252 THE DIRECT-CURRENT MOTOR CH. XI 



the best curve that can be obtained for M=72, and 

 r=3'18, while B is the induction curve for the motors that 

 are actually employed on this line. 



The acceleration and current curves obtained with 

 motors having induction curves represented by A and B 

 respectively, are given in figs. 62 and 63. The dotted 

 lines give the curves for the motors in use on the Chicago 

 elevated railroad, as obtained by actual experiment. For 

 details of the way in which the experiment was carried 

 out, the reader may consult the paper by Mr. Gerry 

 already referred to. 



The weight of the train was estimated at 90 tons, the 

 track was level, and the mean tension at the train was 

 504 volts. The brakes were applied at the end of 77 

 seconds, when 1,930 feet had been covered, and the re- 

 maining distance of 570 feet, making up the total of 2,500 

 feet, was covered in 27 seconds, making the total time 104 

 seconds. 



The acceleration curve for the motors with induction 

 curve A in Fig. 61, is plotted on the same scale as the 

 experimental curve. The initial acceleration is 1'27 f.p.s. 

 per second; the motors speed up in series to 8 '7 feet per 

 second, and then in parallel with the same acceleration to 

 18 feet per second ; the remainder of the curve is obtained 

 graphically from the torque curve. 



The total torque at the start is 26,100 inch-pounds; 

 after deducting 3,180 for retardation we have left 22,920 ; 

 allowing 90 per cent, mechanical efficiency we get 20,600 

 inch-pounds available for acceleration. The curve of 

 accelerating torque is plotted, points on it being obtained 

 from the induction curve A in Fig. 61. Point l> is given, 

 for we know that at full speed the train is moving at 3 7 '5 



