244 



THE DIRECT-CURRENT MOTOR 



CH. XI 



The following table shows the expenditure of energy 

 in foot-pounds in the two cases : 



Total 



387 x 10 3 



229-9 x 10 3 



We must remember that in this example the expression 

 ' train resistance ' means all resistances opposing the 

 motion, including those due to the friction of the gearing 

 and the torque lost in the motor itself. 



The advantage of series winding thus consists 

 mainly in a saving of energy. There is, however, a 

 limit to the amount of energy saved. Thus with the 

 highest possible induction factor, the C 2 R loss is 13 x 10 3 

 foot-pounds. Hence there is a limit of weight beyond which 

 it will not be worth while to go with a view to reducing 

 the heat loss. This is shown by the following table, where 

 the weights are taken as proportional to the induction 

 factors, the heat losses inversely as IP, and the efficiency 

 as the ratio of the sum of the friction and acceleration to 

 the total energy: 



