188 THE DIRECT-CURRENT MOTnl; CH. IX 



travelled in 3 minutes from the start is 5,580 feet. The 

 distances travelled are plotted in Fig. 47. It is noticeable 

 how short a time elapses on Step II. before the final speed 

 is practically reached, and how rapidly the current then 

 diminishes. 



We might suppose that a greater initial accelera- 

 tion could be attained if the resistance at the start were 

 wholly in the motor itself. If we look at the equation 

 for .!/, we see that the greatest possible resistance that 



2 

 the motors can have is given by the equation R= 



when E, T, and S are given. If 11 is greater than this, 

 the equation for M is insoluble. In the case before us 

 the limiting value for R is 4'3 ohms. If we adopt this 

 value and the current is not to exceed 100 amperes per 

 motor, we cannot dispense with a rheostat. Suppose that 

 we make our motors of 4 - 3 ohms each, and have besides 

 a starting rheostat of 0'7 ohm. The expression under 

 the root vanishes, and we find the value of the induction 

 factor to be 49*1, that is to say, M must have this value if 

 the car is to run at the specified speed with the given 

 load. The current required to overcome friction is now 

 o7'2 amperes, leaving 42 - 8 amperes available for ac- 

 celeration. The torque available for acceleration is then 

 2,960 inch-pounds, and the initial acceleration is 0'13 

 f.p.s. per second, or about one-third of its former value ; 

 so that, putting aside the question of the decreased 

 efficiency when running at full speed, we see that the 

 acceleration is nearly one-third of what it is when the 

 resistance is 0'4 ohm. It appears then that an increase 

 in the resistance of the motor involves a decrease in the 

 value of J/, and consequently in the acceleration. 



