302 



ELEMENTS OF ELECTRICAL ENGINEERING. 



excessive momentary flow of reversed current would take place. 

 This difference in the behavior of the field current and armature 

 current of a shunt motor when the supply voltage is suddenly 

 reversed would show itself as a very considerable phase difference 

 between field current (field magnetism) and armature current if 

 the attempt were made to operate a shunt motor from alternating- 

 current supply mains, and the motor would develop but little 

 torque. The shunt motor cannot be operated satisfactorily from 

 alternating-current supply mains. 



In the case of a series motor a reversal of the supply voltage 

 would not be followed by an excessive flow of current, because 

 the armature and field windings are in series ; and there can be 

 no phase difference between armature current and field magnetism 

 of a series motor which is operated from alternating-current 

 supply mains. 



Sparking. The most serious difficulty that is encountered 

 when the attempt is made to operate an ordinary series motor by 

 alternating current is the tendency to excessive sparking at the 



Fig 259- 



brushes. Whenever a brush touches two commutator bars, an 

 armature coil or section is short-circuited ; at the moment of this 

 short-circuit the armature coil surrounds a field pole * as shown 

 by the heavy line in Fig. 259, so that the armature coil is related 



*This particular statement applies to the lap drum -winding, but the effect referred 

 to exists in every variety of armature winding. 



