CH. XI DESIGN OF EAILWAY MOTORS 245 



We may here inquire as to the effect of using 

 other values of v and d than those given by 

 Equation 103. 



An increase in the value of d will have the same effect 

 as a decrease in the value of v ; if then we take three 

 cases, each having r = 4'78, d being respectively 24-3, 33, 

 aiid 40 inches, we shall get the same results as if we took 

 a constant diameter of 33 inches, and velocity ratios of 

 6 - 50, 4'78, and 3*94 respectively. 



In Fig. 60 curves of accelerating torque are drawn 

 for velocity ratios 6'50, 4'78, and 3 - 94, d being 33 inches 

 in each case. The curve for r=4*78 is the same as curve 

 M in Fig. 56. In constructing these curves with different 

 values of r we must remember that an increase in v 

 increases the initial acceleration, but diminishes the final 

 speed and also diminishes the speed when the starting 

 rheostat is all out. 



In calculating the initial acceleration we must also 

 bear in mind that the torque on the motor shaft required 

 to overcome the frictioiial torque on the car axle increases 

 inversely as v. Thus when v is 4*78 the maximum total 

 torque on the motor shaft is, as we have seen, 4,500 

 inch-pounds, from which we have to deduct 750 to 

 balance the frictional resistance, leaving 3,750 available 

 for acceleration at the start, the acceleration in f.p.s. per 

 second is then found by Equation 85 to be 3*12 f.p.s. per 

 second. Now when the velocity ratio is 3*94, the total 

 torque on the motor shaft is 4,500 as before, but from 

 this we must now deduct 910 to overcome the friction on 

 the axle, leaving us with 3,590 inch-pounds available for 

 acceleration. Using this value of t a in Equation 85 and 

 putting r = 3'94, we find the acceleration to be 2*46. 



