ONE OF THE SINGLE-PHASE LOCOMOTIVES ON THE NEW YORK, NEW HAVEN & HARTFORD RAILROAD CO.
Note the two pantograph bow trolleys for collecting the current.
THE SINGLE-PHASE ELECTRIC RAILWAY.
In no other line of electrical activity have developments during the last few years been so rapid as in that of electric railway work, and from all indications the limit has not yet been reached.
Until recent years all electric traction has been dependent upon direct current as a motive power. This is due principally to the fact that the series direct-current motor is admirably adapted for such work, and no alternating-current motor had been developed which could be substituted for it. One of the great advantages possessed by the direct-current series motor is its large starting torque, which may be several times greater than that required to propel a car at full speed. This type of motor is also essentially a variable speed machine, and lends itself very well to wide variations in speed control; consequently, for many years, in this country at least, all advance was made along direct-current lines.
The trolley voltage used at first was from 450 to 500 volts, this being supplied directly to the cars by means of a trolley wire, the rails being used for the return circuit. It is evident from the outset that the comparatively low voltage, necessitating as it did a correspondingly large current for a given amount of power, would place a definite limitation on the use of such a system for anything other than purely local distribution. To overcome this difficulty as far as possible, the trolley voltage was gradually raised to 600 or 650. This of course decreased the required current, thus increasing the scope of the system accordingly. The limit of increase of direct-current voltage on the trolley was reached at about this point, and the fact was recognized that some means must be devised for using a still higher voltage, since there are difficulties to increasing the trolley voltage beyond 600 or 700, due to flashing of the motors, which seems to increase directly with the voltage.
It may be mentioned in passing that one prominent electric traction expert has stated that a direct-current trolley voltage of 1500 can be used, but it remains to be proven whether or not he is correct. A very satisfactory solution of the problem for large city street railway systems and long interurban roads, consists in the use of a combination alternating-current direct-current system in which three-phase high tension alternating current is generated and distributed on high tension lines to substations along the road. It is here stepped down by means of transformers, and then changed to direct current by rotary converters, and supplied to the trolley wire as direct current at the usual voltage of say 600. This system has many advantages, as there is but small loss in the high-tension lines, and these lines can be made comparatively small, thus effecting a considerable saving in investment for copper.
The above mentioned system of distribution is very generally used, and has been found quite satisfactory. The substations can be located at frequent intervals, and the distance that the 600-volt current must be conducted to supply the cars is not great. By this means current can be distributed over wide areas with a small loss, where it would be impossible to use the straight direct-current system of distribution.