32 THE DIRECT-CURRENT MOTOR PIT. II 



-pi 



line, and DB= , it follows that the tangent of the angle 

 It 



AOB is equal to the resistance of the motor. The same 

 diagram could be constructed by setting off .10.6= tan" 'ft, 

 and OD equal to the tension of the line. DB would then 

 be the maximum current. 



The diagram shows us that when the load is so great 

 as to require a current DB to balance it, the whole of the 

 energy from the line is expended in heating the resistance, 

 the line watts being represented by the area OABD. As 

 the load is reduced the motor begins to turn, the line watts 

 is diminished, but some of the energy from the line is now 

 used in doing work, the ratio of the areas DKFG, DKHO, 

 being the ratio of the mechanical watts to the line watts. 

 This ratio represents the proportion of the energy from the 

 line that is used in overcoming resisting torque. 



At the point B the speed is nothing, the mechanical 

 watts is nothing, and the torque has its maximum value. 

 On the other hand, at the point D the speed is a maximum 

 and the torque is nothing. Hence the mechanical watts 

 increases from nothing to some maximum and then decreases 

 to nothing again, the speed increasing as the torque 

 decreases. 



The mechanical watts is a maximum when DK=KB 

 (not necessarily when DKFG is a square), i.e. when the 

 current is half DB, the area of DKFG is then seen to be 

 one fourth of DBAO. Hence the greatest possible 



.E 2 

 value of -w the mechanical watts is -. 



Ll 



E 2 

 For any value of w less than ^ there are two pos- 



MM 



sible values, one with t large and n small, the other with 



