146 THE DIRECT-CURRENT MOTOR CH. VII 



CHAPTER VII 



ACCELERATION 



LET the vertical and horizontal ordinates in Fig. 37 measure 

 respectively torque in inch-pounds and revolutions per 

 second. Let the curve A represent on the given scales 

 the total torque of a motor at different speeds. At o 

 where the torque is a maximum the speed is nothing. Let 

 the curve B represent on the same scales the torque 

 required to overcome all resistance to motion due to the 

 load, friction of the gearing, hysteresis, &c. At the point 

 k where the curves A and B intercept, draw a vertical 

 line cutting the speed base in y. The speed at which 

 the motor will run when uniform motion has been attained 

 will then be given by og. 



For if a curve C be constructed, whose vertical ordi- 

 nates are the difference between those of curves A and 7?, 

 the prdinate of the curve C gives the torque available 

 for acceleration at any speed, being the difference 

 between the total torque and that required to overcome 

 resistance to motion. At (j this difference is nothing, so 

 that at this point the motor has ceased to accelerate, and is 

 therefore running uniformly. The form of the curve 13 is 

 determined mainly by the variation of the friction with 

 the speed, since this constitutes the greater proportion of 



