164 THE DIRECT-CURRENT MOTOR CH. VII 



second ; the torque will then rapidly diminish as the car 

 speeds up ; we may plot the curve as far as 26 feet per 

 second. 



From the records of actual performance we find that 

 the average current at full speed is 70 amperes, giving a 

 retarding torque of 5,760 inch-pounds, equal to a tractive 

 effort of 42,000 pounds on 27-inch wheels. This includes 

 all forces opposing the motion. Since the weight of the 

 train is 35 tons, this corresponds to a tractive effort of 

 19'7 pounds per ton on wheels of 33 inches diameter. 



Construct a curve B giving torque available for accelera- 

 tion, by deducting from the horizontal ordinates of curve A 

 an amount equal to 5,760 inch-pounds; this curve will 

 cut the axis of torque at = 7,610 inch-pounds, and the 

 axis of speed at 21'2 t.p.s. This gives us the final 

 speed of the car, which is equivalent to 14'5 miles 

 per hour. 



Curve B shows that the accelerating torque is con- 

 stant from the start up to 16 f.p.s., that it then rapidly 

 diminishes, until at 21'2 f.p.s. it becomes nothing; the 

 car then ceases to accelerate, and uniform speed is 

 attained. 



We here assume that the retarding torque remains 

 constant at all speeds. This is not strictly correct, but it is 

 probably not far from the truth. If the values of the 

 retarding torques for different speeds can be obtained 

 from experiments, the true form of the curve of accelerating 

 torque can be deduced. No experiments of this kind 

 have been made with these motors ; we shall therefore take 

 the average value, and assume it to remain constant at all 

 speeds. 



Since the weight of the train is 35 tons, each motor 



