Electric railways

by American School of Correspondence; James Raley Cravath

While, as stated, this furnishes a fairly satisfactory solution of the problem, it is far from perfect, as it necessitates the intervention of the rotary converter substation, in which the investment must be large; and moreover the cost of operation is high, as such a station requires skilled attendance on account of the somewhat intricate nature of the rotary converter. The ideal system, therefore, is one which does away altogether with the use of direct current, the power being generated, distributed, and utilized by the motors, as alternating current.

Three-phase induction motors have been used quite extensively and with considerable success in Europe for many years past. The three-phase motor, however, is not entirely adapted for railway work, since it possesses the characteristics of the shunt rather than of the series motor, being a constant speed, not a variable speed machine. Moreover, two trolley wires are necessary instead of one, and still another disadvantage consists in the low power-factor of the three-phase induction motor at starting.

The recent application of the single-phase alternating current to railway work has opened up a new field, which bids fair to supplant all other forms of distribution to a great extent at least, and it is impossible to predict at the present time just what its limitations may or may not prove to be. This has been made possible by the development of a practical commercial single-phase motor, which permits of the use of alternating current on the trolley wire with all its advantages, and yet sacrifices few, if any, of the advantages of the direct-current series motor on the car.

INTERIOR OF SUB-STATION SHOWING ROTARY CONVERTER AND TRANSFORMERS.
The three-phase current is delivered to the transformers where it is stepped down to the voltage required for the rotary converter. In this machine it is transformed to direct current and delivered to the trolley wire.

This motor, which is the latest and most important development in the electric railway field, is of the series commutator type, and does not differ in principle from its direct-current contemporary. It is called the commutator type single-phase motor, and is the one type of alternating-current motor which has the same desirable characteristics for railway work as the direct-current series motor.

Compensating Alternating-Current Railway Motor.

At first thought it may seem strange that a motor built fundamentally on the same lines as a direct-current machine would operate on an alternating current, as it might appear that the motor would tend to turn first in one direction and then in the opposite direction with no resultant motion. This, however, is not the case, because the direction of rotation of a motor depends upon the relative direction of its field and armature currents. If now the field were maintained in a constant direction and the armature supplied with alternating current, then the tendency would be to rotate first in one direction and then in the other, it is true, but as a matter of fact the alternating current is supplied to the field in series with the armature, so that when the direction of current in the armature changes it also reverses in the field. The result is that the relative direction of current in the field and armature is constant and the motor has, therefore, a tendency to turn continuously in one direction as long as the alternating-current power is supplied.

This being true, the question may arise as to why the single-phase motor was not brought to the front for railway work long ago. The answer is that there were certain inherent difficulties to be overcome, and the development of the single-phase motor has been simply the removal of these difficulties, rather than the design of an entirely new type of machine.