CHARACTERISTIC CURVES. 



411 



supplied with constant current. In this case the torque is con- 

 stant irrespective of the speed and the characteristic is a straight 

 line parallel to the axis of speed. This constancy of torque is 

 evident when we consider that constancy of current implies con- 

 stancy of field excitation in a series motor, and therefore constancy 

 of flux <l>, so that the torque is constant according to equation 

 (25), Chapter IV. 



(c) Series motor supplied with constant current at low speeds 

 and connected to constant voltage mains when the speed reaches a 

 certain value. These are the 

 conditions which obtain in a rail- 

 way motor. When the car is 

 started a large resistance is in 

 series with the motor, and as the 

 car speeds up this resistance is 

 slowly cut out so as to keep the 

 current approximately constant 

 at the greatest value that is con- 

 sidered permissible without over- 

 heating the motor. When this 

 resistance is all cut out, the cur- 

 rent begins to fall off inasmuch as the approximately constant 

 voltage between trolley wire and rail is opposed by an increasing 

 counter electromotive force in the motor. The speed-torque char- 

 acteristic of a railway motor, under the above conditions, is a com- 

 bination of the curves shown in Figs. 43 and 44. A typical 

 example of a speed-torque characteristic of a railway motor is 

 shown in Fig. 45. 



Two rigidly coupled series motors connected in series. When 

 two similar series motors are connected in series to supply mains 

 precisely the same current flows through both, and therefore they 

 always develop nearly the same torque whether their speeds are 

 alike or not. If the two motors, so connected, are coupled to- 

 gether mechanically, they run at the same speed, and they share 



O 



Torque 



Running CUTYG 

 (constant applied voltage) 



Starting curve 

 (constant current) 



Fig. 45. 



