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.
OPERATION.
Power Taken by Cars. The amount of power required in the practical operation of a car depends upon so many variable elements that many of the calculations sometimes given for determining the power required by a car are of little value. The theoretical horsepower required to maintain a car at a certain speed on a level, is evidently the tractive effort in pounds multiplied by the speed in feet per minute and divided by 33,000. What the tractive effort per ton of car will be, depends on the condition of the rail and on several other uncertain factors. For street-railway motor cars, 20 pounds per ton is the usual tractive effort assumed as necessary. A calculation of this kind, however, takes no account of the losses in the motors and gears, nor of the fact that the greater part of the power required to propel a street car in practical service is used in accelerating the car from a state of rest to full speed. In interurban service, of course, the power required in acceleration is not so great a proportion of the whole.
Fig. 93. Plotted Data of Road Test.
The safest figures to use in engineering calculations as to the amount of power required, are those taken from actual results obtained in everyday commercial service. The power required by an eight-ton car in service in a large city like Chicago, is in the neighborhood of one kilowatt hour per car-mile run. On outlying lines this figure may be reduced to .7 kilowatt hour, and in the down-town districts may run up to 1.5 kilowatt hours per car mile. Double-truck cars in city service, weighing from 20 to 25 tons, take from 2½ to 4 kilowatt hours per car mile at the power station. Interurban cars around Detroit, weighing about 32 tons, in interurban service, making 25 miles per hour, including stops, in level country, and geared to 43 miles per hour, take about 3 kilowatt hours per car mile at the power station. However, interurban railway conditions are extremely variable.
The reports of several Indiana electric railways show an average power consumption of 1.48 kilowatt hours per car mile for city cars and 5.18 kilowatt hours for interurban cars, including line and distribution losses.
An interurban car weighing 31½ tons and equipped with two 150 horsepower motors, on a test run of 50 miles at an average speed of 39 miles per hour consumed 2.20 kilowatt hours per car mile. This car made 18 stops. A similar car under the same conditions made the same run at an average speed of 26 miles per hour with 44 stops, consumed 2.44 kilowatt hours and a third car, making 12 stops and at a speed of 33 miles per hour, consumed 2.10 kilowatt hours per car mile. These individual car test figures are from measurements taken at the car and do not include line losses.