212 THE DIRECT-CURRENT MOTOR CH. X 



The induction curve for these motors has been given in 

 Fig. 31. The value of M at 1,800 amperes is probably 

 about 165, but the test was not carried up to this point. 



Fig. 54 gives the current curve on a time base, as 

 actually observed, and is taken from a paper by Mr. L. H. 

 Parker in the 'Street Railway Journal' for March 1896. 

 At 0*98 of the final speed the current is 930 amperes ; 

 in the figure this is reached at 45 seconds, from first 

 making the current. But we see that eight seconds 

 was occupied in increasing the current from nothing to 

 800 amperes, which is that required to overcome friction, 

 so that the train could not begin to move until 8 seconds 

 after the current was first made, giving us 37 seconds for 

 the actual start. 



We shall now consider the case where the final speed 

 is specified, and the value of '/', instead of being fixed as 

 before, depends upon the value of d. This is the case 

 in motor cars, where the resistance to motion consists in a 

 torque // at the car axles to which the motors are geared. 



2 

 Since T=-t f we can reduce the value ol T by increasing 



Cv 



the size of the driving-wheel, the value of t f the frictional 



torque, remaining constant. By putting 2/ for T in Equa- 



(L 



Mv 



tion 95 we see that the ratio -- increases slightly with 



a 



an increase of d for a given speed. The frictional current 

 is given by the equation 



- 



(96) - 



From this we see that c, decreases with an increase in d, 



