Page:Popular Science Monthly Volume 74.djvu/223

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The single-phase motor has the same characteristics and operates exactly as the direct current motor and may in fact be operated by direct current—a fact which constitutes one of its greatest advantages. It is, however, heavier for the same output and since it, too, operates at low voltage, a stationary transformer is required on the car or locomotive to reduce the high trolley voltage to the value required. This necessitates a heavier and more expensive motor equipment than the direct current system and acts as an offset to the saving effected in feeding conductors and sub-stations. In this system the feeding conductor is the overhead trolley with catenary suspension and the collecting device is the sliding trolley or pantagraph which is necessary for very high speed and permissible by reason of the low values of current required.

The three-phase system owes its principal value to the fact that the speed variation of the motor is very small throughout its full range of tractive effort. As already stated, the tractive effort of the direct-current and single-phase motors falls off with increasing speed, though not so rapidly as that of the steam locomotive. Owing to this advantage, the three-phase locomotive can maintain its high speed independently of the grade. It operates without transformers on the car with trolley voltages up to 5,000 and in coasting it returns power to the line automatically, its motors acting as generators. It is, however, heavy per unit of output and the system requires two trolley wires and is not adapted to operation on the direct-current installations to be found in many terminals. It has been adopted for one installation in this country in which it is desired to increase the schedule speed on a long mountain division.

In this country the best engineering opinion seems to have united in thinking that the single-phase system is the one best adapted for future application to steam railways. This system is as yet only four years old, yet there are at present over 1,000 miles of railways in the United States operating under it and in this aggregate there are at least five railroads formerly operated by steam. The system has proved most successful in operation, although the first six months' operation of the New York, New Haven & Hartford Railroad have developed so many unforeseen troubles, when applied to such a large enterprise, as to bring upon it much adverse criticism. On the other hand, the high degree of perfection to which the direct-current system has been brought, the greater capacity of the motors of this system and the enormous mileage already installed in tunnels and terminals have resulted in a strong advocacy of this system. Speaking generally for motors of equal weight, that of the direct-current system has 25 per cent, more capacity than that of the alternating-current system. The equipment of a high speed interuban car having four motors of approxi-