TRACTION, Electric. The history of the development of electric traction follows closely that of other great technical developments— starting with experiments on a small scale, hav ing no practical outcome, followed by experi ments on a larger scale, showing the physical possibility of the system, and finally using the experience so obtained in a successful, commer cial application of the system to practical oper ating conditions. It was not until the invention and development of dynamo-electric machines made it possible to obtain' electrical energy at a reasonable cost that the commercial application of electric traction became possible, and even after that, electric machinery had to be im proved before electricity could compete with the existing means of traction. Among the first to study the problem seriously were Siemens, in Germany, and Edison, Field, Daft and Vande poele in this country. Before 1884 a few roads had been built for exhibition purposes and from that time until 1888, although some small roads were equipped with a few cars, yet it had not been proved that electricity could be profitably substituted on tramways for cable or animal traction. In February of the Sprague Electric and Manufacturing Company finished the equipment of the tramways in Richmond. Va., with the Sprague Electric Railway system, and operated the road so successfully that rail road managers from other parts of the coun try were convinced that they had something, to substitute for animal traction that would in crease their facilities for transportation and de crease the cost of operation. From this the development of electric roads in the United States has been rapid. At first horse tramways were electrically equipped, then cables were dis placed by electric trolley roads, then the tram ways were extended further to the suburbs and surrounding towns, and towns were connected by interurbsi roads equipped electrically, run ning at high speeds and offering a sernce im possible with steam locomotives. For city train service on elevated roads, the substituuon of electricity has taken a longer time, as the steam locomotives were efficient and as the changes involved an expenditure which would not have been, advisable unless decided advantages with respect to service and cost of operation were shown.
Direct-current Railway Motors.— On all the electric roads in this country, and on a large proportion of those abroad, direct-current mo tors are used for traction. These motors are of the series type, and are usually connected to the axle through a single spur gearing. The charac
teristics of the direct-current series motor are admirably adapted to traction work. The speed is dependent upon the tractional effort, and the motor slows down when the tractional effort increases, having something of the effect of a variable speed gear; this is very important in practical railway work, for if the motors ran at a constant speed, the energy demanded on grades would be greatly exaggerated and much larger motors would be required. It is also true that this type of motor gives a greater flexibility to the system, as far as speed is con cerned, and this flexibility is important in prac tical operation. To show the difference between a series motor and a shunt motor, suppose the track resistance is 10 pounds per ton weight of the car ; then if the speed were constant (as would be the case with a shunt motor), the power required on a 5 per cent grade would be 11 times as much as on a level— air re sistance being omitted — while with a series motor it would be only three or four times as much.
It might be well to briefly trace the develop ment of electric railroad motors in the United States. The motors used by Sprague in the equipment of the Richmond road had too small a capacity for the work required and very often burned out. They were two-pole machines, of a rated capacity of 7.5 horse power, and at first cirove the axle through a single reduction gear ing; the work, however, was so heavy that it was found necessary to change to a double re duction gearing, the teeth of the pinion stripping on the heaviest grades. These motors were un protected from moisture and from the dust and dirt of the streets. The methods of suspend ing the motor and some of the details of the regulation are still retained. The field magnets of the motor were sleeved on the axle, thus centring the armature on the axle and al lowing a satisfactory relation between the two. The first motors had a double commutator, which was soon abandoned, as it greatly in creased commutator troubles, which were at that time serious. The brushes first used on these niotors were of copper and a number of forms were experimented on, it being necessary to re verse the motor without injuring the commu tator. None of the experiments with copper brushes were successful, and it was not until the intrOduction of the carbon brush by the Company that the commu tator difficulties were largely obviated.