Encyclopædia Britannica, Ninth Edition/Traction, Electric

From volume XXIII of the work.
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TRACTION, Electric.The driving of vehicles by electricity was made commercially practicable by the invention of the dynamo-electric machine, which gave a ready means of producing electrical energy by the expenditure of mechanical work, and by the further discovery that the function of the dynamo could be reversed, that it was capable of acting efficiently as a motor to do mechanical work when supplied with energy in the electrical form. Experiment has shown that when a dynamo is used to produce an electric current, which, in its turn, drives another dynamo serving as a motor, the double con version of energy may be performed with no very serious loss. In favourable cases, when the dynamo and motor are close together, the motor will yield more than 80 per cent, of the work which is spent in driving the dynamo. When they are far apart there is an additional loss, due to the resistance of the conductor which connects them, and a further loss due to its imperfect insulation. The use of high electromotive force, which reduces the first of these, tends to increase the second; it is, however, practicable to keep both within reasonable limits. Early attempts to apply electricity to traction were made by Robert David son, who placed an electromagnetic locomotive on the Edinburgh and Glasgow Railway in 1837, and by Jacobi of St Petersburg, who propelled a boat on the Neva in 1839 by an electromagnetic engine driven by a battery of Grove's cells. The inefficiency and bulkiness of early electromotors, and the cost of producing electric energy when a galvanic battery was the source, made it impossible for electricity under such conditions as these to compete with other methods of traction. A good battery using zinc as the active metal consumes from 1 to 2 lb of zinc per hour per horse-power developed; a good steam-engine consumes from 2 to 3 Ib of coal in doing the same amount of work, and the cost of zinc is about fifty times that of coal. Hence, notwithstanding modern improvements in electro motors, the cost of producing mechanical power by means of electricity, when a zinc-consuming battery is the source, is still prohibitive.

The earliest practical electric railway was constructed at the Berlin exhibition of 1879 by Dr Werner Siemens. At one station was a dynamo driven by a steam-engine. The current was conducted to the moving car through a special rail placed between the ordinary rails and insulated from the ground by blocks of wood. From this rail it passed through a motor-dynamo on the car, and the ordinary rails completed the circuit. Electrical contact with the ordinary rails was made by the wheels, and with the central rail by a pair of brushes made of copper wire which rubbed against its sides. Spur-wheels were used to connect the motor shaft with the wheels and to effect a suitable reduction of speed. The line was half a mile long and of 2-feet gauge. The motor developed about 3| horse-power, and was carried by a separate truck, forming a locomotive which drew a car with 20 passengers at a speed of from 4 to 7 miles an hour.

The success of the Berlin experiment was complete, and Messrs Siemens followed it up in 1881 by the construction of a permanent electric tramway, 1^ miles in length, at Lichterfelde, which has now (1887) been in continuous operation for six years. At Lichterfelde the ordinary rails, insulated by wooden sleepers, are the only conductors. Where roads cross the line the rails are cut out of circuit, and the current is carried past the gap by underground cables, but switches are provided by which the current can be sent into the insulated sections if re quired. Each car takes 24 passengers, and runs at a speed of 12 miles an hour. There is no separate locomotive, the motor-dynamo being on the car itself. In 1882 Messrs Siemens constructed an electric tramway in the mines of Zankerode, in Saxony, and built for it a locomotive able to draw 8 tons at a speed of 71/2 miles an hour. Overhead conductors were employed, consisting of a pair of insulated I-shaped rails fixed to the roof of the workings; the current was conveyed to and from the locomotive by means of a pair of contact carriages sliding on these conductors, and connected with the car by short flexible cables. A similar line was opened in 1883 at the Hohenzollern colliery in Upper Silesia.

The same year witnessed the completion of another pioneer undertaking of the first importance, an electric tramway 6 miles long connecting Portrush and Bushmills, in the north of Ireland. Here the insulated conductor is a special rail, carried alongside of the line on wooden posts at a height of 11/2 feet above the ground. Contact is made by springs shaped like carriage-springs, which project from one side of the car at both ends, so that the length of the car enables continuous contact to be maintained at cross roads, where there are gaps in the conducting rail, past which the current is taken by underground cables. The ordinary rails serve as return conductors. The dynamos are driven by turbines at a station nearly a mile distant from the line; they supply a current of 100 amperes with an electromotive force of 250 volts. The motors are placed on passenger cars; their speed is regulated by means of resistance coils, which the driver of the car switches into the circuit. A similar tramway, 3 miles long, connecting Bessbrook and Newry, was opened in 1885; there also water-power is made use of to drive the generating dynamos. On these lines the train usu ally consists of a motor car with passengers, followed by two or three goods waggons, and the whole working expenses are from 3d. to 4d. per train-mile. The speed is 10 miles an hour.

