OPERATION AND CARE OF ELECTRIC MACHINERY 449 



circuited coils the IR drops in the various coils will be 

 the same. Of course, if there are more coils in one path 

 of the armature winding than in the other the drop per 

 coil will be different. But all the coils in series with each 

 other in one path should have the same drop. If a short- 

 circuited coil is present it will be indicated by a zero IR 

 drop, or, at least, by an IR drop much smaller than that 

 of the other coils. An open-circuited coil is indicated by a 

 zero IR drop across all of the coils in the same armature 

 path, except the open-circuited coil, and the full voltage 

 of the cell across the coil which is open-circuited. 



After all coils have been tested as far as possible the 

 armature should be revolved slightly so that the coils 

 connected to the commutator segments under the brushes 

 may be tested. 



As has been mentioned before a short- or open-circuited 

 coil, on an armature which has been in service, can gener- 

 ally be located by the burned condition of those com- 

 mutator bars to which it is connected. 



Test for a Grounded Winding. A test to see whether or 

 not the winding is grounded may be made by connecting 

 one side of a 110-volt line, through a voltmeter, to the 

 shaft of the armature and connecting the other side of the 

 line to any bar of the commutator. If the meter gives an 

 appreciable reading, it indicates a ground in the winding 

 because, if the winding was perfectly insulated no current 

 could flow through the voltmeter to make it indicate. 

 Whether or not the winding is badly grounded is determined 

 by the magnitude of the reading. If the meter gives a 

 high reading, the winding is poorly insulated; a low reading 

 signifies a well-insulated winding. A well-insulated arma- 

 ture should give, with the ordinary c-c. voltmeter, less 

 than one volt when such a test is made. If the magnitude 

 of the reading is the same as though the meter were con- 

 nected directly to the 110-volt line, it indicates that there 

 ifl a bad ground somewhere in the winding, a ground so 



