248 THE DIRECT-CURRENT MOTOR CH. XI 



torque will be 9,400, less 750 for friction, giving an 

 acceleration of 7 "2 f.p.s. per second; this can be main- 

 tained until a speed of 4-8 f.p.s. is reached, when the 

 motors must be thrown into parallel. The rest of the 

 curve will be the same as before ; if plotted, it will show a 

 gain of one second in covering the given distance. 



If on the other hand we allow only 45 amperes per 

 motor, we obtain the same acceleration curve as in Fig. 56, 

 but as we are able to keep the motors in series until a 

 speed of 5*7 f.p.s. has been reached, the expenditure of 

 energy up to that point is halved, the total expenditure of 

 energy being slightly less than by the first method. 



We see then that the only advantage in taking 90 

 amperes per motor is a gain of one second in time. Since 

 the motors used in the first method have to be designed 

 to carry twice as much current as those used in the second, 

 the latter method is to be preferred. 



As an illustration of the application of the principles 

 of this chapter to the heavier class of railway work, we 

 will take the Metropolitan Elevated Railroad of Chicago. 



Particulars of this railway will be found in a paper by 

 Mr. M. H. Gerry, published in the ' Proceedings of the 

 American Institute of Electrical Engineers ' for 1897. 



. The rolling stock consists of motor cars and passenger 

 cars. The former measure 47 feet in length and when 

 fully loaded weigh 62,000 pounds. They are mounted 

 on locomotive trucks, having 33-inch wheels, with a 

 velocity ratio of 3-18. One truck of each motor car is 

 equipped with two motors. 



The passenger cars are 47 feet in length, having trucks 

 fitted with 30-inch wheels, and when fully loaded weigh 



