Page:Electrical Engineering Volume 1.djvu/355

Rh 2801. The advantages of this system are the absence of the overhead wires or underground conduit of the trolley roads, the complete independence of each car, and the ability of the dynamos which charge the batteries to run uniformly at their full output.

2802. A sufficient number of accumulators are usually carried on one car to run it about 30 miles, or for about six hours, with one charging. Such a battery weighs about 4,000 to 4,500 lb., increasing very materially the weight of the car, which ordinarily weighs, with passengers, about 10,000 lb. The power required to propel this extra weight must be provided, and the wear of the tracks and car-trucks is increased.

2803. In order that the cars shall not stand idle while its battery is being charged, several sets (about 3 sets to each car) are provided, which makes an expensive equipment, and the cost of handling the heavy batteries, when moving them into and out of the cars, is considerable.

2804. The chief disadvantage, however, is the rapid deterioration of the plates. On starting the car, and in ascending steep grades and rounding curves, the accumulators are called upon to furnish currents far in excess of their normal discharge rate, which, added to the continual jolting to which they are subject, makes the disintegration of the positive plates very rapid indeed; and only when the plants have been under the charge of skilled experts has accumulator traction been at all successful in this country.

2805. The Waddell-Entz accumulator would seem to be especially suited to traction work, on account of its light weight and capacity for high rates of discharge; but to operate efficiently, this accumulator must be charged and discharged under special conditions (see Art 2790), which makes its cost of operation high.

2806. Accumulators have been very successfully applied to launches and other small boats, in which the propeller is driven by a suitably connected motor; the battery is