Amongst early electrical railways Mr Volk's short line on the beach at Brighton deserves mention. There the rails themselves act as conductors, and are insulated only by wooden sleepers lying on the shingle. The line has been in operation since 1883, with a working expense of only 2d. per car-mile.

Other English and Continental lines will be referred to later; it is, however, in America that electrical traction has hitherto found its widest development. In 1880 Mr Edison ran an electric locomotive on an experimental track near his laboratory at Menlo Park. Soon after the Chicago exhibition of 1883, at which an electric railway was shown in action, a large number of permanent lines were established. There are now more than twenty electrical tram ways at work in the United States, under the patents of Edison, Field, Daft, Van Depoele, Sprague, and others. Many more lines are projected, and experiments are in progress on the application of electrical traction on a large scale to the elevated railways of New York.

Electric traction by storage batteries.In all the instances which have been referred to above, electricity is employed as a means of transmitting power as it is wanted from a generating station to the cars, through a conductor extending along the track. Another method of effecting electric traction is to carry a store of energy on the car or on a special locomotive, by using secondary batteries which are charged from time to time at the generating station. This system, which was introduced in England by Mr Reckenzaun and on the Continent by M. Julien, has been successfully employed on several lines.

The system of storage, by means of secondary batteries, has the great advantage over the system of transmission through a conductor that it makes each car independent and that it is applicable to ordinary tramway lines. As regards economy of power, we have in the storage system a more complex series of transformation of energy, and therefore a larger number of items of loss. In both, systems alike we have a certain loss of energy at the dynamo and at the motor. A secondary battery yields in the electrical form only about 70 per cent, of the energy given to it. In comparing the two methods, the loss which this involves has to be set off against that which occurs in the transmission system in the process of conduction, an item which may be very small in favourable cases, but which becomes large when there are many cars to be driven, when the line is long, and when, owing to the use of an exposed conductor, the electromotive force has to be kept low. Under average conditions it is probable that the conductor system has a slight advantage over the other in this respect, but the difference is not material, especially as the cost of power is a comparatively small part of the whole working expense of a line. The difference is slightly affected by the fact that in the storage system there is an extra weight to be carried namely, the batteries amounting to about 1/4 or 1/5 of the whole weight, and the tractive force required to overcome friction is increased in a corresponding degree. A serious objection to the storage system is the probable cost of renewing batteries. In respect, however, both of durability and of power (in relation to weight) secondary batteries have of late undergone a marked improvement; and it is likely that the storage system will prove the most applicable to tramways in city streets, where conductors on the level of the road are impracticable and overhead conductors would not be permitted.

Classification of systems.The existing methods of electrical traction as applied to tramways may be classified as follows:—

I. Motor driven by storage batteries, the batteries and motor being carried either (a) in the car itself or (b) on a separate truck forming a locomotive. Eeckenzaun's and Julien's cars, in which the batteries are under the seats, are examples of the first plan, which is in operation on lines at Antwerp, Hamburg, Brussels, and New York. Mr Elieson's tramway locomotive working in London on the North Metropolitan tramways is an example of the second plan. It is obviously preferable, when space can be found on the car itself for the motor and batteries, to place them there rather than on a separate truck. When a separate locomotive is used it must be heavy enough to grip the rails, and the whole weight to be drawn is then considerably greater.[1]

II. Conductor systems, which may be classified thus:—

(a) Those using the ordinary rails as the only conductors. The lines at Lichterfelde and Brighton, already mentioned, are examples of this plan, which is quite inapplicable where the rails are laid flush with the roadway as in city streets.

(b) Those using a third (insulated) rail, above ground. To this class belong the Portrush, the Bessbrook, and several American lines. This plan, like the last, is not applicable to city streets.

(c) Those using one (or in some cases two) overhead conductors. A line of this type has been successfully worked between Mödling and Hinterbrühl, near Vienna, and another between Frankfort and Offenbach, both since 1884, at a cost of about 31/2d. per car-mile. The conductors consist of slotted tubes 1 inch in bore supported on posts 18 feet high and stayed by wires at intermediate points to keep them from sagging. The contact carriages are pistons sliding in the tubes.[2] The Daft lines at Baltimore and other places in America, and the Van Depoele lines, of which some 30 miles are in operation, are mostly worked by means of overhead conductors.

