98 THE DIRECT-CURRENT MOTOR CH. V 



watt curves is not altered by the reversal of rotation or 

 the change in the sign of the induction factor at the origin. 



From the speed and torque curves we may construct 

 a curve of torque on a speed base. In the diagram this 

 curve is shown with the torques measured vertically, the 

 maximum torque being bh, as in the original torque curve, 

 the speeds being now measured horizontally from b, 

 as origin, towards o. This curve gives us the speed at 

 which the motor will run for any given load ; we shall 

 find it useful in determining the conditions under which 

 a motor speeds up from rest ; it also shows the changes in 

 the speed consequent on a change in the load. 



In a series-wound motor, the induction factor is a 

 function of the current, and the torque is proportional to 

 the product of the current and the induction factor, hence 

 the torque varies as some power of the current always 

 greater than one. For points on the induction curve near 

 the origin, the induction factor increases in direct propor- 

 tion to the current, so that the torque curve is here a 

 parabola, the torque increasing as the square of the 

 current. As the induction curve bends over, the torque 

 curve changes in character and is slightly inflected, as a 

 close inspection of the diagram will show, until when we 

 have passed the bend and arrived at the straight upper 

 portion of the induction curve, the torque increases at a 

 rate greater than that of the current by an amount 

 determined by the inclination of the induction curve to 

 the horizontal. 



The form of the torque curve of a series-wound motor 

 has the very important practical result that large torques 

 may be obtained without drawing proportionately large 

 currents from the line. This is a great advantage when a 



