*210 THE DIRECT-CURRENT MOTOR CH. X 



For the present we shall assume that the induction 

 factor remains constant. First let us take the case 

 where the final speed 8 in miles an hour is speci- 

 fied. We have 



Mi- E i j 



From this we see that when R, T, 8, and E are given, the 



ratio r is fixed. Referring now to Equation 93, we find 

 d 



that under these conditions the only way of increasing the 

 acceleration is to increase c a . When the maximum current 

 is fixed we can do this only by reducing the value of c f , 

 and this is given by the equation 



Hence since E, R, r l\ and 8 are all fixed we cannot alter 

 c f We conclude then, that when the final speed, the value 

 of T, and the maximum current are fixed, the initial 

 acceleration is also fixed. 



As an illustration we may take the case of the motors 

 used on the Baltimore and Ohio Railroad. The conditions 

 are as follows : A freight train weighing 780 tons has to 

 start from rest on a grade' of 0'8 percent, and move up the 

 incline at 1O7 miles per hour. The train is to be drawn 

 by a locomotive equipped with four motors, which are 

 permanently connected in series across a 625-volt line. 

 The motors are gearless, the internal resistances when 

 hot, are for the armature 0'0154, for the magnets 0'0055, 

 total O0209 ohm. The maximum permissible current is 



