. The Street railway journal . ne, and its widest departuretherefrom is to be found near the origin where the increaseof / with the increasing current is greatest. Fig. i displaysa typical curve of horizontal efiort, in which the featuresreferred to can be clearly seen. The curve does not in-tersect the origin, because near that point the speed be-comes so high that all the torque developed is needed toovercome the motor friction. The curve of speed roughly approximates a hyperbolaasymtotic to the speed axis. Therefore, a railway motor,or, indeed, any series motor, rapidly gains in speed as th
. The Street railway journal . ne, and its widest departuretherefrom is to be found near the origin where the increaseof / with the increasing current is greatest. Fig. i displaysa typical curve of horizontal efiort, in which the featuresreferred to can be clearly seen. The curve does not in-tersect the origin, because near that point the speed be-comes so high that all the torque developed is needed toovercome the motor friction. The curve of speed roughly approximates a hyperbolaasymtotic to the speed axis. Therefore, a railway motor,or, indeed, any series motor, rapidly gains in speed as theload falls of¥. If the motor had no losses the speed at zeroload would, according to the equation, be infinite and prac-tically might reach a very dangerous figure. As a railwaymotor is never relieved of its mechanical load in service,only such parts of the speed curve as correspond to work-ing load are of interest. Considering the typical curve also shown in Fig. i, wefind that as the current increases the speed 0 20 40 GO 80 100 120 140 ICO ISO 20O 220 AinptTOS Stritt FIG r.—CURVES OF SPEED AND TRACTIVE EFFORT This is due to two causes ; first, the increasing field strengchto which the speed is inversely proportional, and, second,because of the increased armature drop, which entails a re-duction of back e. m. f., and, therefore, of speed, in orderto permit the necessary current to flow. The armature dropfactor is small, compared with the field factor, and hencethe speed curve of a series motor is a rough kind of satura-tion curve plotted in a peculiar way, the knee of the curveoccurring at that point where it turns to approximate paral-lelism with the current axis. From this it follows that thesooner this occurs, as the current increases, the greaterthe reluctance of the magnetic circuit, and other thingsbeing equal, the lightei the motor. It is sometimes con-venient to plot speed curves at different voltages whichproduces a family of approximate
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Keywords: ., bookcentury1800, bookdecade1880, bookidstreetrailwa, bookyear1884
