Identifier: cyclopediaofappl05amer (find matches)
Title: Cyclopedia of applied electricity : a general reference work on direct-current generators and motors, storage batteries, electrochemistry, welding, electric wiring, meters, electric lighting, electric railways, power stations, switchboards, power transmission, alternating-current machinery, telegraphy, etc.
Year: 1916 (1910s)
Authors: American Technical Society
Subjects: Electric engineering
Publisher: Chicago : American Technical Society
Contributing Library: Northeastern University, Snell Library
Digitizing Sponsor: Northeastern University, Snell Library

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uch tests. The curves illustratethe important fact that the speed falls off more slowly at the highspeed, due to the lower coefficient of friction. With air brakes itis possible to so graduate the air pressure as to produce a practicallyuniform deceleration if there is any particular reason for havingthis. The curves show clearly the duration and the distance ofbraking in each case. The very quick stops, or emergency stops,show what can be done if necessary, although such stops are uncom-fortable for the passengers and racking to the equipment. Therate of braking in these cases averages 3.3 m. p. h. p. s., while thatfor the slower stops is 2.3 m. p. h. p. s. The gross braking forces are,respectively, 120 and 84 pounds per ton. Coasting. WTienever a motorman finds that he can make satis-factory speed without power, he cuts the latter off and coasts ordrifts. The car loses speed by the process and this loss of speedcan be calculated by means of the acceleration formula. For example. 234
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ELECTRIC RAILWAYS 139 if the car is on level, tangent track, the decelerating force is simplythe friction. This we have assumed to be 20 pounds per ton. Ifthis be assumed constant, the total decelerating force is 400 pounds,from which the rate of deceleration is calculated as 0.22 m. p. h. p. s.The car will lose 1 m. p. h. of speed every 4.54 seconds. Applications of the Principles of Mechanics to Series MotorsMounted on Cars. In practice it is impossible to secure uniformacceleration in a car because the counter electromotive force of themotors increases with the speed and cuts down the current. If thecurrent could be kept uniform it would be possible to apply a con-stant gross accelerating force. Even then the net accelerating forcewould not be constant because the frictional resistance increaseswith the speed. For the present we shall neglect the latter item,however, as it is not of very great importance except in high-speedcars.

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