The Generation and Distribution of Cur the earlier roads, and, in fact, in many of the roads now operated in this coun try, the current for the motors is obtained from direct-current machines, having a voltage of between 500 and 600 volts. These machines are, as a rule, compounded so that their voltage in creases with an increase of load. There were two reasons for this: One is that the increased voltage compensates to a certain extent for the loss of voltage in the distributing system; the other, and most important reason, is that the strengthening of the field prevents the sparking that might otherwise be caused by the armature reactions. It is the custom to ground the nega tive side of the machines and connect the trol ley wires through what is called °feed wires" to the positive brushes. These feed wires are carried along the line, connected at intervals to the trolley wires, it being usually the custom to divide the road into sections and feed the different sections from different feed wires. At the voltage which has been adopted as the standard— that is, from 500 to 600 volts— the economical distribution when the service is heavy is limited, and if a direct-current system is to be employed for a large area, a number of stations must be installed. For this reason, when the distances are great, it is sometimes necessary to employ a high potential alternating system, reducing it to a 500-volt continuous-cur rent system at centres of distribution called sub stations. On interurban lines, for instance, where the distances are great compared with the number of units in operation, this method becomes imperative, as otherwise the variations of load on the central station would be exces sive, and the cost of power would be practically prohibitive. This method of operation may be described as follows: The central station, lo cated at some point where the greatest economy of operation can be obtained, generates alternat ing currents at a high potential; these alter nating currents are taken to the sub-stations, situated at reasonable distances from one an other along the line, and are there changed to alternating currents of lower potential by means of transformers and then converted into direct currents of the proper potential by means of rotary converters. In this way a large area can be supplied from one central station and a comparatively large number of units can be operated at one time from this station, thus giving conditions of maximum economy. On the sub-stations themselves, there is, of course, a fluctuating load and it is the custom to install storage batteries in connection with the rotary converters to equalize the load; the battery stor ing energy when the load is light and giving it out when the load is heavy.
The Return Circuit and In most of the overhead traction systems, the cur rent passing through the motors goes through the wheels of the car to the rails, and then returns to the station, °bus bars° finally.appear ing at the negative brushes of the dynamos. When a system is operated in this way, the railc are bonded— that is, two adjacent rails are connected by conductors securely attached to the rails and giving a low resistance to the electric current. If the bonding is good, a considerable part of the current passes along the rails while a small portion strays through the earth to other conductors, such as water pipes and gas mains, using these as a return conductor. If the bonding is bad, however, a considerable portion of the return current is carried by the underground pipes, causing them at times serious injury. For this reason, and
also because of the fact that low conductivity in a rail return causes loss of energy, it is of importance that the bonding of the rails should be as thoroughly perfected as possible; it should also be carefully inspected at intervals, as the bonds are liable to break and produce a high resistance between the adjoining rails, thus diverting a considerable amount of current to the underground pipes. The matter of the elec trolysis of underground metallic structures re ceived little attention during the first two or three years of electric railroad development; the effect, however, naturally increased with time and the development of the systems until the serious damage that occurred became evi dent, and means for preventing it was sought These means, as a rule, consisted in connecting different points of the rail circuit to the negative bus-bars of the station, by return feeders, and in making metallic contacts between the rail and pipes at points where the current was leav ing the pipes; also in metallically connecting the pipes near the station with the negauve bus-bars. No electrolytic damages is done when the cur rent leaves the pipe through a metallic conductor ; it is only when the current passes from the pipe through an electrolyte, such as is furnished by moist soil, that corrosion takes place. In some places, the joints of the pipes have been insu lated to prevent the current from passing through them and this is perhaps the most efficient means of avoiding electrolytic troubles.
The most important drawbacks of the induction motor are these; In the first place the motor has practically the same characteristics as a direct current shunt motor and, therefore, does not afford the variation of speed with load that is one of the most important and valuable features of a series motor. In the second place three circuits are necessary to operate the machine and while the track can be used for one of these circuits, the necessity of two wires over each track, the wires having a con siderable difference of potential, makes the prac tical operation of such a traction system diffi cult. Numerous systems have been devised for regulating these motors, but they are either com plicated or inefficient and it is the opinion of the writer that induction motors will not be used for railroad work except under special conditions. The only advantage such a system offers is the high potential that can be used and this advantage is neutralized by the fact that the three conductors necessary make high potentials practically unavailable. Electrifica tion offers a wonderful opportunity for saving in capital expenditure, where expanding de mands new facilities which by other means would be inordinately expensive to provide. The higher sustained speeds of electrically hauled trains on heavy grades offers an ir resistible opportunity to expedite traffic and the flexibility of make-up and movement of multiple-unit suburban trains is a most valu able consideration. The fact that electricity as a source of railroad motive power offers the only- possible connecting link between the water fall and the locomotive gives it an unique po sition, analogous to that of alternating current in the general power distribution field. It is more difficult to prove out the economy of elec tric operation on this basis, but it is certamly sound over-all conservation to use to the limit g form of motive power that cuts clown mate rially the rate at which the supplies of coal and fuel oil are being depleted.