. Electric traction and transmission engineering . comparison of the equip-ments of similar existing installations), and from the motorperformance curves there are plotted curves of speed ofthe car in traversing the entire roadway and of motorcurrent. The former curve enables one to foretell if theprescribed schedule speed can be maintained, allowing areasonable margin for making up delays, and the lattercurve serves as the basis for determining whether theassumed motor can perform the required service withoutsuch extreme heating as to endanger the insulation. 20. Speed. — The velocity of a ca
. Electric traction and transmission engineering . comparison of the equip-ments of similar existing installations), and from the motorperformance curves there are plotted curves of speed ofthe car in traversing the entire roadway and of motorcurrent. The former curve enables one to foretell if theprescribed schedule speed can be maintained, allowing areasonable margin for making up delays, and the lattercurve serves as the basis for determining whether theassumed motor can perform the required service withoutsuch extreme heating as to endanger the insulation. 20. Speed. — The velocity of a car in operation varieswidely from time to time. Starting from standstill, thecar is accelerated, rapidly at first, then more and moreslowly until a uniform speed is attained. After runningat this speed for a definite time, the current is turned off 52 TRACTION AND TRANSMISSION. and the car is allowed to coast, the velocity meanwhilegradually decreasing. Finally the brakes are applied inorder to bring the car rapidly to rest at the next stopping. Fig. 27. point. Here freight or passengers are taken on or dis-charged; thereafter similar runs are performed. 21. Typical Speed Curves. — The velocity of a car atsuccessive instants of time may be graphically portrayedby a speed curve, in which the instantaneous speeds areplotted in terms of time. Such a curve takes the formof a series of lobes, each one representing a run and one ofwhich is shown in Fig. 27. The slope of the curve at anypoint indicates the time rate of change of speed. Thisslope may be positive, zero, or negative, correspondingrespectively to acceleration, uniform speed, or retardation. The speed curve may be considered as made up of fourparts as follows: starting, motor, coasting, and starting part corresponds to the period of manipula-tion of the controller, the acceleration of the car and thecurrent in the motor being kept constant, while the voltageimpressed upon the motor is gradually increased fro
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