142 TIIK DIRECT-CURRENT MOTOR CH. VI 



The same results are plotted in Fig. 36, on a base 

 of miles per hour. The maximum efficiency does not 

 appear to be very different in the two cases, though 

 the precise form of the whole curve is difficult to deter- 

 mine. 



We have seen that when the speed of a car in feet per 

 second, the tension of the line and the tractive effort are given, 



the ratio is fixed ; we are, however, at liberty to take 

 (L 



any values of M, v and d so long as this ratio is maintained. 

 Since e f = '71 ^ -, we cannot reduce the value of c f by simply 



increasing v, for in proportion as we increase v we must 

 decrease M in order to get the proper speed. We can, 

 however, decrease c f by increasing Mv and d at the same 

 time. For an increase of Mv will diminish cy, and will also 

 cause the car to run slower; we may then increase 

 d and bring the speed up to that specified, without thereby 



Td 

 increasing c /} the useful current c = - 71 - being unaltered. 



Hence in comparing the performance of two motors with 

 different induction curves, if we assume that the wheel 

 diameter remains the same in both, a car equipped 

 with motors having high induction curves will run slower 

 than one with lower induction curves, but with better 

 efficiency. But if we are at liberty to alter the diameter 

 of the wheel, we can increase the speed of the former, and 

 make it run at the same speed as the latter, but at the 

 same time with a higher efficiency. In practice, the wheel 

 diameter is usually fixed for a given class of service ; thus 

 33 inches is universally used for street railway work. 

 When this is the case, and the velocity ratios are equal, the 