(d) Those using underground conductors in a slotted channel or conduit. This system, which has the obvious advantage that the conductor is placed entirely out of the way of street traffic, has been introduced at Blackpool by Mr Holroyd Smith, and, in America, at Cleveland by Messrs Bentley and Knight and at Philadelphia by Mr Schlesinger. In the Blackpool line the conductor is split into two parts which run parallel to each other within the conduit on its two sides, and are touched by a contact arm which reaches down through a narrow central slot at the level of the street; an electromotive force of 200 volts is employed. The conduit is placed midway between the rails, but it may be questioned whether, in view of the conditions of ordinary street traffic, a better place for it would not be at one side. Mr Field has proposed a tramway with two conduits, one beside each rail, containing two conductors, one to be charged positively and the other negatively, so that a comparatively high resultant difference of potential is available for the motor although the potential of neither conductor differs to a dangerous degree from that of the earth.

(e) One system remains to be described, which was proposed in 1881 by Messrs Ayrton and Perry as specially applicable to electric railways of considerable length, in which an exposed conductor would give rise to much loss through leakage. Their plan is to use a well-insulated conductor in a closed channel underground. The line is divided into short sections; each of these has an exposed conductor, which may be one of the rails, and this is placed in temporary contact with the insulated conductor as the train passes, by the pressure of the wheels on a flexible rail or stud, or by means of automatic electromagnetic switches. Leakage is thus restricted to the continuous and well-insulated conductor, together with that section of the surface conductor which is in contact with the former at any one time; and the system has the further advantage that it gives the means of providing an automatic block by which successive trains are kept from overtaking one another.

The form and disposition of the motor-dynamo and the mode by which it is connected with the driving-axle of the car are matters in which much variety of practice exists. The question of gearing is complicated by the fact that the frame of the car oscillates vertically with respect to the axles. Spur-wheels, worm-gear, friction gear, belts, multiple-band gear, and chain-gear are or have been used. Mr Reckenzaun's car is carried by two bogie trucks, one under each end, and each bogie carries a motor whose axle, placed longitudinally, drives a central spur-wheel on one axle of the bogie by means of a worm. An advantage possessed by two motors is that, by coupling them in series or parallel, or by using one only, the driver is able to command different grades of power without the use of resistance coils. In cars driven by storage batteries the same object may be secured by various groupings of the cells.

Telpherage.Telpherage.—In all the methods of electrical traction to which reference has been made the road on which the cars run is essentially a railway or tramway of the kind used in horse traction and steam traction. In 1881 the late Prof. Fleeming Jenkin devised a system of electric locomotion in which the vehicles are hung upon what resembles an exaggerated telegraph line. To this he gave the name of telpherage. As developed by the inventor, in conjunction with Messrs Ayrton and Perry, the system is especially adapted to the transport of goods at a slow speed, in localities where the traffic would be insufficient to support an ordinary railway.

The telpher line is a steel rod or cable, suspended from brackets on posts about 70 feet apart; it serves at once as carrier of weights and conductor of electricity. The line may be made rigid, and in that case a^high speed of transit may be attained; but in general the line is flexible and the trains travel slowly in what may be, if the volume of traffic requires it, a nearly continuous stream. Each train consists of a series of buckets or skeps which hang each from a single running wheel or pair of wheels, and are spaced by wooden connecting bars. A small electric motor, which hangs below the line and is geared by spur and chain gearing to a pair of driving-wheels, forms the locomotive. In general, the line is electrically divided into equal sections, which have the same length as a single train, so that the front carriage is always on the section in advance of the rear carriage. The train is furnished with a continuous conductor from end to end, through which it makes electric contact between the section in front and the section behind, and the motor is included in the circuit of this conductor. Two systems of working are used, which enable trains to be run either in electrical series or "parallel." In the series system the successive sections of the line are electrically connected, so long as no train is on them, by means of switches at the joints between the sections, so that the whole forms one continuous conductor. When a train comes on any one section it breaks contact at the joint between that section and the one behind it; the circuit, however, remains closed through the conductor on the train itself, and in this way the motor receives the current which is passing through the line. Other trains at other places in the line receive the same current, each by breaking for the time the ordinary contact between the two sections it touches, and substituting a contact through its own conductor and motor. When a train leaves a section it replaces the switch that makes contact with the section behind. If, however, there are more than one train on the line, an automatic block system is added to prevent one from overtaking another by letting the section which a train leaves stand insulated for a time. No control is exercised from the vehicles themselves; in fact, the trains run without attendants. In the simplest parallel system of telpherage a continuous conductor distinct from the line is stretched alongside of it; the trains make contact between the two. The figure shows another plan, known as the cross-over parallel system, which is suitable where a double line of trains is desired.

