GH. VI EFFICIENCY 127 



the induction curve get the curve of total torque, using 

 the equation =1 -4 IcM. The mechanical efficiency 

 is then the ratio of the ordinates of the curves of 

 useful and total torque. 



These curves have been given in Fig. 21 for the G. E. 

 800 railway motor. The curves of speed and useful torque 

 are from actual tests, the curve of total torque is obtained 

 from the induction curve, which is itself derived from the 

 speed curve. We see that the mechanical efficiency at 

 40 amperes is 83 -5 per cent., and at 20 amperes it is 

 nearly 86 per cent. 



The total torque at 40 amperes is 3,320 inch-pounds 

 since M is 59, and the measured useful torque is 2,780, 

 showing that the torque lost internally amounts to 540 

 inch-pounds. On 33-inch wheels with a velocity ratio of 

 4'78 the useful tractive for this current would be 800 

 pounds ; this figure gives rise to the name of the motor. We 

 cannot at present say how the torque loss is distributed ; 

 this must be determined by a separate test. We see from 

 the curves that in this motor the torque loss increases with 

 the total load. 



In Fig. 31 similar curves are given for the motors used 

 on the Baltimore and Ohio Railroad. In this case there 

 is no gearing, so that the mechanical efficiency is much 

 higher, being about 95 per cent, for 800 amperes. The 

 torque loss increases with the load, but not to such an 

 extent as^ with the geared motor. 



The torque loss in a gearless motor may be 

 divided into three parts as follows : 



(1) Friction. 



(2) Eddy currents. 



(3) Hysteresis. 



