08 THK DIRECT-CrKRENT MOTOR CU. IV 



the changes of E, the speed curves are parallel to one 

 :i IK >t her. If the induction factor is altered as well as the 

 load, the speed curves will be differently inclined to tin- 

 axis of speed (see Fig. 19). 



We shall now consider the conditions of motion when 

 the load - a function of the speed. Suppose that 

 the resistance to motion increases with the speed, and that 

 it can be represented by a curve, where vertical ordhiatrs 

 represent the speed in revolutions per second, and whose 

 horizontal ordinates represent the torque resisting the 

 motion. 



For uniform speed the torque due to the current 

 assisting the motion must be equal to the torque resisting 

 the motion. Hence there will be a certain speed at which 

 the motor must run when it is carrying a certain current. 

 For, if M is constant, a given current means a given 

 assisting torque ; and there is only one speed at which 

 the motor can rim for that torque. If the speed is 

 greater than that given by the curve, the resisting torque 

 will be greater than the assisting torque due to the current, 

 and the motor will be retarded, and if the speed is less 

 than that given by the curve, the resisting torque will be 

 less than the assisting torque, and the motor will 

 accelerate. 



If we place such a motor in series with a circuit in 

 which the current varies, the speed of the motor will 

 depend upon the current. The motor would then act as 

 a current meter, and the total revolutions at the end of 

 any time would depend upon the current that had been 

 passing during that time. If the curve of torque and 

 speed is straight and passes through the origin, the speed 

 is proportional to the current. In practice it is not 



