102 THE DIRECT-CURRENT MOTOR CH. V 



a descending car, the generating action of the motors 

 meanwhile serving as a brake on the motion of the car. 



The only condition under which a series- wound motor 

 can be made to generate a current when running under 

 ordinary circumstances is that the induction curve should 

 not pass through the origin. This condition is frequently 

 obtained in practice owing to the existence of residual 

 magnetism. In Fig. 22 curves of torque and speed are 

 given for a motor in which the induction curves for 

 ascending and descending values of the currents are deter- 

 mined by the amount of hysteresis. In the figure the two 

 induction curves are similar, but displaced horizontally. 



The speed and torque curves for the ascending and 

 descending portions of the induction curve are plotted 

 in the usual way. There is a portion of the torque 

 curve where the torque is negative, namely where the 

 current and the induction factor are of opposite sign. 

 Taking the descending portion B, we see that the verti- 

 cal line drawn through the point b, where the current 

 is equal to 10 amperes negative, is an asymptote to 

 the speed curve, hence, when the current is nothing, the 

 motor will be running at such a speed that the induced 

 tension made up on the fesidual magnetism is equal to the 

 tension of the line, and for speeds above this the motor 

 will deliver current into the line, but this current cannot 

 exceed 10 amperes. 



If the magnets of a motor are made of steel, the resi- 

 dual magnetism may be considerable, and a current may 

 be sent into the line when the motor is running at a high 

 speed, but the current will be small. If, however, the 

 motors are disconnected from the line, and have their 

 armature terminals connected through a low resistance, 



