptable to main line conditions, where infrequent stops and long runs at high speeds are the rule.
The adoption of electric traction on such a route as the Brighton main line would be a benefit in several ways. It would lead to a faster express service, as the high overload capacity of the electric motor enables it to take small account of gradients. It would also lead to a more frequent service, as the electric system is free from the conditions which force a steam railway to try to concentrate traffic on a limited number of long trains. Further, it would, by reducing the time lost in stopping and starting, bring the average speed of stopping trains much closer to that of express trains. All these improvements—assisted, probably, by lower fares—should lead to a great increase in the volume of traffic, thus reproducing the characteristic results of electric traction on suburban lines.
By ADAM GOWANS WHYTE, B.Sc.
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