184 THE DIRECT-CURRENT MOTOR CH. IX 



CHAPTER IX 



CONTROL 



THE action of a motor during the time in which it is being 

 brought up to speed from a state of rest is determined by 

 the method of control. When there is a single motor a 

 rheostat is used, the resistance of which can be varied by 

 a switch woi'ked automatically or by hand. The maximum 

 resistance of the rheostat must be such that when connected 

 in series with the motor at rest, the current from the line 

 does not exceed the maximum safe current that the motor 

 can carry. If the torque due to this current is greater 

 than the frictional and other torque resisting the motion, 

 the motor will begin to speed up, and the current will 

 decrease. We may, however, keep the current constant 

 by reducing the resistance as the motor speeds up. This 

 process can be carried on until the resistance is cut out 

 completely, after which we cannot prevent the current 

 from getting less ; the motor will then speed up according 

 to the law expressed by Equation 69. 



When two motors are used we find more than one 

 possible method of control. If we take as an example a 

 motor-car driven by two motors, there are the following 

 alternatives : 



I. The motors may be started and run in parallel. 



