80 THE DIRECT-CURRENT MOTOR CH. II 



due to the current is equal to that due to the load. The 

 process of speeding up will be discussed in a subsequent 

 chapter. We shall for the present confine our attention 

 to the conditions of uniform motion. 



When the motion is uniform, i.e. when the speed is 

 neither increasing nor decreasing, the torque due to the 

 cuiTent must exactly balance that due to the load, for if it 

 were greater or less than this there would be a torque 

 available either for acceleration or retardation. It follows 

 that for uniform motion the current is determined by 

 the load, and is given by Equation 5, where t is the 

 torque due to the load, i.e. the sum of all the torques resisting 

 the motion. The current is thus independent of the 

 tension of the line and of the speed. 



As an illustration we may take the case of a railway 

 motor driving a loaded car. Suppose that the tension 

 of the line falls from 500 to 200 volts. Since the current 

 depends on the load, which is unaltered by the decrease 

 of the tension, the motor will take the same current at 

 200 volts as it did at 500 volts. 



In Fig. 10, let DO be drawn to represent the maximum 



E 



possible speed in revolutions per second, and DJ! the 

 M 



E 



maximum current --- amperes. Join BO, then the inter- 

 /t 



cept of any vertical ordinate between DB and BO repre- 

 sents both the speed and the induced tension for any current, 

 since, if M is constant, the induced tension is proportional 

 to the speed. When the load on the motor is such that for 

 steady motion the current given by Equation 5 is UK 

 amperes, the speed is given by KF and the induced volts 

 also by KF. Since DO is equal to /, the tension of the 



