CH. XI DESIGN OF KAIL WAY MOTORS 239 



Draw 1*7 to represent an acceleration of 3'12 f.p.s. per 

 second. This is the acceleration for 45 amperes per motor 

 with 3f=71. The total torque is now 4,500 inch-pounds, 

 and the whole retarding torque is 750 inch-pounds, 

 so that the accelerating torque is more than twice 

 what it was when M was 35 '5. The speed of the car 

 when the starting rheostat is all out is 12'1 f.p.s. The 

 acceleration curve can now be constructed from the 

 torque curve by the method described in Chapter VII., and 

 continued until the area included is equal to 500 feet. 

 Draw dh at 45 amperes and construct the current curve 

 from the speed curve. The energy expended is represented 

 by the area pclhqr. 



A comparison of the curves for constant and variable 

 induction factor shows us in the first place how great a 

 saving of energy is effected by the use of the series 

 winding, the area of the current curve for the series-wound 

 motor being 0'6 of that for the motor with constant 

 induction factor. 



In the second place we see that there is a small 

 saving of time effected by using series winding. The 

 amount of this saving will depend on the shape of the 

 induction curve. We may generally assume that in a 

 well-designed motor the use of series winding will save 

 about 5 per cent, in time. Hence we may use in our 

 equations a value of t about 5 per cent, greater than the 

 time actually specified, and thus obtain a smaller value for 

 the starting current. 



The current curves for the two types of motor are 

 plotted in Figs. 58 and 59. At the point / the whole of 

 the energy is being expended in heat. The heat loss at 

 any point may be calculated by taking the corresponding 



