126 THE DIRECT-CURRENT MOTOR CH. VI 



get a second curve, Jik, giving us the useful torque. 

 In the figure we have assumed that the torque loss is 

 constant, and we have taken it abnormally large in order 

 to make the diagram clearer. 



The mechanical efficiency is the ratio of the ordinates 

 of the curves of useful and total torque. This efficiency 

 is nothing at the point h, and increases with the total 

 torque. If we divide the ordinates of the curve of 

 mechanical watts in the ratio of the useful and total 

 torques for each current, we shall get the curve of useful 

 mechanical watts. We can also construct a curve of 

 mechanical efficiency. 



We have to make an arbitrary distinction in deciding 

 what are to be counted as internal losses. In an ordinary 

 belt connected motor it is obvious that these should include 

 friction of bearings and brushes, hysteresis and eddy current 

 losses, but when the motor is fitted with gearing as most 

 railway motors are, it is not so easy to draw the line and 

 say what losses are to be counted as internal losses. If 

 the motor and its gearing form part of a self-contained 

 equipment, the gear friction is usually counted in with the 

 internal losses. 



We have also assumed that the torque increases with 

 the current, and have drawn the curve of total torque 

 a straight line, as in a motor with constant induction 

 factor. But all that follows is equally true for a series- 

 wound motor, since the curves of heat and line watts are 

 not affected by the variation of the induction factor. 



The mechanical efficiency at diiferent currents or loads 

 may be found by the following method. Determine by a 

 test the curve of useful torque, and also the speed curve. 

 Deduce the induction curve from the latter, and from 



