CHARACTERISTIC CURVES. 415 



slight dissimilarity between the two shunt motors is greatly ex- 

 aggerated in its effect upon the distribution of load between the 

 two machines by these internal electromagnetic actions. 



It is instructive to show, by means of the speed-torque curves 

 of two shunt motors, how a given load will be distributed between 

 them. Thus AA and BB, Fig. 48, are the speed-torque curves 

 of two shunt motors, the curve BB being drawn to the left of 

 the origin. The dotted portions of AA and BB correspond 

 to generator action. When the two motors are coupled together 

 mechanically their speed is the same. Under these conditions a 

 given total torque P'P is shared unequally by the two motors 

 (P 1 C = torque of B y and PC = torque of A) unless the two mo- 

 tors are exactly alike. 



38. Series motor characteristics as used in electric railway engi- 

 neering. The characteristics of a motor, supplied, for example, 

 from constant voltage mains, may be expressed not only in terms 

 of speed and torque, but also, if desired, in terms of speed and 

 current, or in terms of torque and current, inasmuch as a definite 

 value of current always corresponds to a given torque or to a 

 given speed. 



Fig. 49 shows the characteristic of a single railway motor 

 expressed in terms of speed of car in miles per hour and current 

 taken by the motor. The speed of the motor in revolutions per 

 second bears a definite ratio to the speed of the car in miles per 

 hour ; this ratio is known when the gear ratio and size of car 

 wheels are known. 



Fig. 50 shows the characteristic of a single railway motor ex- 

 pressed in terms of tractive effort and current taken by the motor. 

 The tractive effort bears a definite relation to the motor torque ; 

 this relation is known when the gear ratio, size of car wheels, and 

 motor friction are known. 



The speed-current and tractive effort-current characteristics of 

 series motors are of great importance in electric railway engineer- 

 ing, since by their means the details of performance and power 