Cross-over Parallel System of Telpherage.

There A1, B2, A3 . . . form successive sections of one line, and B1, A2, B3 {{...|3)) of another. A1 A2, A3 . . . are electrically continuous, and are connected to one pole of the dynamo. B1 B2, B3 . . . are also continuous, and are connected to the other pole. Thus the sections of each line are alternately positive and negative. Any train, such as P or Q, bridges the gap between two sections and receives a current which suffers reversal as the train passes from one section to the next. It is to be regretted that space does not admit of any description of the details of telpherage, many of which present the utmost ingenuity. The system was shown to be practicable by experiments on an experimental line at. Weston. The first telpher line on a commercial basis was erected in 1885 at Glynde, in Sussex, and has been maintained in operation notwithstanding many difficulties inseparable from so completely novel an undertaking.

Propulsion of boats.The electrical propulsion of boats, by means of storage batteries, has been the subject of several successful experiments, but has not found systematic application. In this connexion reference should be made to a scheme proposed by Ayrton and Perry for the haulage of boats on canals or of waggons upon roads. Their proposal was to have a conductor ranged along the towing path, or along the side of the road. A motor running on this was to pull itself along and drag the boat or waggon after it.

Aerial navigation.In aerial navigation, storage batteries working an electric motor have been used to drive the propeller of a "dirigible" balloon.

Theory of motors.Space does not admit of more than the briefest reference to the theory of electric motors. A motor may be regarded as a dynamo acting to produce an electromotive force e which is opposite in direction to the externally impressed electromotive force E. The resultant electromotive force is E–e, and on this, together with the resistance of the circuit, the strength of the current C depends. The electrical power supplied is CE, and of this the motor utilizes Ce. The efficiency is e/E. It is easily seen, as was first shown by Jacobi, that the power developed by the motor (Ce) is a maximum when c = 1/2E. But this condition of maximum power involves that half the energy supplied is wasted; to secure higher efficiency, motors are in practice run at much less than their maximum power, so that e may approach more nearly to equality with E. The field magnets of motors, like those of dynamos, may be wound with coils in series with the armature coil, or with coils forming a shunt to the armature, or with a combination of both. A very important part of the theory deals with the automatic regulation of speed by the use of compound winding. In a paper of fundamental importance with regard to this part of the subject, Messrs Ayrton and Perry[3] have shown that a motor may be made to run at constant speed under varying loads when the external electro motive force is constant, provided that a differential combination of direct shunt and reverse series winding be employed, the shunt coil serving to energize the magnets and the series coil to reduce their magnetism to a certain extent when the current in the armature is increased. The proportion of series to shunt winding necessary for this result depends on the relation of the resistance of the armature to that of the shunt coil, and it is an easy deduction from the theory that, when the resistance of the armature is negligibly small, the speed of a simple shunt- wound motor driven by means of a constant external electromotive force is sensibly constant, a result which has been experimentally demonstrated by Mr Mordey (Phil. Mag., Jan. 1886). It is shown in the same paper that a similar means of governing may be used when the current passing through the motor is kept constant, instead of the external electromotive force. The principle of differential compound winding to secure automatic regulation of speed has been applied in several American motors, notably by Mr Sprague.

Details of most of the electrical tramways and railways mentioned in the text will be found in the journals Electrical Review, Electrician, and Electrical World (New York) of the dates referred to. See also The Electric Motor and its Applications, by T. C. Martin and J. Wetzler (New York, 1887). The Portrush line is described by E. Hopkinson and A. Siemens in a paper read before the Society of Arts, April 1883. For telpherage, in addition to articles in the journals named, see Fleeming Jenkin, "On Telpherage," Jour. Soc. Arts, May 1884; also Professional Papers of the Corps of Royal Engineers, Chatham, vol. x., 1884. (J. A. E.)

  1. For a comparison of the weights to be drawn and the tractive force required in different systems, see a paper by Mr Reckenzann, Elect. Rev., May 21, 1886.
  2. For details of the construction and working expenses of these and other lines, see the valuable paper by Mr Reckenzaun, Jour. Soc. of Arts, April 20, 1887. Statistics of American lines will be found in a paper by T. C. Martin, read before the American Institute of Electrical Engineers, May 18, 1887.
  3. " Electromotors and their Government," Jour. Soc. Tel. Eng., 1883.