Electric railways

by American School of Correspondence; James Raley Cravath

ALTERNATING-CURRENT SYSTEMS.

So far this paper has been devoted almost entirely to electric railway systems employing 500-volt direct-current motors on the cars, since this is the system almost universally employed on electric railways at the present time. There are, however, several systems employing alternating-current motors on cars, which have already been used experimentally and to some extent commercially. Some of these give promise of coming into extensive use.

Three-Phase Motors. On several roads in Europe three-phase induction motors are employed. These induction motors are operated by three-phase alternating current taken direct from the trolley wires. As three conductors are necessary, two trolley wires are used, with the rails as the third conductor. The two principal objections to the system are the necessity of two trolley wires, and the fact that the induction motor operates very much like a direct-current shunt motor in that it is a constant-speed motor and not adapted to variable-speed work. The power factor is low in starting; that is, a great volume of current is taken, although, owing to the voltage and the current not being in phase, the actual energy consumed is small.

Single-Phase Motors. The Westinghouse Electric & Manufacturing Company has brought out a railway motor adapted to operate on single-phase alternating-current circuits. This motor is very similar in construction to the ordinary series-wound 500-volt direct-current railway motor. It has, however, more field poles than the ordinary direct-current motor; and the pole pieces are laminated to avoid heating of the iron by eddy currents caused by the influence of the alternating current. There are also other special features in the design that reduce the sparking at the commutator, which sparking was for several years the greatest obstacle to the use of alternating-current motors of this kind. In the Westinghouse system the current is taken from the trolley wire at high potential, and is reduced by an auto-transformer on the car. This auto-transformer is connected with an induction regulator so arranged that a low voltage can be supplied to the motor in starting or for slow running, and this voltage increased to increase the speed. There is thus no need to reduce the trolley voltage by wasting part of it in a rheostat, as is the case with direct-current motors; and the efficiency during acceleration is, therefore, higher with this alternating system than with the direct current. Several other single-phase railway motors are also being worked out at the present time, including that of the General Electric Company.

Alternating-Current Motor Advantages. There are two great advantages secured by the use of an alternating-current railway motor. The first is a reduction in investment and operating expenses by doing away with substations containing rotary converters. Such substations are necessary on long lines of railway operating with direct-current motors. The second advantage is that, owing to the fact that a high tension current can be used on the trolley wire and reduced by a transformer on the car, the difficulties of collecting a large amount of energy from a trolley wire are much reduced.

First, in regard to the substations, it will be seen that with the alternating-current motor system, high-tension current can be conducted from the power house to substations along the line which contain nothing but static transformers. Since these transformers have no revolving parts they do not require the constant attendance that a rotary converter does. Furthermore, the investment in rotary converters is entirely dispensed with, and this makes a considerable reduction in the total cost of the distribution plant. With the alternating-current system, current is fed direct to the trolley wire from the secondary terminals of the transformers at the substations.

As regards the advantages of carrying a high voltage on the trolley wire, it will readily be seen that, since the amount of power, or the watts required by a car, is equal to the product of the voltage and current, an increase in the voltage reduces the volume of current necessary. By having high voltage on the trolley wire, even a large car can be operated with a small volume of current, and this current can be taken through an ordinary trolley wheel without difficulty. Where 500 volts is the pressure used on the trolley wire, there is considerable flashing and burning of trolley wheel and wire when large cars and locomotives are run, owing to the heavy current conducted; and this has been one of the principal reasons for the adoption of the third rail instead of the trolley on certain roads. Even with the third rail, the volume of current that must be conducted to large electric locomotives involves some difficulties in the way of heated contact shoes and considerable loss of energy. The use of high voltage on the trolley wire, with transformers on the car to reduce the voltage to a safe pressure for use on the motors, overcomes many of the difficulties that would otherwise be found in the use of electricity for heavy railroad work.