148 TIIK DIHECT-rniKKNT .M<)T<1; CH. VII 



the whole resistance to motion. Experiment shows that 

 the friction is greatest at the moment of starting, and that 

 after the first few revolutions it diminishes and continues 

 very nearly constant for all speeds. 



if the initial retarding torque od is greater than the 

 maximum total torque oa, the motor cannot move. The 

 current that is taken from the line when a motor is 

 started depends simply upon the tension of the line and 

 the resistance in circuit at the moment : if there is a heavy 

 load on the motor, that is, if the resistance to motion is 

 great, the motor will start up slowly, if at all ; if the load 

 on the motor is light, the motor will start up quickly. 

 Thus the load on the motor does not affect the 

 current taken from the line at the moment of 

 starting, but only the rate at which the motor speeds up. 



The acceleration at any instant is proportional to the 

 accelerating force. If n is the speed of the motor in 



revolutions per second, the acceleration will be 



tit 



The accelerating force will be proportional to the 

 torque available for acceleration, and if the induction 

 factor remains constant, it will be proportional to the 

 difference between the total current flowing through the 

 motor at any moment, and the current that the motor 

 will take when uniform motion has been attained. If we 

 denote the former by c, and the latter by c f , we can 

 write the equation for acceleration thus : 



where /, is a constant depending, as we shall see later, 

 upon the value of the induction factor, the moment of 



