THE DIRECT-CURRENT MOTOR CH. XI 



limit. AVe shall thus not only fulfil the conditions as 

 regards time and distance, but we shall do so in the most 

 economical way. 



We may here notice that the inclination of the 

 induction curve to the horizontal may be written 



(HO). 



AVhere p is the number of polar divisions of the armature 

 connected in series, A the number of surface conductors, 

 the number of turns per pole, each carrying the 

 whole current, and <j the permeance of each polar gap in 

 centimetres. 



It will however generally happen that the weight 

 limit requires a velocity ratio that is practically unattain- 

 able even with the largest values of d. We have here the 

 same problem that occupied our attention in a former 

 chapter, namely, to get the greatest ratio of v to d. For 

 single reduction spur gearing this ratio is limited by the 

 clearance between the car axle and the ground. If we take 

 a 33-inch wheel we could not make v as much as 7'15, 

 the value required to get the best results. We shall here 

 suppose that the largest possible value of v is 4- 78. We 

 shall then have a bent induction curve, such for instance 

 as that given by the line abf. In what follows we shall 

 take this as the induction curve of the motor. 



Construct the curve dp of total torque on a speed base, 

 measuring torque horizontally and speed vertically. By 

 deducting the retarding torque, 750 inch-pounds on the 

 motor shaft, from each horizontal ordinate of this curve we 

 get the curve of torque available for acceleration. This has 

 been drawn at kl in Fig. 50'. 



