CH. V SERIES-WOUND MOTOES 101 



these losses, for when the motor is running under 

 normal conditions, the frictional torque has to be de- 

 ducted from the total torque, so that the torque mea- 

 sured at the rim of a brake wheel represents the total 

 torque diminished by the amount of the torque losses. 

 Hence when the torque curve obtained by a brake test 

 is plotted, it will be found to lie below the curve of 

 total torque as obtained from the equation =1'41 elf. 

 In this chapter we shall take the speed curve as the basis 

 for the construction of the curves of induction and total 

 torque. 



If the induction curve of a series-wound motor 

 passes through the origin, we cannot make it generate 

 a current by increasing the speed. This is seen from the 

 form of the speed curve. For instance, it is not possible to 

 utilise the energy of a car descending a grade by sending 

 current into the line, as might be done if the motors were 

 shunt wound. This is a consideration that may assume some 

 importance in certain cases. Thus in the system of elec- 

 tric railroads radiating from the city of Baltimore, there 

 are grades of one in sixty extending for a distance of as 

 much as five miles. In order to run at fifteen miles an 

 hour up such a grade with a seven-ton car, we should need 

 about four horse-power for friction and track resistance, 

 and eleven for the grade. On the descending journey, we 

 should have to take current from the line only when 

 starting up after a stop ; for the rest of the time the motors 

 would be cut out and the brakes on. If the motors are 

 series wound, they are now useless either for restoring 

 energy into the line, or for acting as brakes. If shunt- 

 wound motors were used, a considerable proportion of the 

 power required for an ascending car could be supplied by 



