124 ELEMENTS OF ELECTRICAL ENGINEERING. 



motor, curve B is a speed-power characteristic of a differential 

 compound motor, and curve C is a speed-power characteristic of 

 a cumulative compound motor. 



() The series motor. Fig. 93 shows a speed-torque char- 

 acteristic of a series motor. This curve shows the enormous 

 rise in speed at light loads (very small torques). 



&400 



800 



25 50 75 iOO 125 

 Torque in % of full rated torque 



Fig. 93. 



The characteristics of street railway motors, as usually plotted, 

 are as follows : (a) The speed-current curve in which the ordi- 

 nates represent the speed of the car in miles per hour, instead of 

 the speed of the motor armature in revolutions per minute ; and 

 (^) the tractive-effort-current curve in which the ordinates repre- 

 sent the net tractive effort in pounds weight exerted, instead of 

 the torque in pound-inches developed by the motor armature. 

 Such characteristic curves involve the gear ratio of the motor, 

 and the diameter of the drive wheels on the car. 



The curve marked " miles per hour " in Fig. 94 is a speed- 

 current curve of a 2 5 -horse power Westinghouse railway motor, 

 and the curve marked " tractive- effort 33-inch wheel " is the trac- 

 tive-effort-current curve of the same motor under the following 

 conditions : The electromotive force between trolley wire and 

 rail is 500 volts, and the motor is connected between the trolley 



