96 ELEMENTS OF ELECTRICAL ENGINEERING. 



the function of the machine is changed from generator to motor, 

 so that the mechanical force with which the field acts upon the 

 armature, is reversed ; but the force action opposes the motion 

 of the generator, and helps the motion of the motor. There- 

 fore since the force action is reversed, the motion must be in the 

 same direction, whether the machine operates as generator or as 

 motor. 



The series dynamo, with given connections of field to brushes, 

 runs as a motor in a direction opposite to that in which the 

 machine must be driven to enable it to build up as a generator. 

 This is evident from Figs. 77 and 78 which show that, with given 



field winding supp]y 

 field winding main ^ 



ooq.-- 



lj armature 



maia supply main 



Fig. 77. Series Generator. Fig. 78. Series Motor. 



connections of field to brushes, the relative direction of current 

 in armature and in field is the same, whether the machine is oper- 

 ating as a generator or as a motor. Therefore the mechanical 

 force with which the field acts upon the armature is unchanged 

 in direction when the function of the machine is changed from 

 generator to motor ; but the force action opposes the motion of 

 the generator and helps the motion of the motor. Therefore the 

 motion must be reversed, since the force action is unchanged. 



Direction of running of a motor. The direction of running of 

 a motor, with given connections of field winding to brushes, is the 

 same, irrespective of the direction of the current supplied to the 

 machine. This is evident when we consider that the reversal of 

 the supply current, which is represented by the heavy arrows in 

 Figs. 76 and 78, reverses the current both in the armature and 

 in the field, and that the reversed field of course exerts a force 



