96 THE DIRECT-CURRENT MOTOR CM. V 



reversed, the curve below the axis being similar to that 

 above it. Construct the reversed induction curve. 



The product of the current and the corresponding 

 induction factor gives us the torque, which can be 

 plotted on the current base on the given scale. Thus for 

 the maximum current of 100 amperes the torque is 9,720 

 inch-pounds, set this off at the point ft ; this will be a 

 point on the curve of torque on a base of current in the 

 dynamo. Construct the rest of the curve. 



We shall find that this curve differs from the torque 

 curve of a motor of constant induction factor, since the 

 axis of current is a tangent to it at the origin. The torque 

 thus obtained is the total torque for any given current ; it 

 will be greater than that observed in a test by the amount 

 required to overcome friction and other torque losses. 

 Since the induction factor is reversed with the current, the 

 sign of the torque remains unchanged, the form of the 

 curve being the same as that on the other side of the 

 origin. Here we see an important difference from the 

 torque curve with constant induction factor. 



The speed curve may now be constructed. For any 

 current od, the heat-drop will be represented by 7rc, so 

 that the motor must run at such a speed that the induced 

 tension is equal to cd. The induction factor for this 

 current is dg, the speed will therefore be given by the ratio 

 of cd, the induced tension, to dg, the induction factor ; 

 using the given scale of speeds, we find that the ordinate 

 of the speed curve at this point will be df, and equal to 

 520 revolutions per minute. 



In this way the complete speed curve can be plotted ; 

 it will pass through the point Z>, and the axis of speed will 

 be an asymptote. It differs in a marked way from the 



