CH. X TIME CURVES 223 



Now when c u is given, we can reduce the total current 

 from the line by reducing c f . Hence,' in both motor cars 

 and locomotives, we should make Mv as large as possible, 

 to reduce c f , and then insert in Equation 102 the values 

 of M and v that have been chosen, and find the best dia- 

 meter. This will enable us to cover the given distance in the 

 least possible time. In choosing values of M and v we 

 must remember that the weight of the motor increases 

 with M. 



In the case of a motor car, we cannot further reduce the 

 frictional current cy by decreasing d. We can, however, 

 thereby diminish the time during which the motor is 

 drawing the maximum current from the line. This will, 

 of course, increase the whole time occupied, since the 

 diameter found from the equation gives the least time. It 

 may, however, be worth while to sacrifice a few seconds if 

 we can reduce the time during which the maximum current 

 is being drawn from the line. 



In the case of a locomotive, we can save current in two 

 ways by decreasing d ; first because c f decreases with d, 

 and secondly because the period of acceleration is thereby 

 shortened, as with motor cars. The reduction of the 

 diameter of the driving-wheel is therefore of greater 

 importance in the case of locomotives than of motor cars. 



Example 51. A motor car weighing 40 tons is 

 equipped with two motors controlled on the series-parallel 

 method, each having a resistance of 0*4 ohms. The tension 

 of the line is 500 volts, the frictional torque 6,600 inch- 

 pounds on each axle, the maximum current per motor, and 

 from the line 200 ampares, and the distance to be covered 

 700 yards. 



The following table gives the time occupied and the 



