CHARACTERISTIC CURVES. 417 



consumption of an electric car or train, with a given equipment 

 of motors may be predetermined ; or conversely, with given 

 schedule and road-bed, a proper selection of motors for the given 

 service conditions, may be made. 



The following example illustrates the application of series 

 motor characteristics to the calculation of the amount of power 

 delivered to the car wheels by a given motor equipment, and the 

 efficiency obtained under specified conditions. 



Example. A fifteen ton car is equipped with two similar series 

 motors of which the characteristic curves, with 500 volt supply 

 (no starting resistance in circuit), are shown in Figs. 49 and 50. 

 With the motors connected in parallel the car travels at a uni- 

 form speed of fifteen miles per hour up a 6.25 per cent, grade. 

 It is required to find the current taken by each motor, the total 

 tractive effort developed by both motors, the total power delivered 

 to both motors, the net power represented by the total tractive 

 effort at the given speed, and the efficiency of the car equipment 

 under the specified conditions. 



Solution. From the curve in Fig. 49 we find the current per 

 motor corresponding to fifteen miles per hour to be 80 amperes, 

 so that the total current delivered to the car is 160 amperes, 

 which at 500 volts represents a power input of 80 kilowatts. 

 From the curve in Fig. 50 we find the corresponding tractive 

 effort per motor to be 1,125 pounds, so that the total tractive 

 effort is 2,250 pounds. The power in kilowatts represented by 

 this tractive effort at the given speed is the total power delivered 

 by the two motors to the car wheels, and is found to be : 



2,250 X 15 x 5,280 746 i 



- x - X - = 67.14 kilowatts 

 3,600 550 1,000 



The efficiency is 67.14/80, which is 0.8393 or ^3-93 percent. 



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