CH. XI DESIGN OF RAILWAY MOTORS 247 



The acceleration curves are obtained from the torque 

 curve by the graphic method, and are continued until the 

 enclosed area represents 500 feet. The corresponding 

 current curves are also shown. 



We see that as the velocity ratio increases the energy 

 expended, given by the area of the current curve, decreases, 

 but the time occupied increases. Thus we gain half a 

 second by putting on a 40-inch wheel at the expense of a 

 considerable increase in the energy used. On the other 

 hand, we save energy by putting on a 24-inch wheel, but 

 at the expense of two seconds of time. There is a minimum 

 limit to the time in which the distance can be covered, 

 which in this case is about 26 seconds, but the increase in 

 the size of the wheel for each fraction of a second gained 

 becomes greater as this limit is approached. 



Hence if we have allowed for the saving of time effected 

 by the use of series winding, the given distance will be 

 covered in the given time, and any other values of v or of 

 d than those given by Equation 103, will involve either a 

 longer time or a greater expenditure of energy. 



We shall complete our discussion of this example by 

 considering to what extent our results are affected by 

 the use of the series-parallel controller. 



We can adopt either one of two methods. We may 

 take the same current from the line as before namely, 

 90 amperes, the whole of this current going through each 

 motor in series, or we may take the same current per 

 motor as before namely, 45 amperes, the current from 

 the line being thus halved as long as the motors remain 

 in series. Let us compare these two methods. 



Suppose that the induction factor of each motor is 

 increased to 74 when 90 amperes is passing, the total 



